WSPC05392

• File Number: 8283.200
• Description: Colorado River Basin-Colorado River Computer Models-Colorado River Decision Support System-Ray
• State: CO
• Water Division: 5
• Date: 11/3/1994
• Title: Consumptive Use Modeling-Task Memorandum 1.14-8-Consumptive Use Model Terms
• Water Supply Pro - Doc Type: Report/Study

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Task 1.14 - Evaluation of Cousumptive Use Models

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Task Memorandum 1.14-8
Consumptive Use Model Terms

1.0 Issue

To cut down the confusion caused by having multiple terms with slightly different
meanings the following definitions for consumptive use terms are recommended for CROSS.

2.0 Discussion
2.1 The following terms are for agricultural consumptive use:

Potential Evapotransoiration (ETp)

Potential Evapotranspiration (ETp) is the potential rate for a specific crop at which water
can be removed from the sail or plant surfaces assuming the soil is fully saturated. By
applying a time period to the ETp rate a potential depth of irrigation water can be
calculated that is exclusive of precipitation or stored soil moisture required consumptively
for crop production.

Crop Coefficient (Kc)

Crop coefficient is the ration of evapotranspiration occurring with a specific crop at a
specific stage of growth to reference crop evapotranspiration at that time. This definition is
not for the basal crop coefficient K used in the Blaney-Criddle evapotranspiration method.
Basal crop coefficients represent conditions for when the soil surface is visually dry, so
that evaporation is minimal but the availability of soil water does not limit plant growth or
transpiration, Where as, mean crop coefficients include wet soil effects.

Actual Crop Consumptive Use (CCl!)

Crop consumptive use, often referred to as evapotranspiration, is the total amount of water
used for vegetative growth, transpiration or building of plant tissue, plus the unavoidable
evaporation of soil moisture, snow, and intercepted precipitation associated with plant
growth for any specified time and given water supply limitations.

Effective Rainfall (ER)

Effective rainfall is precipitation (rain and/or snow) falling during a growing period which
meets a portion of the crop's water requirement. Effective rainfall does not include
precipitation unused by crops such as surface runoff and deep percolation past the root
zone.

Carry Over Soil Moisture (SM)

Carry over soil moisture is moisture which is stored in the pores of the soil root zone
during the winter while the plant is dormant and becomes available to meet the
consumptive use requirements of crops during the growing season.

10/6/94 - ell Model Terms (1.14-8)

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