Water Yield

the ability of rocks saturated to their full moisture capacity to give up part of the water by free flow under the influence of gravity.

The water yield equals the difference between the absolute and the maximum molecular moisture capacity. The amount of gravity water (that is, the water able to move in interconnecting pores, cavities, and cracks under the influence of gravity) in liters that can be obtained from 1 cu m of rock is known as the specific water yield. The water yield of rocks is denoted by the water yield coefficient, expressed as a fraction or a percentage. Average water yield coefficients are 0.10-0.15 for sandy loam and fine sand, 0.25-0.35 for coarse sand, 0.02-0.03 for sandstones, and 0.008-0.10 for fissured limestones. The level of the water yield coefficient depends on the grain-size distribution and porosity of the rock, as well as on the viscosity of the water. Water yield is determined by laboratory techniques or calculated from observations of water flow, trial pumpings, and data on the utilization of ground water.