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2362 a mixture of saline ground water and agricultural drainage. .!/ Figure 4 illustrates a typical subsurface flow regime where natural vegetation is adjacent to irrigated land. The effect that natural vegetation has on the salt balance was suggested by a correlation between the amount of water used by natural vegetation and the tonnage of salt retained in each of three subareas into which Boyle divided the Reservation for analysis. Table 2 shows that the percentages of water use and salt retention are roughly the same. The boundary between the North and Central Subareas is formed approximately by the East Drain, and the Main Drain below their confluence. The boundary between the Central and South Subareas is formed approximately by the Mesa Drain, and the Lower Main Drain below their confluence. The drains are shown on the Frontispiece Map. This rather superficial correlation omits the ef- fects of other influences on the salt budget, but it helped in discern- ing the effect of vegetation. In order to isolate the effects of vegetation, an estimate was made of the salt contained in the combination of percolating drainage and river water that is used by native vegetation. The amount of vegetation that was supplied directly by subsurface inflow from the river was estimated by inspection of aerial photos. The results were as follows: Northern Area Central Area Southern Area Estimated Salt Retention Caused By Natural Vegetation In 1974 21,000 tons 97,000 tons 60,000 tons 178,000 tons The effect that natural vegetation had each year from 1965 to 1976 was estimated by determining the remaining vegetation each year, and applying an average unit water use and an average salt .!/ 735 mg/l dissolved solids is equivalent to one ton per acre-foot of water. 15