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<br />
<br />STATEMENT OF THE PROBLEM AND
<br />RESEARCH ORlECTIVES
<br />
<br />In an era of increasingly scarce, degraded, and
<br />expensive irrigation water supplies in the Western
<br />United States and in other arid regions of the
<br />world, 'water planners are increasingly being asked
<br />for accurate predictive estimates of how crop
<br />production varies with the quantity and quality of'
<br />water supplied. Consequently, they are increasingly
<br />handicapped by the fact that production functions
<br />which clearly show expected relations between crop
<br />yields, and water supply at all levels are not
<br />available.
<br />
<br />For a long time, the need for such production
<br />functions has been felt by economists, agricultur-
<br />ists, engineers, and other water planners. The
<br />guidance these functions could have provided water
<br />users would also have been of value. Neither need
<br />was urgent, however; because both planners and
<br />I users have in effect avoided use of water production
<br />functions by setting as their practical goal the
<br />provision of a known adequate supply of good
<br />quality water. The above procedure was the most
<br />reasonable to follow at the time because 1)
<br />relatively plentiful, low cost, high quality water
<br />supply conditions made it possible and, 2)
<br />technology had not advanced sufficiently for
<br />researchers to develop accurate water production
<br />functions.
<br />
<br />Today conditions are drastically changed, and
<br />the best information available on the relationships
<br />between crop yields and water supplies are grossly
<br />inadequate. However, .on the plus side the
<br />technological barriers to development of more
<br />accurate water production functions have been
<br />erased, principally by two particular types of
<br />instrumerits.
<br />
<br />The first of these, the neutron meter, enables
<br />accurate, repeated, non-destructive measurement
<br />of soil water content on a volumetric basis under a
<br />, growing crop in the field. The second is the large
<br />lysimeter capable of daily measurement of evapo-
<br />transpiration by the crop iri every growth stage
<br />from planting to maturity, with total elimination of
<br />water loss by percolation below the root zone.
<br />
<br />The study reported here is the first known to
<br />the writers to have fully utilized this new capability
<br />to produce crop-yield water-use information with
<br />broad applicability to many of the increasingly
<br />difficult questions involved in water resources
<br />planning and management. These questions
<br />address such topics as the fol1owing:
<br />
<br />1. Allocations of water to agriculture versus
<br />other uses.
<br />
<br />2. Assigning priorities among potential water
<br />projects, And allocation of itrigation water
<br />supplies among established project areas,
<br />3. Design criteria for water storage, convey-
<br />ance, distribution, and application systems.
<br />4. Simulation of the effects of different water
<br />and salinity management prog\'ams on crop
<br />yields. ,
<br />5. SimUlation of production from different
<br />cropping patterns in accordance with project
<br />w.ater supply, soils, salinity conditions, and
<br />climate.
<br />a, Selection among crop types and
<br />varieties.
<br />b. Optimization of land area devoted to
<br />each crop.
<br />6; Planning of strategies for use of a limited
<br />water supply, a saline water supply, or a
<br />Saline soil condition, or combination of
<br />these,
<br />7, Planningqropping patterns and manage-
<br />ment systems to maximize production in
<br />rainfed agriculture in accordance with
<br />climate and soils.
<br />8. &t>nomic analyses of alternatives involved
<br />in ,the abov~ and related questions, with
<br />assessment ofbhpacts on income as opposed
<br />to investment cdsts,
<br />
<br />the objectives of the studyl10w from the
<br />foregoing, description of currel1t needs of water
<br />planners' and managers. They are to:
<br />
<br />1. Dev,~lop pfilductfon functions for "particular
<br />crolfll which reflect inflUences on' yields of
<br />different water ~upply le"'(saM of moisture
<br />tension within the root zone at different
<br />stages of crop growth,
<br />2. Developproduction functions for particular
<br />crops which rel1ect inl1Ul!l\c6s on yields of
<br />salinity conditions within the root zone.
<br />3. Formulate and test generally applicable
<br />mathematical models for predicting, crop
<br />yield~ as a function of soil moisture and
<br />sa\initY,~onditions across a broad spectrum
<br />of climate and soil types.
<br />
<br />Objective 1 specifical1y, afplies to project
<br />C-5189, objective 2 to project B. 21, and objective
<br />3 to the amalganla:tion of the two projects. The two
<br />research(Jroposals through which support for this
<br />work "'ti~obtained envisioned a broad 5-year
<br />phased study, with the work covered by this report
<br />involving primarily data collection and preliminary
<br />model testing. In accdrdance with the proposal,
<br />further field WOrK and model development and
<br />testing are ptanMd in subsequent phases of the
<br />overall 5-year st\ldy.,
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