Soil and the Hydrologic Cycle Read Ch 6 Brady and Weil Quiz 6 on Monday, Oct. 15

Most of the earth’s water is contained in ice and groundwater salty (oceans) Most freshwater is

 A temporary storage point for eventual use by plants  A filtering medium for water  Medium for groundwater recharge  An evaporation platform Role of soils

Watershed Area drained by a single system of streams that is bounded by ridges Outlet Ridge

catchment Two or more watersheds draining at the same outlet

Cycling of water from earth’s surface to atmosphere and back Driving Force Solar Energy

Fate of precipitation and irrigation water Interception Infiltration Surface runoff Percolation Soil storage Evaporation Transpiration

Water Balance Precipitation = Evaporation + Transpiration + Soil Storage + Discharge (runoff) Soil surface Plant uptake and use Evapotranspiration P = ET + SS + D

Class-A Evaporation pan Potential Evapotranspiration (PET) = pan coefficient x pan evaporation Evaporation from the pan integrates the effects of relative humidity, temperature, wind speed, and other climatic variables Dense, well-watered plants transpire water ~65% as fast as it evaporates from an open body, thus PET = 0.65 x pan evaporation (Lower for less dense planting)

Potential Evapotranspiration: If soil water content is maintained at a high level, PET tells us the maximum rate at which water will be lost from soil as evaporation and from leaves as transpiration PET is dependent upon temperature, relative humidity, cloud cover, wind speed, etc PET = 0.65 x pan evaporation PET ranges from: 1500 mm per year in hot, arid areas 40 mm per year in very cold regions

Climatic FactorCrop water need HighLow Temperaturehotcool Humiditylow (dry)high (humid) Wind speedwindylittle wind Sunshinesunny (no clouds) cloudy (no sun)

 Soil water deficit is the amount of available water removed from the soil within the crop's active rooting depth  Likewise it is the amount of water required to refill the root zone to bring the current soil moisture conditions to field capacity  Soil water decreases as the crop uses water (evapotranspiration) and increases as precipitation (rainfall or irrigation) is added  Expressed in soil water deficit, evapotranspiration increases the deficit and precipitation decreases it

Plant Water Stress: When soil water content is low, plant will not be able to withdraw water from soil fast enough to satisfy PET Actual ET < PET causes plant stress (i.e., wilting) Water deficit = PET – actual ET In arid areas without sufficient irrigation, water deficit will be large and plants will suffer. LAI: leaf area per unit land area LAI increases – Transpiration increases but evaporation decreases

Control of Evapotranspiration Plant selection Planting density or spacing (relate to LAI) Weed control (herbicide, biological, mowing) Mulches (plastic, organic matter, plant residues) Tillage practices (conservation tillage) Fallow years in dryland cropping areas