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'~-::J I\~ ~ ~ Q';l PROCEDURE t The data to be used to derive the desired production functions were to be collected at four field sites representing a wide variety of soil and climatic conditions. General procedures as agreed for all experimental sites will be described first, followed by additions or modifications at specific sites. GENERAL PROCEDURES IN 1974 The procedure used in 1974 was influenced by the funding dates for the projects, April 1 for the water study and July 1 for the salinity study. Normal planting dates at Yuma are February or March and April or May at the other three sites. In the interest of effectively using the 1974 growing season, a decision was made to initiate the water project even though neither project was yet funded, and following fun~ ing, to prepare for inclusion of and concentration on the salinity aspects of the amalgamated study in 1975. Accordingly, the initial working session of the four research groups was convened at Yuma in mid-February. Operational decisions on experimental design, treatments to be applied, measurements to be made, techniques to be utilized, and many related matters were agreed upon at the Yuma meeting. A listing of the more important decisions follows; 1. Study crop-<:orn was selected, with locally adapted high yielding varieties specified. 2. Management of other than treatment varia- bles-to be optimal to the degree feasible. 3. Experimental soil-representative of better agricultural soils of the area, without a water table affecting the upper 3m of the profile, and free of other known major problems. 4. Initial soil water content-all experimental plots, regardless ofsubsequent treatment, to be brought uniformly to field capacity before or at the time of planting. 5. Desired soil water content and evapotrans- piration (ET) history at the time differential irrigation treatments began-soil water content to be uniform in all plots, with drainage of excess water complete, and with evapotranspiration requirements having been fully met to date. The choice of the first treatment irrigation date was based on meeting these criteria. 6. Division ofthe growing season in accordance with important growth stages-three major growth periods were designated for purposes of sequencing treatments and for measuring and analyzing data and reporting results. Periods selected were the vegetative growth period from planting to first tassel, the pollination period to blister kernel, and the maturation period to physiological maturity. 7. Experimental design-the "Line Source" continuous variable design developed by Hanks and associates (1974) at Logan was adopted. In this design all irrigation after establishment of the crop is from a single sprinkler line parallel to the rows through the center of the plot(s). The closely spaced (6.1 m) sprinkler heads are a type which throws a triangular water pattern such that the maximum application occurs at the sprinkler line, tapering evenly away as one moves outward in either direction. Finally, at a distance of approximately 15 m, no irrigation water is applied at all. Therefore, water application level is inherently a variable in this design. 8. Treatments, variables, and measurements- the approach taken overall in this study was to establish a wide array of measured irri- gation regimes, to determine the associated evapotranspiration regimes which occurred, and to measure the resultant dry matter and grain yields from each. Figures 1, 2, and 3 show the experimental design to achieve this variation. Figure 1 is a schematic illustration of a line source plot, viewed end-on. The figure shows 1) the expected water application pattern and the variation in its relative adequacy with respect to meeting ET requirements with distance from the sprinkler line, 2) ratios of actual (ETa) to maximum (ET m) Et levels, 3) associated ET deficits, and 4) resultant yield fractions relative to the maximum Y M expected when ETA = ETM = 1.0. Figure 2 shows the wetting pattern in a single 3