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Basal Crop Coefficients (Kcb) <br />Basal crop coefficients represent conditions when the soil surface is visually dry so that evaporation is <br />minimal, but the availability of soil water does not limit plant growth or transpiration. <br />Consumptive Use (CU) <br />In general, consumptive use is the estimated amount of water leaving a managed water system in a <br />specified area because of human activity. There are a few assumptions that are acknowledged along with <br />this definition: <br />Water in the upper basin diverted outside of Colorado's Western Slope water system is assumed <br />? <br />not to be returned to the system in any way. <br /> <br />Some portion of what might be considered water use will not be consumptive use. In other <br />? <br />words, some portion of the water delivered to a municipal area or irrigated field will return to <br />the system in the form of treatment center discharges, sewer runoff, ground water recharge <br />and/or precipitation. Where possible, these amounts will be included in consumptive use <br />estimates for the CRDSS consumptive use model. <br /> <br />The selection of a small area for calculating consumptive use may not account for all the <br />? <br />interactions of water injection and withdrawal from the area's water system. <br />Deep Percolation <br />Deep percolation is the amount of water that passes below the root zone of crops, exclusive of any <br />upward movement of water from below the root zone. <br />Evaporation (Evap) <br />Evaporation is the physical process by which a liquid or solid is transformed to the gaseous state. In <br />irrigation, evaporation is usually restricted to the change of water from liquid to a gas. <br />Evapotranspiration (ET) <br />Evapotranspiration is the combined process by which water is transferred from the earth surface to the <br />atmosphere; evaporation of liquid or sublimation of solid water plus transpiration from plants. <br />Field Capacity <br />Field capacity is the moisture percentage, on a dry weight basis, of a soil after rapid drainage has taken <br />place following an application of water, provided there is no water table within capillary reach of the root <br />zone. This moisture percentage usually is reached within 2 to 4 days after an irrigation; the time interval <br />depends on the physical characteristics of the soil. <br />Leaching Requirement (LR) <br />The leaching requirement is the fraction of water entering the soil that must pass through the root zone in <br />order to prevent soil salinity from exceeding a specific value. <br />3 <br />A275 01.09.95 1.14-8 CSU IDS <br />