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<br />d <br /> <br />4a <br /> <br />of the younger alluvium. The upper fine-grained zone contains <br /> <br />the shallow water system, a minor part of the groundlvater reservoir. <br /> <br /> <br />It receives return flow from irrigation water which constitutes <br /> <br /> <br />the principal source of recharge in both the Mesa and Valley <br /> <br />sub-areas. <br /> <br />The important groundwater units are the coarse-gravel zone <br /> <br /> <br />and the overlying fine-grained aquitard unit. The variation in <br /> <br /> <br />permeability of these two units is of special importance in <br /> <br />assessing the effects of pumping and leakage between the two <br /> <br />units. <br />