For the circuit in Fig. 12.40(b), let V, V, I = 20 A, and

Lecture 4  What is a Year o Tropical or Solar Year ­ Time difference between spring equinox ­ 365.242 o Sidereal Year ­ Orbit Sun once with respect to distant starts ­ 365.256 days o Timing of equinox in the orbit is changing  One view of Water: o A series of inputs and outputs, climatic and human  Inputs:  Precipitation (22% over land, snow, and rainfall)  Infiltration (water that makes it through the soil)  Percolation (water from soil to rock and subsurface runoff ­ overland flow, streams) o Outputs:  Evaporation (Loss of water from surface)  Transpiration (Water loss from plants)  Evapotranspiration includes both terms  ~14% of global evapotranspiration is over land  Potential Evaporation (PET or POTET): o The amount of water that would evaporate under optimum conditions o If there was water, how much would evapotranspire o Actual Evapotranspiration (AET or ACTET) ­ Determined by subtracting a “deficit” (DEFIC) from the POTET  Storage: o Moisture exists in soil in three forms o Hygroscopic Water is a molecular layer attached to soil particles and is not available for use o Capillary Water is held within the soil and IS AVAILABLE for withdrawal from the savings account o Gravitational Water is big surplus; it saturates the soil and recharges groundwater  The Natural Balance ­ Evapotranspiration = Transpiration + Evaporation  Balance ­ Utilization is withdrawal, recharge is a deposit  Groundwater:  Lies beneath the Earth’s surface between 0 and 13,00 feet depth  Usable groundwater is “stored” (found) in aquifers  Aquifers: o Zone of aeration is the area just beneath the surface ­ air pockets allow water to move through capillary action o Zone of saturation is here all the soil pore spaces contain water o Water table is the contact point between zone of aeration and zone of saturation o Permeability refers to whether or not soil/rock can permit water flow o Aquifer is a rock layer that is permeable to groundwater flows in usable amounts o Influent vs Effluent describes the interaction between the water table and surface streams  Aquifers and Human Modifications:  Groundwater pumping can change the aquifer “drawdown”  Area where the water table is lowered takes the form of a “cone of depression”  When water is pumped faster than it can be recharged, it’s called “groundwater mining”  Aquifers can collapse when they lose the support of water ­ this creates a problem with subsidence  Aquifer Examples:  Ogallala Aquifer: o Largest known aquifer in the world 174,000 mi cubed BILLIONS of dollars in agriculture activity o Retreating glaciers from Ice Ages charged it up for several hundred thousand years o In the past 100 years, we’ve dropped the water table about 100ft  Drought:  A physical and social construct  Meteorological Drought ­ Dry conditions of low and precipitation and high temperature  Agricultural Drought ­ Soil­moisture and weather affect crop production  Hydrologic Drought ­ Water supply (streams, reservoirs, groundwater) drops  Socioeconomic Drought ­ Comprehensive measure...not enough water to meet demand of plants, animals, and humans