The specific yield of a water-bearing formation is defined as the ratio of (1) the volume of water, which after being saturated, it will yield by gravity to (2) its own volume (O. E. Meinzer, Outline of ground-water hydrology, U.S. Geol. Sur. Water-Supply Paper 494, p. 28, 1923). It is a measure of the quantity of water that a formation will yield when it is drained by lowering of the water-table. The determination of the specific yield is essential in many quantitative ground-water investigations.

In 1931 a pumping-test was made in the Platte River Valley, in Nebraska, to determine by Thiem's method the permeability of the water-bearing sand and gravel that underlie the Valley. This test, which constitutes a part of a cooperative investigation by the Conservation and Survey Department of the University of Nebraska and the United States Geological Survey, was briefly described in the 1932 Transactions of the American Geophysical Union (p. 393). The well was pumped continuously for 48 hours, and measurements of the discharge were made every 30 minutes. During the period of pumping and after pumping stopped about 5,000 measurements were made of the depths to the water-surfaces in 80 observation-wells, which were located on lines radiating from the pumped well to distances up to 1,200 feet. Instrumental levels were run to the measuring points on each of the wells so that the altitudes of the water-surfaces could be determined for any time. From the data obtained in this test the average coefficient of permeability was computed to be about 700. In the present paper a method is presented for determining the specific yield of the water-bearing material from the data collected in this test, and the results of the computations are given. This method for determining specific yield was suggested by Meinzer (O. E. Meinzer, Outline of methods for estimating ground-water supplies, U.S. Geol. Sur. water-Supply Paper 638-c, p. 136, 1932).