IRRGTN & DRAINAGE ENGR
IRRGTN & DRAINAGE ENGR BAEN 464
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Date Created: 10/21/15
Application Uniformity Example 51 EXAMPLE 51 DEPTH MEASUREMENTS 9quot o N 01 N h N h N o N O i 1 1 5 1 1 N N A 01 1 w w w w P F 1 w w w w w w w w w w w DEPTH inches 2 P 01 P o 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 CAN NUMBER DEPTH inches Application Uniformity Example 51 30 25 20 15 8 9 EXAMPLE 51 LOW QUARTER OVERALL MEAN 1 6 MEAN 158 19 19 19 17 17 17 39 10 11 12 13 14 15 NUMBER IN RANKING 16 17 18 19 20 Application Uniformity Example 51 EXAMPLE 51 9quot o OVERALL M EAN 196 LOW QUARTER MEAN 1 ER N on DEPTH inches 01 1 C 1 N o 1 3901 1 quoto 21 21 21 20 20 6 7 8 9 10 11 12 NUMBERIN RANKING 13 14 15 16 17 18 19 20 Q RELATJONSHJP OF UJJJFORJUJJT AJJD EFFJCJE Percent of Field Area 0 10 20 30 40 50 60 70 80 90 100 DESIRED APPLICATION DEEP PERCOLATION ACTUAL DEPTH APPLIED ADEQUACY 0F IRRIGATION I A O A h A N AlEAALDETWYI A O DEPTH APPLIED 00 10 20 30 40 50 60 70 80 90 100 AREA 0o I Figure 1 Application Depths I 16 L4 7 12 AdEAALDEPTH39 Q8 06 Q4 02 DEPTH APPLIEDMEAN DEPTH 10 20 30 40 50 60 70 80 90 100 AREA Figure 2 Relative application depths I 00 02 04 06 08 10 12 14 16 18 20 APPLICATION DEPTH AVERAGE DEPTH IRRIGATION APPLICATION UNIFORMITY 10 20 30 40 50 60 70 80 CU84 PERCENT OF FIELD AREA 90 100 Figure 4 DimensionleSSappli catidn depths z 6 6 i g i S 1 is CU 80 Df gtc a B Q 18 RELATIVE DEPTH OF APPLICATION RATIO TO MEAN 24 22 18 167 14 12 08 06 04 02 CU70 CU90 CU95 CU80 0 10 20 30 40 50 60 70 80 90 FRACTION OF AREA Figure 6 Dimensionless application depths for different values of CU I 100 gtuu0gtOz muuOmZOlt Ammv u Q no gtuu0gtOz muuOmZOlt Ammv u ltltgtmN mONmu 2ltgtmN gtuumu mg n gtmmgt gt Agtmmgt w gtmmgt 3 393 m 03320 0 gtUvnmoz mini 2 Z 0 E 02 5 04 4 06 g cu 80 j m 08 V I lt t 1 g 39 2 18 2 E 2 f I ADEQUACY80 7 a 2 2 t ADEQUACY70 7 lt5 39 f at ADEQUACY60 A 24 ADEQUACY90 i a 26 1 o 20 40 so so 100 FRACTION OF AREA 02 04 06 08 12 14 16 18 22 24 26 Figure 9 Use of the term ADEQUACY I APPLICATION EFFICIENCY sloooococoo UIOU IOU39IO APPLICATION EFFICIENCY o g 8 8 5 A 01 40 0 10 20 30 40 50 DEFICIENTLY WATERED AREA DISTRIBUTION EFFICIENCY 100 3 gt 90 E a 80 H B 70 LE LT so a 50 I cu 94 D cu90 on 40 CU84 E at CU80 E 30 cu 74 a at cu70 Q 20 o 10 20 3o 40 50 DEFICIENTLY WATERED AREA Unformigz of Agglication ACTUAL DEPTH APPLIED no1ounvuuInIanncoiI DESIRED APPLICATION 3 395 395 339 a I I I I I I I I 20 30 40 so 60 70 so 90 100 Percent of Field Area Depth of Water Consequences of Nonuniformity 39 ACTUAL DEPTH APPLIED STRESS J DESIRED APPLICATION 0 80 10 20 30 40 50 60 70 90 100 Percent of Field Area Adeguacz of Irrigation K ACTUAL DEPTH APPIJED DESIRED APPLICATION I I I I I I 30 40 60 60 70 80 90 100 Percent of Field Area Adeguacz vs Efficiency increased d of application DESIRED APPLICATION i3 3 395 i a a 0102030405060708090100 Percent of Field Area Increasing the depth applied increases the adequacy and reduces crop mar ulna Howevnr leaching increases and the application ef ciency deem Imgroved Uniformigz Less Stress and Less My poor uniformity good uniformity IDGI I DIOiIUG tI OII I f 3 D o I H a o o DESIRED APPLICATION l l 1 I I 20 3 so so 70 80 Percent of Field Area Depth of Water Uniformity vs Efficiency 39 ACTUAL DEPTH APPLIED STRESS DESIRED APPLICATION ADEQUACY OF IRRIGATION 10 20 30 40 50 60 70 80 90 Percent of Field Area 100 Application Efficiency Example Given Find CU 80 SWD 12 inches Adequacy 50 1 Irrigation Application Efficiency Ea 2 Gross Irrigation Depth d9 inches APPLICATION EFFICIENCY APPLICATION EFFICIENCY 0 10 20 30 40 DEFICIENTLY WATERED AREA 50 Figure 10 Use of the term APPLICATION EFFICIENCY I Application Efficiency Example Solution 1 Using Figure 5 Irrigation Application Efficiency Ea 900 2 Gross Irrigation Depth dg inches SWDEa 120090 133 inches Application Efficiency Example Given Find CU 80 SWD 12 inches Adequacy 80 1 Irrigation Application Efficiency Ea 2 Gross Irrigation Depth d9 inches APPLICATION EFFICIENCY APPLICATION EFFICIENCY 0 10 20 30 40 DEFICIENTLY WATERED AREA 50 Figure 10 Use of the term APPLICATION EFFICIENCY I Application Efficiency Example Solution 1 Using Figure 5 Irrigation Application Efficiency Ea 740 2 Gross Irrigation Depth dg inches SWDEa 120074 162 inches Application Efficiency Example Given Find CU 80 SWD 12 inches Adequacy 50 1 Irrigation Application Efficiency Ea 2 Gross Irrigation Depth d9 inches RRGA TION SYSTEM PERFORMANCE SYSTEM CAPACITY REQUIREMENTS mmwmrymrs In addition to meeting the seaonal and annual Irrigation requirement Irrigation systems must be able to supply enough water to prevent crop water stress during a shorter time period The Is the rate of water supply that the irrigation system must provide to prevent water stress and thus yleid loss The system capacity must account for the crop water use and the ef ciency of the system SYSTEM CAPACITY REQUIREMENTS SYSTEM CAPACITY REQUIREMENTS The system must account for both the net NETSYSTS39W system capacity on and gross system capacity 0 The net system capacity ls determined by the supply rate needed to provide crop water needs to maintain a soil water balance above a specified level that will reduce or minimize crop water stress The gross system capacity Includes the combined effect ofcrop needs and system performance NEI39 SYSTEM CAPACITY 0 NET SYSTEM CAPACITY 0 Dohrmlnlng Ibo NET SYSTBI CAPACITY II dif cult and pond on a number dhcbn Th m t WNW ve IPth I to pmld lrrlgatlon nynhm must aupply nnough wahr quotWWII WW 0 IIIIOHM mlxIllIIIm m1 prolongd podod to am an cupcud or quotpeakquot consumptlv uu rob of GINrim batman ovapotunsplrauon tho crop In this can ralnfall or stored poll demand and quotInnquot niobium Is not considered m39 Md 39quot u quot 9 quotMm 39339 PP Tho proadum alvon In Tabla 58 can b usod M anquot quot 9 mm39 ublwtb u to u mm punk comump vo m Ilmlh on cforop m Tlhla5l PEAK DAILY CROP ET FORMULA u ulmum monthlycrop nvponnplrnuon E NM 35313 35 1 15 3 35 3 35 13135 11 115 1 an a par Irrlnl on In 109 7009 13 23 24 23 23 31 33 35 31 43 42 44 43 43 1 O d 15 13 23 25 21 23 32 34 33 33 41 43 45 41 1 23 13 23 25 21 23 31 33 35 31 33 41 44 43 A d m n 25 13 22 24 23 23 33 32 34 33 33 41 43 45 A 33 13 22 24 23 23 33 32 34 33 33 43 42 44 A 35 13 21 23 25 21 23 31 33 35 31 33 41 44 A 339 391 I 3 3933 392 3923 31 33 393 393 33 31 IE quot 53 11 21 23 25 23 23 33 32 34 33 33 43 42 A WEE ETdrb 39Inpoakddlycrop ETJnldqy 55 11 21 22 24 23 23 33 32 34 33 33 43 42 A r 3 33 11 23 22 24 23 23 33 32 34 33 33 43 41 1 E39l39l In the madman month crop Et Inlmonth dquot In the not depth of Irrigation In NET SYSTEM CAPACITY Anothor hchnlquo Io dourmin the NET SYSTEM CAPACITY utlllns dally crop watt yum out ONO II 3000th for Ihls mothod Tho Ibllowlng Figure Illustraha this technique PROBABILITY OF DAILY ETVI LENGTH OF PERIOD 0 a 03 DAILY ET RATEI lnlday PROBABILITY OF SMALLER ET DAILY CROP EI inlday WATER AMOUNT INCHES SOUTH CENTRAL NEERAGKA MAV JUNE JULY AUG SEPT TIME days MONTH EFQIT mews WHEN ET gt 33mm 1 43quot SUPPLY DEF39C39T FOR A 500 GPM SUPPLY ON 130 ACRES DEFICIT NEEDED FROM SOIL TO SOUTH CENTRAL NEBRASKA MAINTAIN ET WATER AMOUNT INCHES NET SUPPLY CAPACITY 500 GPM 85 EFFICIENCY MAY JUNE JULY AUG JUNE JULY AUG MONTH MONTH WATER AMOUNT INCHES 500 GPM ON 130 ACRES 85 EFFICIENCY WATER AMOUNT INCHES 4 SOIL WATER DEPLETION 500 GFM ON 130 ACRES Q 85 EFFICIENCY 10DAY SUPPLY DEFICIT WATER AMOUNT INCHES l FEHIoanEM aurPLv usncw JUNE JULY AUG MONTH CUMULATIVE DEPLEI39ION JUNE JULY AUG MONTH MAY SEPT SEPT CUMULATIVE DEPLETION EXCEEDS ALLOWABLE DEPLETION FOR 500 GPM Em APAGIT V MU 39N SYSTEM CAPACITY 0quot AND O 8mul on modal usan I dally analyala ofaoll An mph of nequ m Is an In FIGIJRE 510 In the bxtAND THE FOLLOWING FIGIJRE JUNE JULY AU MONTH OWN SVITEII WAG AME RRGA quotON WACquotREQUIREMENTS IRRIGATION SYSTEM CAPACITY REQUIREMENTS NEBRASKA Q n AELQ I 100 39 1 DII100 a Hm a not I altow allow cum DMn on IncIn mm SYSTEM CAPACITY REQUIREUENTS 139 mImm quot 7 7quot 1 Iron malty iSt m39mwwM allure0mm m l 10 1 030 aquot Horn 101100 a 044lnlduy SYSTEM WAGITY REQUIREMENTS 03 o o c 39o m sVsTEM Morleran c o o L o 3 NH 5mg camcw gymqt a m a Example 57 Solution Uu gun 510 quotmombntdbpleuo n 15mm x4ft x 50 quot15 4 050 l 30 MOMS Pram momma 760 Win capacity I approximatin 022 Inlay Sample 51 Sandyme Sol 15 In of walla owa br perfo ot ofsoil pm 00m root tom pm M4 10939 and manq om htullmblo 39dlplotlon 0160 want Reguln39d Mot Wmnlpnlv nudod at a 95 probablllty loyal Soluiion Us Flour 510 SYSTEM CAPACITY REQUIREMENTS 0 on e E WICBIIEIL TY LEVEL g 5 eg 025 933 E 4 E E A 4 gnaw a 0 quot 3 0 5 3 H 0 l0 2 u x o z a 5 M Iwmar qwemwhr dbllvm39lqesasmw bdlhqlqdcdlng n39d 39lrfnlna an qteaoss svsmwmm mztonowl nuamln MI woulo dn WE VE o sunnimhm chniclmgmd E n tuniopmym ahmw Ed o slz 1 Th9 n39et capacity has boon glyoninxlngh a poi day and thla musth39e omm39b I flew rat poium de36qu 1030 mm a walnl fxidayl h r 0 51 OPmInI39Q Imw39v39ulm uphovi mugs miuppnmoh C lele may I 85 on both39 nldgand ismquot ndownaowgmuunw 39 Muind M dbphargo 1510 Wall Eatmph Solution 2 The grouolmo lw eld biglvon by Q 51 g ml r tuw 101 I 919 pmloan spree coins 0 97 36 mann migpmlo a 381733quot Mnml yn K um In nsm u my u pmsnlmmmllwmllsm n kxmlmnlhmnghwl nu lnm Smmmvhcwlupu n pmuxvla uv 1bSDA ammm WNW Hnnu wllwmsx x my Kmmmm mywmwmmPM ml mm a WWWquot mlmuwmn MW v mmwnn mnhu mum ynm hrmpumkmum a wmxrmmvlhb rmpnamvmmp mllnn m s sm Mawrslnmschrmr vy mm anmlll mum mH mmme Mrwudmxmllw w mmmmm Msmm mmmn Mm n m Mum ww er mm um The following materials were taken from the following TEXT CLASS NOTES IRRI GA T I 0N PRINCIPLES AND MANAGEMENT a text by Dr Dean Eisenhauer Dr Derrel Martin and Dr Glenn Hoffman Biological Systems Engineering Department University of Nebraska Lincoln Nebraska M2002 Chapter 5 Irrigation System Performance 5 1 Introduction Management of irrigation systems must be based on the desired objectives or outcomes consistent with economic energy environmental labor water and resource constraints Goals can vary from maximizing pro t producing a contracted yield optimizing water resource use or maintaining the quality of produce or the landscape Managers cannot achieve these goals without considering the performance of the irrigation system The investment and operational costs must be weighed against the bene ts or returns generated There are many regulations and safety issues that irrigators must also consider This chapter explains basically how irrigation systems operate de nes parameters used to quantify performance describes basic requirements all systems must provide gives a range of attributes for systems and provides some safety and legal considerations for management Characteristics of speci c systems are detailed in later chapters The key here is to understand the basic systems and their relative performance 52 Types of Systems There are three general types of irrigation systems 1 sprinkler 2 surface and 3 microsystems including drip and trickle 521 Sprinkler Irrigation Sprinkler irrigation systems are used for agricultural or horticultural production and for landscape or turf applications The principles of operation are the same for all applications even though the management objectives may differ Sprinkler systems can be divided into ve basic types single sprinkler systems solidset systems moved lateral systems moving lateral systems and microirrigation systems Single sprinkler systems are designed to irrigate an entire area with only one sprinkler that is moved or automatically moves across the area Examples range from the single lawn sprinkler that is placed throughout the yard to automatically moving systems equipped with a large gun type sprinkler that throws water hundreds of feet The performance of the single sprinkler systems depends on placing the sprinkler at the proper location for the correct amount of time The systems generally apply water beyond the irrigated area to ensure that the desired land is adequately watered However the single sprinkler system is quite versatile and widely used for irregularly shaped land area In many cases pressure and energy requirements are quite high for single sprinkler systems leading to high operational costs The basic components of lateralbased sprinkler systems are the mainline that is a pipe network designed to carry water from the water source to the lateral The sprinkler devices are located on the lateral pipelines When the lateral is placed permanently in one location in the eld the system is called a solidset system Generally the lateral and mainline of solidset systems are installed under the soil surface and the sprinklers are raised above ground with pipes called risers or the sprinkler devices are specially designed to pop up above the soil when water pressure builds in the lateral Solidset systems are widely used on lawns landscapes golf courses and some agricultural and horticultural applications The systems can be very ef cient since each sprinkler in the system is only used in the area it was designed to irrigate The systems are easily automated and can apply any depth desired Because of the extensive piping network and large number of sprinklers needed solidset systems usually require large capital investments To reduce investment costs a single lateral could be moved across a portion of the irrigated area and used in multiple locations The earliest and simplest of these moved lateral systems is carried by hand and is called a handmove system The lateral can also be moved by pulling the lateral across the eld which is called a skidtow or towline system Laterals can be mounted on wheels that suspend the pipeline above the crop These systems are called sideroll systems because the wheels are rolled across the eld to reposition the lateral Moved lateral systems require less to operate Because of the large labor requirement the laterals are usually left in one location for 8 12 or even 24 hours Thus the systems usually apply large depths of water each irrigation Automated systems have been developed to move the lateral across the eld Examples of moving lateral systems include center pivots linear or lateral move systems and automated sideroll systems These systems still use one lateral to irrigate a large area but since the lateral moves at a controlled velocity the depth of water applied can vary over a wide range Moving lateral systems are usually much more expensive than moved lateral systems but labor requirements are substantially reduced and water is used more ef ciently 522 Surface Irrigation Several types of surface irrigation are used depending on topography soil texture and the types of crops grown Surface irrigation systems are used primarily on agricultural or orchard crops With surface irrigation the water is distributed across the eld as it ows over the soil surface There is less control of the depth of water applied than with sprinkler systems because the soil in ltration rate determines the rate of water application rather than the supply system as with sprinklers Surface irrigation methods generally have lower pressure requirements than sprinkler irrigation and therefore are less expensive to operate per unit of water applied The installation costs of surface systems may be lower than for sprinklers if leveling is not necessary Two common problems occur with surface irrigation To irrigate uniformly water must advance across the field quickly This means that some water will run off the end of the field Some states have regulations stating that the surface runoff must not leave the field The second problem is that surface irrigation is laborintensive Irrigators are generally unwilling to invest the time needed to efficiently irrigate This results in excessive applications leading to water losses as runoff or deep percolation Recent developments in surface irrigation may allow producers to irrigate more efficiently and avoid the excessive labor requirement A surface irrigation system must contain some type of water supply mechanism which may be a quotheadquot ditch gated pipe or buried pipelines with valves at the surface A variation is the use of siphon tubes to deliver water from a supply ditch located along the top of the field The simplest surface irrigation method consists of ooding from a ditch in which the water level in the ditch is allowed to rise until the ditch overtops Water spreading is similar to ooding from a ditch except that the path of the water over the irrigated field is not controlled Whatever water supply device is used water will run across a constrained portion of the field This area of the field may be constrained by small dikes in border irrigated field or the furrows in furrow irrigation Sometimes an area is leveled and surrounded by small dikes This type of system is called basin irrigation If the field is nearly level in both the direction of ow and the transverse direction the water that would run off the field may be blocked and forced to stay on the field Ifthe field has much slope in either direction a runoff recovery system can be used to reuse water that runs off the field 523 Drip Trickle or Micro Microirrigation systems may consist of a lateral containing drip emitters or microsprinklers or a lateral with out ow continuously along its length soaker hose Microirrigation is unique in that the discharge devices are intended to irrigate individual or groups of plants and not the entire area In landscape applications the ow rate from each emitter may be quite small while in orchard applications several devices may be required to apply the needed irrigation Microsystems are usually permanently installed and can be quite expensive Labor requirements are minimal although maintenance may be high for systems with low ow rates Microirrigation systems are popular on highvalue crops in locations where water is expensive or in short supply Emitters and microsprinklers have very small orifices or outlets Since the orifices are small it is necessary to prevent the plugging of the microsystem by soil particles or microorganisms such as bacteria Microsystems work best when the system can be left in place for multiple seasons Microsystems are among the most expensive methods of irrigation They are generally not applicable to row crop production due to the expense and the need to remove the system each season Currently research is underway to apply microirrigation technology to row crop irrigation by either burying microlaterals or by dragging microlaterals through the crop using a linear move or center pivot as the supply manifold Microirrigation is used extensively for landscape applicationespecially for trees and shrubs Additional considerations of these systems include 1 high efficiency because evaporation loss is small since the whole plant area is not wetted 2 apply water at very low rates so runoff is applicable for very steep slopes 3 are easily automated to minimize labor and 4 require extensive piping systems of mainlines and laterals 53 Management Relationships Achieving management objectives requires that water be applied at the proper time rate and quantity and in the desired location However irrigation systems are not perfect and some water applications may be inappropriate or impractical Some areas may receive more water than others while some water is simply lost How should an irrigator respond to inefficiency and nonuniformity How does a management change affect operation and performance To address these questions relationships must be developed to quantify performance 531 Ef ciency Irrigation systems are not completely efficient The major ways water can be lost from an irrigated field are illustrated in Figure 51 For irrigation systems such as sprinklers that throw water into the air while irrigating some evaporation occurs while the droplets are in the air or once they reach the crop surface Research suggests that there is little evaporation of the drop while in the air and that evaporation from the crop canopy may be more significant If wind blows droplets may be blown outside of the land to be irrigated Thus drift losses may be important When water is applied at a rate that exceeds the infiltration rate of the soil water begins to accumulate on the soil surface If the water builds up sufficiently it will begin to run off the area where applied or off of the field The runoff water could also infiltrate at a lower elevation in the field leading to poor uniformity When water is applied to the field in excess of the soil water depletion SWD the excess water may leach from the soil profile a quantity called deep percolation Irrigation water that remains in the soil at the end of the growing season may also be lost if offseason rains would have replenished the root zone anyway Thus there are many ways applied water can be lost from the crop root zone The manager must minimize losses where possible yet some losses will occur In this case the manager must know how much water might typically be lost to adequately supply crop needs DR 1 CANOPY EVAPORATION 9N EVAPORATION L 27 1 I i I l OPEr i DEEP PERCOLATION Figure 51 The application ef ciency E has been used to describe the portion of an application that is available for crop use The application ef ciency is usually de ned as the fraction of the applied water that is stored in the root zone and is available for crop water use The water stored in the root zone is o en called net irrigation and the total amount applied to the eld is called grass irrigation Thus the application ef ciency is de ned as E dndg 51 Where Ea application ef ciency dn net irrigation depth and dg gross irrigation depth The application ef ciency can be expressed as either a decimal fraction ie ranging from 0 to 10 or a percentage ranging from 0 to 100 The application ef ciency is the result of system characteristics management soil and crop conditions and ultimately the weatherespecially rainfall Therefore there is a broad range of application efficiencies 533 Application Uniformity Irrigation systems are not capable of applying exactly the same depth of water to every location in the field For moved lateral systems the points along the pipeline that receive the highest pressure generally apply more water These points are usually in lowlying areas or near the inlet to the lateral For furrow systems the downstream end is usually drier than the head end of the furrow Some emitters can become partially clogged reducing the discharge at some locations in trickle systems The end result of these problems is that some areas of the field receive more water than other areas For many irrigation systems the depth applied at a point is nearly the same as the depth infiltrated at the point Thus nonuniform application leads to nonuniform depths of infiltration and ultimately to varying levels of soil water in the root zone This nonuniformity severely affects system performance so information about the uniformity of application is needed to manage effectively The uniformity can be measured for all irrigation systems Sprinkler systems containers like rain gauges are placed in a grid format in the field The irrigation system is then operated for a period of time and the depth of water caught in each container is measured For trickle or drip systems the volume of water emitted in a given time is measured for all emitters on a lateral For surface irrigation experiments can be conducted to determine the depth of water that infiltrates at points along the field To evaluate uniformity a method is needed to compute a performance parameter from field test data The two most commonly used methods are the distribution uniformity DU and the Christiansen coefficient of uniformity CU The DU is a simple method used to indicate the uniformity of water application d DU dz 52 where dLQ average lowquarter depth of water received and dZ mean depth infiltrated The value of dLQ is the average depth of application for the lowest onequarter of all measured values when each value represents an equal area of the eld You can determine the lowquarter depth by ranking observed depths and computing the average for the smallest 25 of the values The CU is also used to evaluate application uniformity When each observation represents the same area the CU is determined as quot dd CU1EIZ 1 1 n d 53 where di depth of observation i dZ mean depth in ltrated for all observations and n number of observations d1quot d Note that the ratio I z is the mean deviation Thus another way to write n Equation 53 is 100l mean deviationmean depth infiltrated Equation 53 was developed to interpret data collected with catch cans placed under sprinkler irrigation Typically water depths in the equation were amounts caught in the cans not in ltrated water Since the distribution of the in ltrated water is really what is of interest we ve replaced catch depth with in ltration depth in Equation 53 If there is no runoff in ltration equals the catch depth When the CU is greater than 70 the depth applied is usually distributed with sprinkler and microirrigation systems When this holds approximate relationships between DU and CU can be written as CU 37 63 DU 54 DU 00159 CU 059 55 The following example illustrates computation of uniformity indices Example 51 Given A sprinkler system was evaluated using 20 catch can containers The depth caught in each container is given below Can No Depthginl Can No Depthgin Can No Depthginl 24 8 15 15 17 1 2 17 9 23 16 20 3 21 10 20 17 23 4 19 11 19 18 18 5 18 12 21 19 21 6 22 13 17 20 19 7 13 14 24 M Compute the distribution uniformity and Christiansen39s coef cient of uniformity Solution Rank the data in ascending order and compute dZ and dLQ d d 11 1 l d 11 inch 1 d 11 inch 1 d 11 inch 1 d inch dzl dzl dzl dzl 1 13 066 6 18 016 11 20 004 16 22 024 2 15 0 46 7 19 006 12 20 004 17 23 034 3 17 026 8 19 006 13 21 014 18 23 034 4 17 026 9 19 006 14 21 014 19 24 044 5 18 016 10 20 004 15 21 014 20 24 044 dLQ average of1 to 5 158 in dZ average of 1 to 20 196 in Then compute the individual deviations I di dZ I and the sum of deviations Zldidzl 470 Then DU dLQdz158196081 CU1001ZIdidZIndz1001 88 47 20 x 196 Using Equation 54 shows that an estimated CU from the DU calculation would be CU3763 DU3763 08188 The DU and CU terms are used differently for different irrigation systems Typically CU values are used for sprinkler and microirrigation systems while DU has become more popular for surface systems However some organizations use DU exclusively for all irrigation systems Methods used to measure the uniformity of center pivot irrigation systems are unique and a modified Christiansen s coefficient of uniformity is used The uniformity of a center pivot is measured by placing containers along two radial lines The cans are usually placed with uniform spacing from 5 to 15 feet apart along the line Then the pivot is operated so that the lateral passes over the containers Since the pivot operates in a circular fashion a container located far from the pivot point represents more area than one close to the pivot point Therefore a revised CU is used for pivots The Heermann and Hein coefficient of uniformity is ordinarily used Heermann and Hein 1968 El di 39 dz 1 Si CUH 100 1 56 n 2 di Si i1 where Si distance from the pivot point to the container and dZ weighted mean infiltration which is equal to 57 Uniformity values are not used like efficiencies rather they provide an index of performance The optimal value of CU or DU depends on the cost of irrigation water value of the irrigated crop costs of drainage or water quality impacts and the cost of system andor management renovation This analysis is very system specific and is described in subsequent chapters Rules of thumb have been used to judge an acceptable uniformity For moved lateral systems a CU of 80 or DU of 70 is commonly the lowest acceptable uniformity For center pivots a CUH 90 is often achieved For furrow systems a DU of 60 may be the lowest acceptable value 534 Adequacy How should an irrigator react to nonuniformity If the average infiltration equals the average soil water depletion each irrigation then about half of the field will receive more water than needed to refill the crop root zone and deep percolation will ultimately occur The other half of the field will not receive enough water to refill the root zone and plant water stress may occur The irrigation manager is continually faced with this tradeoff between excessive deep percolation and plant water stress The management decision affects profits and application efficiency An important variable is the adequacy of irrigation Adequacy of irrigation is the percent of the field that receives the desired depth or more of water Adequacy of irrigation can most easily be evaluated by plotting a frequency distribution as shown in Figure 52 The curve is developed by grouping field measurements of infiltration depth in descending order and computing the fraction of the field area that receives at least a given depth of water The point where the curve intersects the desired depth indicates the percent of the field that is being adequately irrigated In Figure 52 80 of the land received the desired depth of infiltration or more so those areas were adequately irrigated The remaining 20 of the field experienced some stress a Pancaumon us EDAI noNDEPm t n FLFA VHWGAYION APPLICATION DEPTH l r l 1 1 l l a 1a 20 an 40 50 EU 70 an an we 39 PERCENT OF FIELD AREA Figure 52 Cumulative frequency distribution of eld in ltration depth indicating adequacy of irrigation The proper adequacy of irrigation depends on many factors and probably varies throughout a growing season With an existing irrigation system the manager can most easily vary the average depth of application to change the adequacy This amounts to simply moving the distribution curve in Figure 52 up or down To change the shape of the distribution curve may require system modi cation Which is usually impractical during the season Of course if an irrigator increases the average depth applied more deep percolation Will occur There is a direct link between application ef ciency and 39formi 535 Application Efficiency of the Low 3 QuarterUni cation of Efficiency and Uniformi It is important that all water quotlossesquot during application be considered in an ef ciency calculation These losses include evaporation and dri run deep percolation due to nonuniform infiltration deep percolation due to excessive application and conveyance losses Deep percolation occurs whenever in ltration exceeds the SWD Excess in ltration can be caused by both the nonuniformity of application and excessive application The percolation caused by the nonuniformity comes about because the manager must make a decision as to how much of the eld should be adequately irrigated A common albeit somewhat arbitrary approach is to use the average low quarter depth as the quotmanagement depthquot Managing according to the average low quarter depth results in approximately 90 of the eld being adequately irrigated and potentially about 10 being deficitly irrigated Conservation of mass requires that the following rules be true if conveyance losses are ignored dg dz d dev 58 where dg gross depth applied dZ average depth in ltrated dr depth of runoff and deV depth of evaporation and drift Rearranging Equation 57 results in dz dg d dev 59 Also it can be shown that 0amp9 DUd 510 where dLQ average depth of low quarter and DU distribution uniformity expressed as a decimal How much of the lowquarter water is effective will depend upon the quantity of in ltration relative to the SWD Figure 53 illustrates 4 scenarios In 53a the in ltrated water is perfectly uniform and is equal to SWD In this case where de effective depth of irrigation No deep percolation would occur in this hypothetical example In Figure 53b the in ltrated water is still distributed perfectly uniformly but due 12 to excessive application in ltration exceeds SWD In this case dLQ dz and dz SWD The excessive application can be caused by irrigating too frequently or the system is operated too long for the existing SWD The interval between irrigations can be increased as long as SWD does not exceed the allowed de cit AD a concept discussed in Chapter 6 Nonuniform in ltration is illustrated in 53c Here the tile SWD de In this case deep percolation is not due to excessive application but is due to the nonuniformity of the in ltration The majority of the eld approximately 90 experiences deep percolation because of the management decision to only allow about 10 of the eld to be de citly irrigated Figure 53d illustrates the case where there are deep percolation losses due to both excess application and nonuniform in ltration The gure illustrates the division of the two losses In this case d2 SWD The effective depth of water is the amount of water that will be used in irrigation scheduling management Its utility will be illustrated in Chapter 6 Percent of Area Percent of Area 1C0 O 10 0 n1 Uniform no excess Uniform excess application SMD Deep perc O SMD a 5 i Q Q a k as W l Infiltration l Deepperc Infiltration a b Percent of Area Percent of Area 0 50 1C0 0 50 100 O Ci 1 Non uniform no excess Nonuniform excess application Deficitly SMD irrigated SMD Infiltration 4 Depth l M eep peril nonuniform y C Figure 53 Distribution of in ltration and deep percolation under 4 scenarios Figure 53 can be summarized by the following equations IdeQ s SWD then d 1 511 Ide gt SWD then at SWD 512 The application efficiency of the AELQ is then AELQ 100 513 W where dW depth applied from the original source Determination of the depth of water from the original source is straightforward except when runoff recovery is part of the system Either Equation 31 or 33 can be used for the calculation of dW Without runoff recovery dW and dg are equal dW is always equal to the volume of water taken from the original source such as a well divided by the total land area irrigated Runoff recovery is a common practice in surface irrigation Either open systems or closed systems are used These are discussed in detail in Chapter 10 When conveyance losses are ignored the relationship between dW and dg are Closed runoff recovery water reused on same eld dw dg d R 514a d3 1 R K Open runoff recovery water reused on different eld d d 3 w 1 R Rt 514b where RT runoff ratio ddg and R1 return ratio the depth of water returned reused divided by the depth of runoff Example 52 In Problem 51 the DU was 081 and dZ equaled 196 inches If dg 22 inches runoff was zero and SWD 16 inches determine the system s AELQ and dev Given dZ 196 in SWD 16 in dg 22 in dr 0 DU 081 Find deV AELQ Solution Using Equation 57 devdgdZdr dev 22 in 196 in 0 024 in Using Equations 410 411 413 and 414 d AELQ e 100 dW 19 00x04 Since dLQ lt SWD de 159 in 081196 159 in Since dr 0 dW dg 22 in Thus 159 in AELQ 100 72 22 m Example 53 Repeat Example 52 if SWD equaled 12 inches Solution Now dLQ gt SWD thus Equation 511 applies and de SWD 12 in Thus AELQ 100 55 l 22 39n 536 The Scheduling Coefficient Another term that is an index of irrigation efficiency is the scheduling coefficient SC It is commonly used for a description of turf sprinkler systems It is used to calculate how long a system needs to apply water with the realization that the water will not be applied perfectly uniform For example if the goal is to apply 05 inches of water and the sprinkler system applies 025 inches per hour it would take 2 hours to apply the desired depth if the water were distributed uniformly across the irrigated area But it usually is not Thus to adequately irrigate the desired proportion of the lawn the sprinkler must be run longer than 2 hours Assuming 90 adequacy is the goal SC z 515 thus SC is just the inverse of DU Example 54 A lawn sprinkler system was tested and shown to have a DU of 067 If the average depth caught in the cans dz was 15 inches and the sprinkler had been running for 5 hours determine the scheduling coefficient SC the dLQ and the number of hours the sprinkler would have been running if the pattern had been perfectly uniform Find dLQ SC Time if uniformity had been perfect Solution Using Equation 59 dLQ 06715 10 in Using Equation 514 SC 15 in10 in 15 A SC of 15 says the sprinkler had to run 50 longer than necessary because of uneven distribution Thus with perfect uniformity the time to operate would have been Time 5 hours15 33 hours 537 Chemical Leaching Losses Deep percolation losses not only decrease irrigation efficiency they result in chemical movement or loss below the root zone Deep percolation due to nonuniformity is designated Vdpl For an adequacy of 90 and normally distributed water application depth the deep percolation due to nonuniformity is given by V4 V2 1 F1 516 where V2 dZ A volume in ltrated dZ average depth of water infiltrated A total irrigated area and F1 factor Table 54 Deep percolation due to excessive average irrigation depths and or irrigating too frequently excessive application is denoted Vsz and Ide9 5 SWD VHF o 517 IdeQ gt SWD Vdp 095 A dLQ SWD The total deep percolation Vdp equals the sum of Vdpl and Vdpz ie 18 V 1sz apIV m 518 The depth of deep percolation ddp V d4 7quot 519 The amount of chemical lost with the leachate can be calculated by CI 0226 X C X dab where Cl chemical loss lbac C concentration of the chemical in the leachate ppm and ddp depth of deep percolation acin ac Example 54 Find the nitrate leached in pound per acre for the eld illustrated in Example 51 if the average annual concentration of nitrate in leachate is 20 ppm of NO3N Find Solution Determine the amount of nitratenitrogen leached from the eld Since we re to calculate this in lbac assume that A 1 acre From Table 54 F1 078 Using Equation 514 Vdpl dz A 1 F1 196 in1 acl 078 043 acin Vsz 095 AdLQ SWD 0951 ac159 in 12 in 037 acin Vdp Vde l Vde 043 acin 037 acin 080 acin V 08 acin ddp 7394quot 08 in 1 ac C1 0226 x 20 ppm x 08 do 36 lbac 20 Table 54 Relationship of CU DU and F1 for a 90 adequacy CU F CU F 70 046 82 067 71 048 83 069 72 049 84 071 73 051 85 073 74 053 86 075 75 055 87 077 76 057 88 078 77 058 89 080 78 060 90 082 79 062 92 086 80 064 94 089 81 066 96 093 98 096 A check of the average depth of deep percolation can be made by determining the depth of percolation at each can and then averaging For example the depth of deep percolation in Can No 1 is 12 inches 24 inches caught12 inches SWD For Can No 20 it is 07 inches 19 12 For the 20 cans the deep percolation is Can No Deep Perc in Can No Deep Perc in 1 12 11 07 2 05 12 09 3 09 13 05 4 07 14 12 5 06 15 05 6 10 16 08 7 01 17 11 8 03 18 06 9 11 19 09 10 08 20 07 Averaging the 20 depths we get an average deep percolation of 076 inches which compares well with the 08 inches calculated by the other method 538 Conveyance Ef ciency Water can also be lost in the delivery system used to supply water to the eld or land area or used to convey water within the farm or eld 21 Losses are most signi cant for canal or ditch systems that convey water over long distances through permeable soils Water can be lost due to seepage from the canal or other conduit by evaporation from exposed water surfaces and by evapotranspiration from phreatophytes along the conveyance system In long delivery systems water can be lost because of operational problems If an irrigator originally requested water delivery but later decided not to take the full supply some water might quotspillquot from the system Alternatively a few irrigators might need or request water yet the canal cannot deliver water with such small ows Thus excess ow is required to supply the requested amount The conveyance efficiency is used to describe the efficiency of the delivery system The conveyance efficiency is defined as the amount of water delivered to the irrigated area divided by the total amount of water supplied or diverted from the supply either reservoirs rivers or groundwater dg E 7 520 where Ec conveyance efficiency dg gross depth of waterapplied and d5 depth of water supplied The conveyance efficiency can be either a decimal fraction or a percentage Measuring water losses in canals and other delivery systems is difficult and expensive and for most management purposes the conveyance efficiency can be estimated Several efficiency terms have been used depending on where the delivery system is located Doorenbos and Pruitt 1977 divide the efficiency of an irrigation project into three components supply conveyance efficiency EC field canal efficiency Eb and field application efficiency Ea Conveyance efficiency and field canal efficiency are sometimes combined and called the distribution efficiency Ed where Ed Ec x Eb The combination of the field canal and application efficiencies is often called the farm efficiency where Ef Ea x Eb The application efficiency can be estimated from the methods described earlier in this section Factors affecting conveyance efficiency include the size of the irrigated area type of delivery schedule used to deliver water types of crops canal lining material and the capabilities of the water supplies The field canal conveyance efficiency is primarily affected by the method and control of operation the type of soils the canal transects the length of the canal and the size of the irrigated block and fields The farm efficiency is very dependent on the operation of the supply system relative to the supply required on 22 the farm Doorenbos and Pruitt 1977 present approximate efficiencies for various conditions as summarized in Table 55 A procedure used in USDASCS s Washington State Irrigation Guide can also be used to estimate seepage losses WAIG 1985 The method gives a range of expected seepage losses depending on the transport material in the delivery system Figure 57 The range is dependent on the amount of nes in the material In addition the following losses may be expected Ditch side vegetation 05 1 loss per mile Buried pipeline 001 015 ft3f t2 depending on the age and type ofpipe An example calculation of the season water loss from an earthen ditch follows 23 Table 55 Conveyance eld and distribution ef ciencies for various types of systems adapted from Doorenbos and Pruitt 1977 Project Conveyance characteristics ef ciency Continuous supply with no substantial change 90 in ow Rotational supply for projects with 7000 to 80 15000 acres and rotational areas of 150 to 800 acres and effective management Rotational supply for large projects gt 25000 acres and small projects lt 2500 acres with problematic communication and less effective management based on predetermined delivery schedules 70 based on arranged delivery schedules 65 Irrigation eld Field characteristics ef ciency Irrigated blocks bigger than 50 acres with unlined canals 80 lines canals or pipelines 90 Irrigated blocks smaller than 50 acres with unlined canals 70 lined canals or pipelines 80 For rotational delivery Project district systems with management distribution and communication adeguacies of ef ciency Adequate 65 Suf cient 55 Insuf cient 40 Poor 30 24 New TYPE OF MATERIAL IN CONVEYANCE SYSTEM Concrete Ditch and Above Ground Pipe Deteriorated Gravelly Sand Gravel SEEPAGE Losst It2 Iday Figure 54 Method to estimate seepage losses from irrigation delivery systems adapted from WAIG 1985 25 Example 5 6 Given Soils Loam Ditch length 1320 ft Flow area 25 ftZft measured wetted perimeter Time water in the ditch 180 days Stream size 25 cfs Find Conveyance loss Solution Use Figure 57 to nd the seepage loss ofa loam soil 123 ft3ft2day Use average values to compute the seepage loss Flow Area x Length x Loss Time 43560 Zac 25 x 1320 x 123 X 180 43560 168 Vegetation loss at 1 of the total ow for the period per mile X ow X days X length miles X 2 acftcfsday 001 X 25 X 180 X 13205280 X 2 23 acft Total loss seepage loss vegetation loss 168 225 191 acftyr The accuracy with this method is no better than 05 acrefeet so the estimated loss is 19 acrefeetyear 54 System Capacity Requirements In addition to meeting the seasonal irrigation requirement irrigation systems must be able to supply enough water to prevent crop water stress during a shorter time period The system capacity is the rate of water supply that the irrigation system must provide to prevent water stress The system capacity must account for crop need and the ef ciency of the irrigation system These computations are distinguished by the net system capacity Qn versus the gross system capacity Qg The net capacity is determined by the supply 26 rate needed to maintain the soil water balance above a specified level that will reduce or minimize water stress The gross capacity is the combined effect of crop needs and system inefficiency Net and gross capacity are related by the application efficiency and the percent downtime D for the system Qquot Q AELQ i 521 100 100 where Qg gross system capacity Qn net system capacity AELQ application efficiency of low quarter and Dt irrigation system downtime 55 Net System Capacity Determining the net system capacity is difficult Irrigation systems must supply enough water over prolonged periods to satisfy the difference between evapotranspiration demands and rainfall Water stored in the crop root zone can supply part of the crop demand However the volume of water that can be extracted from the soil cannot exceed the amount that will induce crop water stress A careful accounting of the soil water status is required if soil water is used to supply crop water needs during periods when the crop evapotranspiration demands are larger than the irrigation system capacity plus rainfall Some irrigation designs have been developed to completely meet peak ET needs without reliance on either rain or stored soil water Other techniques intentionally rely on stored soil water to meet crop requirements The most conservative method is to provide enough capacity to meet the maximum expected or quotpea quot evapotranspiration rate of the crop In this case rain and stored soil water are not considered in selecting the system capacity This design procedure relies on determining the distribution of crop ET during the year The ET during the season varies from year to year With the peak ET method the maximum daily ET for each year is determined Then the annual maximum daily ET rates are ranked and plotted The system capacity required to meet peak daily ET 70 of the time ie in 7 of 10 years is acceptable for this method A method to predict the daily peak period ET rate for general conditions was presented by the USDASCS 1970 as shown in Table 56 This relationship should only be used for general estimates and where more localized peak data are not available 27 Table 56 Peak daily crop ET rates as related to maximum monthly ET for the crop during the season and the net depth applied per irrigation Net Maximum monthly crop evapotranspiration ETm inmonth 3225 5 I 6 I65 7 I75 8 I85 9 I95101051111512 in Peak daily evapotrans1iration ETd inday 10 20 24 26 28 31 33 35 37 40 42 44 46 49 51 15 19 23 25 27 29 32 34 36 38 41 43 45 47 50 20 18 23 25 27 29 31 33 35 37 39 41 44 46 48 25 18 22 24 26 28 30 32 34 36 39 41 43 45 47 30 18 22 24 26 28 30 32 34 36 38 40 42 44 46 35 18 21 23 25 27 29 31 33 35 37 39 41 44 46 40 17 21 23 25 27 29 31 33 35 37 39 41 43 45 45 17 21 23 25 27 29 31 33 35 37 39 41 43 45 50 17 21 23 25 26 28 30 32 34 36 38 40 42 44 55 17 21 22 24 26 28 30 32 34 36 38 40 42 44 60 17 20 22 24 26 28 30 32 34 36 38 40 41 43 Formula used ETn1 0034 ETml 9 dh39 9 The peak ET method is based on selecting a system capacity that can supply water at a rate equal to the peak ET for a period However it is unlikely that several periods with water requirements equal to the peak ET will occur consecutively The crop water use during the combined time period can come from the irrigation supply or from rain and stored soil water Therefore the capacity could be reduced if rain is likely or if stored soil water can contribute part of the ET demand Relying on soil water can reduce capacity requirements in two ways First the soil water can supply water for short periods of time when climatic demands exceed the capacity The soil water used during the short period can be stored prior to its need or be replaced to some extent during the subsequent period when the ET demand decreases When the irrigation capacity is less than the peak ET rate there will be periods of shortage when crop water use must come from the soil or rain Figure 58 However during other periods the capacity may exceed the ET and the water supplied during the surplus period can replenish some of the depleted soil water Figure 58 The second way soil water can contribute to reduced capacity requirements is through allowable depletion AD This is the amount of water that can be depleted from the soil before crop stress occurs The minimum capacity that maintains soil water above the allowable depletion during critical periods of the season can be used to design the irrigation system An example of the effect of net capacity on soil water mining and the magnitude of soil water depletion during the season are shown in Figure 59 28 Figure 58 An example of the shortage and surplus periods for a system Where the DAILY EVAPOTRANSPIRATION 9 NET SYSTEM CAPACITY 025 inchesday AVERAGE CROP EVAPOTRANSPIRATION 0275 inchesday DAILY CROP EVAPOTHANSPIHATION Inchesday o 5 15 20 10 TIME days capacity is less than the average ET during a peak water use period WATER AMOUNT INCHES CROP EVAPOTRANSPIRATION C DEFICIT SUPPLY CAPACITY 392 500 GPM J 700 GPM 3 h v K 39 ALLOWABLE DEPLETION 4 CUMULATIVE SOIL WATER DEPLETIONl FOR A 500 GPM SUPPLY CAPACITY 1984 39 I MAY i JUNE JULY 1 AUG SEPT DATE Figure 59 Diagram of the 10day ET rain and soil water deficit plus the soil water depletion pattern over a growing season as affected by gross system capacity Based on a 130acre field and 85 application efficiency 30 The positive bars in Figure 59 represent the amount of rainfall and ET during 10day periods After midMay ET exceeds rain The de cit bars represent the difference between ET and rain The largest 10day de cit occurs in midJuly Without considering the use of soil water the irrigation system would have to supply the de cit in that period The peak 10day irrigation requirement would be 33 inches per 10days or 624 gpm peracre For the 130acre eld shown in Figure 59 the net capacity requirement for the peak 10day period would be 810 gpm and using an 85 application ef ciency AELQ the gross capacity requirement would be approximately 950 gpm The amount of water that a 500 gpm capacity system with an 85 application ef ciency can supply is also shown in Figure 59 The net capacity for this system is Qquot 500 gpm X 14acinhr X 24 hr X 1 085 017 inday 0 gpm day 130 ac The 500 gpm capacity falls short of meeting the ET in late June and soil water would be depleted The 500 gpm capacity falls short of the 10day deficit from early July through late August resulting in a cumulative depletion of 4 inches Suppose that the AD before stress occurs is 3 inches for the crop and soil in Figure 59 With the 500 gpm capacity system the soil water would be depleted below the allowable level in late July and the crop would suffer yield reduction Obviously 500 gpm is inadequate for maximum yield at this site The net supply capacity for a 700 gpm system is also shown in Figure 59 Here the system can supply the 10day de cit for all but 20 days in late July The cumulative soil water de cit for the 700 gpm system would be about 125 inches with proper management That depletion is well above the AD and should not reduce crop yield This example shows that the maximum cumulative soil water depletion would be approximately 4 125 and 0 inches for gross capacities of 500 700 and 950 gpm respectively Clearly the opportunity to utilize available soil water substantially reduces the required system capacity Simulation programs using daily time steps to predict the soil water content have been used to determine the net system capacity when soil water is intentionally depleted Some models such as by Heermann et al 1974 and Bergsrud et al 1982 use the soil water balance equation to predict daily soil water content von Bemuth et al 1984 and Howell et al 1989 used crop simulation models to predict the net capacity to maintain soil water above the speci ed allowable depletion and the capacity needed to maintain yields above a speci ed percentage of the maximum crop yield The capacities determined using soil water andor crop yield simulation are usually very dependent on the available soil water holding capacity of the soil An 31 example from the results of Heermann et al 1974 is shown in Figure 510 To use the procedure of Heermann et al 1974 the allowable depletion of the soil pro le must be determined The allowable depletion is the product of the allowable percent depletion and the available water in the crop root zone The use of Heermann et al s procedure for a sandy loam soil is illustrated in the following example Example 5 7 Given A sandy loam soil that holds 15 inches of available water per foot of soil depth Corn root zone depth of 4 feet Management allowable percent depletion 50 Find The net system capacity needed at a 95 probability level Solution The allowable depletion is computed as l5inft X 4ft X 05030in From Figure 510 the net capacity is approximately 022 inches per day 32 025 020 NET SYSTEM CAPACITY inchesday 015 010 Figure 510 Design net capacity required for corn grown in Eastern Colorado to maintain soil water depletion above a specified depletion for 3 design probabilities adapted from Heermann et al 1974 PROBABILITY LEVEL 99 95 uuunux 2 3 4 MANAGEMENT ALLOWED DEPLETION inches 33 Like peak ET methods net capacity determinations using soil water simulation require analysis of several years of data to de ne the design probability level Data from the simulation models has been analyzed in two ways Heermann et al 1974 and others used a version of an annual extreme value analysis They kept track of the maximum annual soil water depletion for given capacities Compiling these data for numerous years and analyzing using an appropriate statistical procedure gives the probability that the driest soil condition will be less than the speci ed allowable depletion von Bernuth et al 1984 kept track of the number of days that the soil water depletion exceeded the speci ed depletion Combining several years of data provide a data base to develop the probability that the soil water depletion throughout the year will be less than the speci ed allowable depletion Thus the procedures are quite similar only the probabilities have different meanings The gross system capacity does not include onfarm conveyance losses If the delivery system for the farm contains major losses then the capacity at the delivery point on the farm should be increase The conveyance ef ciency EC is used to compute the farm capacity Cf amp EC 9 522 where Qf farm system capacity and Ec conveyance ef ciency The example below illustrates the use of the procedure to compute a farm system capacity 34 Example 58 Given Find Solution 1 2 3 wever the losses in the conveyance system must also be supplied by the pump A farm has an irrigation system with a net capacity of 03 inches per day There are two elds as shown below Each eld is 80 acres in size and both are irrigated with siphon tubes The application ef ciency is 65 for both elds The system is shut down about 10 of the time Determine the discharge needed from the well Net capacity for the farm is expressed in inches per day so convert to ow rate per unit area gpmac 450 gpm X day 57 gpmac acinhr 24 hr Qn 030 inday X The gross capacity for each eld is Q 57 mac 065 X1 01 97 yamac X 80 ac 780 gpm Concrete Canal Ec 90 m Directian of Flow Earlh Canal Er 80 eld 1 Hal 2 Ho Discharge needed for Field 1 is Qfl 780 gpm 080 975 gpm Discharge for Field 2 would be sz 780 gpm 090 867 gpm The well must supply the ow to each eld plus the loss in the main supply canal Q 975 867 090 2047 gpm or about 2050 gpm 56 Operational Factors An irrigated area is often subdivided into tracts of land called sets or zones A set or zone is the smallest subdivision of the total area that can be irrigated separately The term set is often used for agricultural systems The set is the area of the eld that is irrigated at one time or by a terminal section of the delivery system For example the land area irrigated while a lateral is stationary would be a set for a moved lateral sprinkler system The block of furrows supplied water at one time would be a set for a furrow system In landscape and turf applications the total area is divided into zones The plumbing of the sprinkler or microirrigation systems is such that the whole zone is irrigated at one time It would not normally be possible to irrigate a portion of a zone The size of a set is often uniform in size but does not have to be The size ofzones may vary considerably depending on the geometry of the landscape The length of time that water is applied to a set is called the application time The time between starting successive sets in the field is called the set time The application time and the set time may be the same if the irrigation system is not stopped to change sets Some systems require that the laterals drain before they are moved Then the set time is longer than the actual application time To apply the desired depth of water the application time must be correct For automated systems the set time can vary for each set or zone depending on the water requirement For manually moved systems the set time may be less exible It is common that the set time is adjusted to fit the labor schedule For example a 12hour set time is very common for furrow or moved lateral sprinkler systems even though less water may be required at times of the season An in exible set time can lead to over irrigation and deep percolation if adjustments in ow rate are not made The amount of time between successive irrigations is called the cycle time or irrigation interval For example suppose a furrow irrigated field is irrigated one per week The cycle time would be 7 days The time during the irrigation interval that the irrigation system is not operated is called the idle time Suppose that the furrow field just mentioned could be irrigated in 5 days The idle time would then be 2 days Idle time is very similar to the downtime used to determine system capacity They would be the same if the application time and the set time are the same If some time is needed to change sets then the downtime will be larger than the idle time When systems are supplied by an irrigation district you will often here the terms duration and rotation used The duration is the time that water is provided to the farm The rotation is time between times when the water is provided If the whole field is irrigated each time water is provided the rotation time is the same as the cycle time For example an irrigator might receive water for 4 consecutive days and then be without water for 10 days In this case the duration would be 4 days and the rotation would be 14 days 36 57 System Characteristics Characteristics of irrigation systems are listed in Table 57 The values listed in this guide are average quantities for the respective systems The table is useful in the preliminary stages of developing and managing irrigation systems The actual value of the various parameters can vary considerably depending on both design and management There has been much written and said about the selection of irrigation systems to fit speci c properties of a site Some factors affecting the selection of water application method are listed in Table 58 The reader should consider these criteria to be general Since this text deals with managing irrigation systems it is important to operate the system as efficiently as possible The practioner will find that many systems have been installed and operated quite economically even though they do not conform to traditionally defined limits on the irrigation method 37 Table 57 Typical characteristics of V arious irrigation systems Nominal Maximum Pressure Labor Initial Application application System slope required required cost efficiency depth type psi hracirrig 39 ac in Surface Furrow gated pipe without reuse 3 5 10 05 10 120 350 40 70 20 60 Furrow gated pipe with reuse 5 10 05 10 150 500 60 85 20 60 Furrow siphon tube 3 0 10 15 120 350 35 65 20 60 Graded border 2 4 0 10 02 10 120 350 50 85 15 60 Level basin 0 0 10 005 05 250 500 70 85 15 60 Sprinkler Hand move 20 50 70 05 15 80 350 60 80 10 60 Solidset No limit 50 70 005 01 400 1250 60 85 05 40 Sideroll amp towline 10 50 70 01 03 150 350 60 80 10 60 Boom 5 60 80 02 05 200 400 55 75 15 40 Traveler 5 15 70 100 01 03 200 400 55 75 15 40 Center pivot 10 20 20 70 005 015 150 400 75 90 025 20 Comer pivot 10 20 30 70 005 02 250 500 70 85 025 20 Linear move 5 8 20 50 005 03 250 600 75 90 02 25 Micro drip trickle Point source No limit 20 50 005 02 250 1250 70 90 Small Lateral source No limit 20 50 005 02 250 1250 70 90 Small 38 Table 58 Factors affecting the selection of a water application method Water Factors affecting selection application m ethod Land Water 1ntake Water tolerance W1nd lnne rate of nil action Sprinkler Adaptable to both Adaptable to any soil Adaptable to most Wind may affect level and sloping intake rate crops Typical application ground surfaces systems may efficiency and promote fungi and uniformity disease on foliage and fruit Surface Land area must be Not recommended Adaptable to most No effects leveled or graded to for soils with high crops May be slopes less than two intake rates of more harmful to root crops percent for most than 25 inches per and to plants which systems It is some hour or with cannot tolerate water times possible to extremely low intake standing on roots ood steeper slopes rates such as peats or that are sodded mucks Trickledrip Adaptable to all Adaptable to any soil No problems No effects micro land slopes intake rate Below surface Land area must be Adaptable only to Adaptable to most No effects subirrigation level or contoured soils which have an crops Saline water impervious layer tables limit below the root zone A A quot 39 controllable water table Subsurface Land area must be Adaptable only to Adaptable to most NO effecm Irrigation level or graded to finetextured soils crops Saline water limit slopes with moderate to tables limit good capillary A A quot 39 m nv em ent REFERENCES Bergrud FG JA Wright HD Werner and GJ Spoden 1982 Irrigation system design capacities for west central Minnesota as related to the available water holding capacity and irrigation management Technical Paper No NCR 82101 American Society of Agricultural Engineers St Joseph Michigan Doorenbos J and W0 Pruitt 1977 Guidelines for predicting crop water requirements Irrigation and Drainage Paper No 24 Food and Agriculture Organization United Nations Rome Italy Heermann DF HH Shull and RH Mickelson 1974 Center pivot design capacities in 39 eastern Colorado Journal of the Irrigation and Drainage Division American Society of Civil Engineers 100IR2127 141 Howell TA DA Bucks DA Goldhamer and JM Lima 1986 Trickle irrigation for crop production Developments in Agricultural Engineering 9 Elsevier New York pp241279 Howell TA KS Copeland AD Schneider and DA Dusek 1989 Sprinkler irrigation management for comSouthern Great Plains Transactions of the American Society of Agricultural Engineers 312 147 160 James LG 1988 Principles of Irrigation System Design John Wiley and Sons Inc New York New York Replogle JA and MG Bos 1982 Flow measurement umes applications to irrigation water management In D Hillel ed Advances in Irrigation Academic Press New York NY 1147217 USDAARS 1970 Irrigation Water Requirements Technical Release No 21 Rev 2 USDASCS von Bernuth RD DL Martin JR Gilley and DG Watts 1984 Irrigation system capacities for corn production in Nebraska Transactions of the American Society of Agricultural Engineers 272419424 Walker RW 1979 Explicit sprinkler irrigation uniformity ef ciency model ASCE Vol 105 No IR2 pp129136 WAIG 1985 Washington State Irrigation Guide Part WA686 Farm Distribution Systems USDASCS 40
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