FLUVIAL GEOMORPH ESS 426
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Date Created: 09/09/15
LECTURE SECTION 6 Hillslope Hydrology Runoff processes the paths of water on the hillslopes A schematic View of the altemative fates of precipitation on a hillslope A SCHEMATIC OF HUNOFF PROCESSES EVAFOTHANSPIRATION PRECIPITATION A t I max gman Lu 5 lt 5 395 IgtRUNOFF H 2 Z lt I 0 deep groundwater discharge quotbaseflowquot HILL SLOPE CHANNEL G412 51 Spring 2002 Two fundamentally different runoff paths are implied by this picture 1 Horton overland ow HOF the movement of runoff over the ground surface where and when the rate of precipitation the rainfall intensity abbreviated RI exceeds the rate at which the soil can absorb water the in ltration capacity abbreviated IC So HOF occurs where and when RI gt IC The difference between RI and IC is known as the precipitation excess Both terms are measured as rates e g inches per hour or mm per hour Many factors can affect the precipitation excess at a particular site Changes in RI a function of the storm variability of the storm intensity Changes in IC a function of the soil properties what covers the soil and time coarsegrained soils also called coarse textured soils namely gravelly or sandy high in ltration capacity many inches per hour finegrained soil fine textured soils predominantly silty or clayey low infiltration capacity a few inches per hour or even less IC is increased by abundant organic matter by root holes and worm holes by a welldeveloped soil structure wellformed soil aggregates with cracks and spaces between them IC is decreased by the impact of raindrops by compaction by animals or machinery or by paving in which case IC gt 0 G412 62 Spring 2002 In addition most soils decline in IC over time as the soil quotwets upquot during a storm and a some of the pores become filled with water b fine sediment is mobilized on the ground surface and clogs some pores and c clay particles swell up from adsorbed water blocking off some passageways LII l l Rainfall 416 cmhr Runoff Ii li f in l I l l I 0 20 40 60 80 100 120 140 Rate of rainfall in ltration and runoff cmhr Minutes from onset of rainfall The second of the two major runoff paths is actually more of a quotfamilyquot of runoff paths 2 Subsurface ow SSFThe movement of runoff predominantly below the ground surface This will occur wherever the soil39s 1C gt 0 and so takes place to some degree even when HOF is also taking place However we tend to reserve this term and we see the effects of this process in the landscape only where there is little or no HOF and thus RI lt IC This is the dominant runoff process in temperate humid regions such as the Pacific Northwest where lush vegetation has produced a porous soil structure with infiltration capacities of many inches per hour and where the predominant storm pattern is one of longduration lowintensity ltlquothr rainfall G412 63 Spring 2002 quotHortonquot Overland Flow Rainfall Intensity gt zn39s intiitration Capacity Shallow Subsurface Flow Saturation Deep Return Subsurface Flow Flow Groundwater Four distinct but related runoff processes are de ned by this picture Shallow subsurface ow SSSF or quotinter owquotthe movement of subsurface water in a relatively shallow zone within a few feet of the ground surface roughly parallel to the ground surface Deep subsurface ow GW quotgroundwater owquotthe movement of subsurface water through deeper substrata beneath the soil zone and relatively unaffected by any but the grossest elements of the surface topography G412 54 Spring 2002 G412 Law permeability at depth There is often but not always a physical basis for the division of subsurface ow into quotshallowquot and quotdeepquot components Surface soils are almost always more permeable as a result of biological activity and chemical weathering than their parent geologic materials and so water will move more rapidly in these upper layers and quotleakquot only slowly into the lower ones However in soils underlain by thick deposits of very permeable material there is no quotinter owquot at all High permeability at depth 39 e If Hillslope a 0 I MW deposuts 0 Saturation overland flow SOF also called quotreturn owquot the reemergenee of subsurface water that occurs where the soil layer is completely saturated and so no additional water will quot tquot within it Most common near the toe of slopes where the accumulated water collected from the entire upslope hillside is most voluminous This is a timedependent condition the longer that rain has fallen the more water will be in the soil layers groundwater tables will rise and a greater area will be subject to saturation Direct precipitation onto saturated ground DP wherever the ground is saturated any rain that falls will not infiltrate even if the soil39s intrinsic C is greater than the rainfall intensity There just isn39t room for any more This rain will run off as rapidly as if the soil39s in ltration capacity were 0 but this process is not equivalent to Horton overland ow although we can at least locally calculate the rate of runoff as though it were HOF 65 Spring 2002 Note how the level of the water tab1es Will change during the course of a storm Prestorm groundwater table Poststorm flow paths G412 6 6 Spring 2002 Because the predominant type of runoff or the runoff regime depends on the interplay of rainfall intensity and infiltration capacity we can recognize regional climatic and topographic settings Where each of these runoff process should dominate Ihin MHIL quot gentle CUIIL aHL DlrL Cl precipitation and fuulslopci return iluw dominate widc valley 351 hydrograph subsurface hullmns soils SimIn ow icsx important of hIgh in low pctmcnhilny in Horton overland aw dominates hydrugm It contributions from subsurface slotm nvt are less important Slurp slmlghl u and direct hillslnp tiucp pr39ClPllullUn cri pcrmciihic HJI39I39O mile hulloms Tupogruphy and mile Arid to subhumui Humid climate climate thin dcnsc ugetunnn Vegclaliun or dislurhud hy immann linmm cgulzumn umi Limi uw gt G412 57 Spring 2002 Runoff characteristics common to the Horton overland ow regime l G412 Brief lagtopeak the time between the maximum rainfall intensity and the maximum stream discharge Rainfall intensity and stream discharge Time hours HOFDominated Basins Basin Area miz LagtoPeak l 20 min 10 40 min 100 60 min The runoffproducing zones those parts of the ground surface from which runoff will reach the stream during the course of the storm are located wherever the infiltration capacity is lower than the prevailing rainfall intensity In semiarid and arid climates these will be found over the entire basin Where landuse changes have affected infiltration capacity these areas will be found as linear features roads and other compacted tracks and patches 6 g building sites without any systematic relationship to the drainage features of the watershed Hillslope erosion see next section mainly will occur by the detachment of surface soil particles by overland ow sheetwash and rilling and gullying 68 Spring 2002 Runoff characteristics common to the subsurface ow regime 1 Long lag to peak All Basins Basin Area miz LagtoPeak SSF LagtoPeak HOF l 1 hr 20 min 10 4 hr 40 min 100 10 hr 60 min G412 The runoff producing zones are primarily those parts of the basin where the soil is saturated up to the ground surface These are concentrated in topographic lows particularly at the base of long footslopes adjacent to streams lakes wetlands or other perennial water bodies Most importantly the size of those zones will expand during the course of a storm and also during the course of the rainy season They are known as Variable Source Areasthey are the quotsource areasquot for runoff and unlike the runoffproducing areas under the HOF regime they quotvaryquot in size during the course of a single storm and during the changing seasons In areas where the subsurface ow regime predominates hillslope erosion will re ect the movement of saturated soil massescreep landslides and other modes of mass failure next section Locally there may be some detachment of surface soil particles by overland ow as with HOF but these areas are normally limited in size and quantitatively not very important 69 Spring 2002 w gl 39igggA j amour mlthml m mm Fig 1 9 n Map Hi Mllllmlrd mu hnvung expzuwun ur umglv mmmm m 41 mm rm huxm h Hump minimum mph mu i mm mm mm Hw mu m mm m nllHZlLd l x u m mug mm in mm hm AM u mmmlcd m hu r m dorm um I r h Ht and mu huh m wmr mm hurl Mun In Hm gmuml mm from Dunne and Leopold 1979 Note that in general surface ow paths Horton overland ow and saturation overland ow move water rapidly and ef ciently to channelsso even temporary increases in rainfal intensity are clearly re ected by increases in stream discharge In contrast subsurface ow paths move water only slowly with much diffusion of any temporary peaks in the rainfall intensity and some losses to deeper groundwater systems The channel response to a burst of rainfall thus may be weak or entirely absent Note also that the water chemistry of the runoff where subsurface ow paths predominate may be very different from that where Horton overland ow occurs fewer surface contaminants more opportunities for ltration by the soil but also a much greater contribution from the groundwater chemistry G412 610 Spring 2002 Other hydrologic relationships in watershed l stcharge C15 if l Ravnlalll Inches per hour 20 440 500 520 Lagtopeak in a small basin Ramlall mches per hour Aug 5 Aug Aug 5 Aug 9 ngn nu Four am of ow at Sugar ka Ohio mum palwge of a Hood on a meam draining 3m square mun Afm How and lungbein Floods Lagto peak in a large basin G412 511 Spring 2002 200000 Valley in ow Discharge cfs Valley out ow 0 I 1 Sept 20 21 22 23 24 25 26 Time days Figure 10L2 In ow and out ow hydrographs for the valley bottom lands and the channel of the Delaware River above Port Jervis New York during the hurricane ood of September 1938 The drainage area is 3076 sq mi The in ow hydrograph represents the computed time distribution of water provided from the basin to the valley bottom lands The out ow is the measured ow in the channel and shows the attenuation of peak due to storage in the valley and the channel The shaded area represents the volume of water stored in the channels and valley oors of the catchment before the peak out ow occurred From H K Barrows 1942 EOS American Geophysical Union Transactions vol 23 pp 483 488 Copyrighted by American Geophysical Union G412 612 Spring 2002 UPSTREAM l Translation only 39a bl Reservoir only Inwhurgc quw a I Actual channel Time unils G412 5 u prngUUZ Basin Size amp Hydrologic Response B a 23 A 8 398 A time G412 614 Spring 2002 G412 615 Sp ngZOOZ t1me Emor mo amp IIIIIIIIIIII p ww Xx I I I I I III 3 IIIIIIIIIIIIIIIIIIII 22 I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I o Storm Size amp Hydrologic Response