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I I I I I I I I I I I I I I I I I I I a case-by-case basis. LA WMA' s consultants do not believe it will be necessary to apply these well depletion factors to well pumping that affects the Arkansas River below the diversion facilities for Buffalo Canal since this pumping does not affect Colorado surface rights and its depletion to usable Stateline flow will be calculated with the Kansas H-I Model3-1 . It was not necessary to apply the center pivot sole-source well depletion rate in this analysis because only a few LAWMA members operate center pivots above the Buffalo Canal, and these center pivots are used on land that historically received ditch water. The increment of additional depletion with these center pivots is small in relation to the depletion from the historical flood irrigation of the lands. The differences in the depletion rates are justified because the consumption of water used for supplemental irrigation is significantly less than that from sole-source water. Supplemental well pumping is required to make up only part of the total irrigation requirement, most of which is satisfied by surface water. The irrigators generally pump more than is required strictly to satisfy this part of the total irrigation requirement. Since water is often pumped into a lateral and commingled with the available surface water, one reason the irrigators pump the additional water is to increase the available head so that the irrigation can be accomplished quicker and with less labor. Another reason may be that the irrigators are uncertain about the adequacy of their future surface supplies, so they pump their wells just to be sure. When the surface water does become available, then the overall irrigation efficiency goes down. Another may be that the wells are pumped so that the irrigators can finish out a run of water. For various reasons, the irrigation efficiency of well water used to supplement surface water, and therefore the consumption, is less than for well water used as a sole-source for irrigation. Table 5 shows the 1990-94 average pumping amounts and estimated depletions for the wells that potentially affect Colorado surface rights. This well pumping averaged 41,893 acre-feet annually, and the well depletions averaged 13,827 acre-feet annually. The well depletions during April through October averaged 9,099 acre-feet annually, which except for the commercial pumping in the Amity to Lamar segment, were 66 percent of the annual well depletions based on ground-water response functions from the Kansas H-I Model. The 3...1 The Kansas H-I Model is a computer model that was used by Kansas in Kansas v. Colorado to evaluate the impact of changes in certain hydrologic and institutional conditions of the flow in the Arkansas River at the Kansas-Colorado stateline. 13