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Soil and the Hydrologic Cycle

Soil and the Hydrologic Cycle. Read Ch 6 Brady and Weil Quiz 6 on Monday, Oct. 15. Most of the earth’s water is. salty (oceans). Most freshwater is. contained in ice and groundwater. Role of soils. A temporary storage point for eventual use by plants A filtering medium for water

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Soil and the Hydrologic Cycle

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

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

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

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

  5. catchment Two or more watersheds draining at the same outlet

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

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

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

  9. Potential Evapotranspiration (PET) = pan coefficient x pan evaporation 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) Class-A Evaporation pan Evaporation from the pan integrates the effects of relative humidity, temperature, wind speed, and other climatic variables

  10. 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

  11. 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

  12. 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

  13. 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

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