About 70% of the annual precipitation on the Walnut Gulch Experimental Watershed in southeastern Arizona occurs as thunderstorm rainfall during the 3-month period from July through September. The summer thunderstorms produce high intensity, short duration rains of limited areal extent. Records from long-term stations give good estimates of the average annual and seasonal precipitation for a region, but networks of recording gages are necessary to describe individual storms and seasonal rainfall on finite watersheds. The optimum rain gage density varies inversely with watershed size and directly with the required accuracy. For example, to correlate rainfall and runoff, a 1-mi2 watershed with a length-width ratio of 4 requires a network of three recording rain gages. For watersheds of approximately 120 acres or less, the optimum network for rainfall-runoff correlation is one recording rain gage. Generally, the number of gages required per unit area decreases as the watershed size increases up to about 10 mi2. A network of gages located at 1.5-mile intervals is necessary to correlate adequately the thunderstorm rainfall and runoff for watersheds of 10 mi2 and more.
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