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. ;i . ' . The empirical constants in equation 2.1; 0,014 and 26.4 were adjusted to 0.0116 and 18.6, respectively. The adjustments were made by USBR. The later adjustment does not result in significant differences in ETp 'estimates from equation (2.1), although it tends to overestimate ETp fOT temperatures below 630F. The monthly potential evapotranspiration estimates are presented in Table 2.3. , The seasonal water use, Wet. is obtained by applying a crop coefficient such ,that th Wet = t Xc ETp i=tp (2.2) , where tp and th are pla.nting and harvest dates, respectively. The crop co- efficient K is a dimensionless number related to the stage of growth c and independent of loca.le. Monthly and seasonal water uses(evapotranspiration) for .the crops of alfalfa, pasture, corn, small (spring) grain, a.nd beans are pres~nted in . Tables 2.4 through 2.8. Monthly values of the crop coefficient Kc for each crop are entered at the bottom of each table. 2.5 . Leaching Requirement Since (a) all waters ~ontain some dissolved solids, (b) evapot~an- spiration is a process that removes pure water, and (c) some soils contain more salts than can be utilized by plants, some provision must be made to regularly remove the salts that are concentrated in the soil by evaporation and transpiration. Since leaching is the only practical way of maintaining a favorable salt concentration, it is considered a necessary and beneficial \lse of water. - Equations and numerical solutio~s are available for calcu- Lating nonsteady state leaching requirements. For practical purposes, :he voiume of water required for leaching under essentially steady state ,