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I. INTRODUCTION
General
Increasing demands for water in the state and nation require water
resources planning to evaluate water needs and alternatives of development.
As a portion of an overall program of planning, there must be more efficient
utilization of water resources. To assist in the planning effort
and to provide a guide for efficient agricultural use, this publication
contains estimates of the consumptive use of irrigation water by agricultural
crops throughout Wyoming.
The study represents a badly needed updating of consumptive use estimates prepared by Byron R. Tomlinson of the Soil Conservation Service published by the University of Wyoming Agricultural Experiment Station as Bulletin 303 in 1951. The Wyoming Water Planning Program, under the general supervision of Floyd A. Bishop and other water resource planning and conservation agencies, is in need of this type of information on a statewide basis. This report should assist all persons needing irrigation water use data. Using this information, the consumption of irrigation water can be estimated for most areas in Wyoming, and when the field irrigation efficiency and conveyance losses are known, the diversion requirement for irrigation can be estimated.
The consumptive use estimates contained herein are for average climatic conditions. When temperature, precipitation and other climatic factors are above or below average during the growing season, the consumptive use will be somewhat higher or lower for that season than the value given.
Method of Estimation
The Blaney-Criddle method of calculating consumptive use was used in
this study. Blaney and Criddle found that the water consumptively used by
plants during their normal growing season was closely related with mean
monthly temperatures and daylight hours. Coefficients have been developed
to calculate the consumptive use at places where temperature and precipitation
are measured. Fifty-six U. S. Weather Bureau (U.S.W.B.) stations
in Wyoming, with long-term records or records of a sufficient number of
years, to estimate normal conditions of temperature, precipitation and
other climatic measurements, were chosen for use in this report. (Map p. 3)
Other Methods
There are several other commonly used methods to estimate crop consumptive use.
These include the Lowry-Johnson6, Thornthwait10 and Penman
7 Methods. The newest method being developed is the modified
Jensen-Haise5 This method apparently provides an accurate estimate
of consumptive use but requires solar radiation data and crop coefficient curves.
Because there are only two U.S.W.B. weather stations in the State where solar
radiation is measured, and because coefficient curves have not been
developed for all crops growing in Wyoming, the Jensen-Haise Method
requires considerable transposition and estimation of data for use in
Wyoming. For these reasons, and because of readily available climatic
data, ease of use, and broad acceptance of the Blaney-Criddle Method, the
Blaney-Criddle Method was chosen for this study.
Definitions
Definitions of some of the terms used in this report are as
follows:2, 8
Consumptive Use (Evapotranspiratlon): The volume of water evaporated and transpired from soil and plant surfaces per unit land area. Expressed in inches, feet, or acre-feet per acre.
Effective Rainfall: That part of the precipitation falling on an irrigated area that is effective in meeting the consumptive use requirements.
Consumptive Irrigation Requirements: The depth of irrigation water, exclusive of precipitation, stored soil moisture, or groundwater that is required consumptively for crop production.
Irrigation Efficiency: The percentage of applied irrigation water that is stored in the soil and available for consumptive use by the crop. When the water is measured at the farm headgate, it is called farm irrigation efficiency; when measured at the field, it is designated as field irrigation efficiency; and when measured at the point of diversion, it may be called project efficiency.
Irrigation Requirement: The quantity of water,, excivislye of precipitation, that is required for crop production. It includes surface evaporation and other economically unavoidable wastes. (Also termed "diversion requirement".)
Crops
Consumptive use calculations were made for the crops known to be grown
in the vicinity of each Weather Bureau station. The consumptive use of
other crops that might be introduced into an area was estimated so that
planners would have the information. The growing season was the primary
basis for determining what additional crops should be considered. For
example, row crops were not considered in areas where the growing season
is too short for these crops.
The consumptive use of alfalfa, grass (assumed to be all varieties of grasses for hay crops, pasture, and lawn grass) and small grains was calculated at all stations since these are present statawide. Other crops include beans, corn, potatoes, and sugar beets. These forage and row crops represent the majority of the irrigated crops in Wyoming. The consumptive use of vegetables could be considered comparable to beans.
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