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001360 IRRIGATION RETURN FLOWS 'lhere seems to be more confusion over the amount and timing of re- turn flows than over any other part of the subject of interaction of surface water and groundwater. Much of tilis confusion has resulted from the tendency to overestimate return flows. It is frequently stated that the total surface diversions below Pueblo are twice the amount of the inflow ,at ~leblo. This has been explained to be the result of diversion and re-di.version of return flows. However, this concept would require return flews of nearly 60% each time the water is diverted. This is obviously much too high for the Arkansas Valley, since very few water users are diverting enough water to make 60l return flow possible. The answer is found in closer examination of the water records. At times during many of the months of heavy diversions in Water Districts 14 and 17, more water will leave the area than enters, and much of that which is diverted is runoff fram heavy rains on the tributaries. When rain effects are eltminated, it can be seen 'that total diversions between Pueblo and the state line are only about 130% of the net depletions of the river. 'lhis means that the average return flow is less than 25%. As conditions are now, the length of time required for return flows to reach the river is both an advantage and a disadvantage. One of the advantages is that during dry years when relatively little of the applied water reaches the water table, there is still some return flow from wet years. However, this effect tapers off with t~e so that return flow becomes relatively unimportant after the first year of a long dry period. A second advantage of the delay in return flows reaching the river is that much of the return flow from June diversions by ditches near the river reaches tile river during August, when the water shortaRes are most critical. Another advantage is that the delay in return flows reaching the river also results in a delay in the effects of wells intercepting return flow. This means that for wells which are far enough away from the river, their pumping will not affect the river during the current irrigation season. A major disadvantage of the delay in return flows is that much of the return flow reaches the river outside of the irrigation season. Making this water available when really needed would considerably increase the benefits which can be obtained from our water supply. The accompanying graph illustrates these points. Since it is in the dry years that we need to do the most to inlprove our water supply, this graph has been based on the poorer one-half of the water years in Water Districts 14 and 17. This is the area between Pueblo and the John Martin Dam. It can be seen that return flows account for a larger percentage of the diversions during dry years than they do the average of all years. The reason is that only It small percentage of the water diverted during wet years is from return flow even though return flow reaching the river is somewhat greater than in dry years. Because of the time delay for return flows, the greatest amount of return flow reaches the river during the months following the period of water surplus.