The Golden State: Where & How to Live, Secure, Visit, Enjoy and Thrive in California

Sprinkler Irrigation

Rate Of Evaporation From Free Water Surfaces

The evaporation from free water surfaces is measured at a number of Weather Bureau stations, and at many other places in California and elsewhere. The evaporation measured at these stations has been found to depend largely upon the type of evaporation pan used, and upon its exposure. For this reason various organizations, such as the Weather Bureau, have standardized their evaporation equipment

to facilitate comparisons. Tests have also been made to determine the coefficients by winch the evaporation from various types of puns must be multiplied to obtain the equivalent from a lake surface. For the standard Class A Weather Bureau pan the coefficient most generally used is 0.70.22 A Class A Weather Bureau evaporation station has been maintained at Davis since 1926. Besides the regular equipment, a recording evaporimeter is used so that instantaneous rates of evaporation can be obtained. The average daily evaporation at Davis for a fourteen-year period was as follows: May, 0.27 inch; June, 0.31; July, 0.35; August, 0.32; and September, 0.24. According to evaporimeter records, the maximum hourly rate sometimes exceeds 0.05 inch and averages about 0.04 during the afternoon.

Since water is sometimes applied with sprinklers at rates as low as 0.10 inch per hour, an appreciable evaporation loss may occur during and immediately after an application. Even with application rates of 0.25 to 0.50 inch per hour, more than 10 per cent of the water may evaporate as it is applied during the afternoon. The evaporation loss at night, however, is usually very low.

Interception And Subsequent Evaporation Of Water From Plants

When crops are sprinkled, part of the water is intercepted by the foliage and later evaporated without reaching the soil. Studies of rainfall interception by various investigators indicate than an appreciable amount may be caught by trees and other plants, especially when the rain occurs in small storms. The determination of rainfall reaching the soil under a vegetative cover is difficult, and results obtained are not always consistent. Furthermore, the interception is generally reported in per cent of rainfall, and therefore depends upon the intensity and duration of the storm. Clark23 made determinations of the maximum interception capacity of many plants. From his data it appears that few crops can retain 0.1 inch of water, although his attempts to measure interception by catching water in pans under the vegetation indicate much larger losses. Interception of rainfall and that for water applied with sprinklers should differ principally in the amount that evaporates during the application. The evaporation rate while sprinkling may be high, whereas during a rain evaporation rates are generally very low. The presence of water on the foliage should tempo rarily reduce the rate of evaporation from the soil and the rate of transpiration from the leaves.