WSPC05258

• File Number: 8283.200
• Description: Colorado River Basin-Colorado River Computer Models-Colorado River Decision Support System-Ray
• State: CO
• Water Division: 5
• Date: 9/30/1994
• Title: Consumptive Use Modeling-Task Memorandum 1.14-9-Consumptive Use Prototype-August and September Update
• Water Supply Pro - Doc Type: Report/Study

\lJitJu.JJ I. J;

Task 1.14(b) - Consumptive Use Prollltype Development

will be verified using the USBR report for 1985.

. The development of the SCS-BC method is presented in Jensen et al (1990). The
earlier research was published in Blaney et al (1952), and later as described in the
U.S. Department of Agriculture Soil Conservation Service's, Irrigation Water
Requirement, Technical Release no. 21, revised September 1970 (TR21-70)
(USDA, 1970).

Enhanced USBR BC

. This is an enhancement of the original USBR as presented in the FORTRAN
program XCONS2. The primary addition to the Original USBR BC is that a soil
budget has been added to the equation. The soil budget is calculated based on
effective precipitation estimates and soil type. A winter soil moisture budget has
also been added to the original USBR BC, this allows for entering an efficiency
factor and for considering winter precipitation contributions to the soil moisture
budget.

PM Reference Crop Method for Alfalfa

. The development of the routines for this method by the consumptive use team were
based on routines developed by W.W.Wheeler and Associates and additional
routines designed for this project. Reference crop equations estimate ET rates for a
reference crop and then apply the rate to other crops using crop coefficients. The
Penman-Monteith (PM) equation is based on a standard alfalfa and grass reference
crop equation, as presented in Jensen et al (1990).

.PM Reference CroP Method for Grass

. This method is the same mathematically as the previous method, but use grass as
the reference crop instead of alfalfa.

3.2.2 Data Inputs

The accuracy of ET estimates depend on collecting good, representative
meteorological and other data and accurately selecting a portion of the data to be used
for ET estimation (Le. correctly selecting the data representative of the land area to be
estimated). The accuracy of ET estimation also depends heavily on the physical laws
governing the processes of evapotranspiration from crops. The most important data for
estimating ET are climate, water supply, and plant growth characteristics.

Climate

. Accurate estimation of ET requires climate data such as effective precipitation,
monthly mean temperature, minimum, maximum and dew-point temperature, wind'
run, solar radiation, and daylight hours (depending on the ET estimation method).
The BC methods use effective precipitation and monthly mean temperature to
estimate evapotranspiration. The PM methods use effective precipitation;
minimum, maximum, and/or dew-point temperature, wind run, solar radiation, and
daylight hours.

. The effective precipitation is the actual amount of precipitation available for use by
plants.

9/30/94 - September U pda te (1.14-9)

5

IDS Group, CSU