Specific Yield Storage Equation

Abstract

Abstract Specific yield is part of the total porosity of a porous rock or sediment. The total porosity (total pore space) of a rock or sediment formation is much larger than the specific yield. Total porosity includes the fraction of pore space that is interconnected (called “effective porosity”) and porosity due to isolated pores. The effective porosity itself includes specific retention and specific yield. Specific retention is that volume fraction of water that is held back by adhesion and capillary forces, when an aquifer is drained. Specific yield is the amount of water that is actually available for groundwater pumping, when sediments or rocks are drained due to lowering of the water table. Specific yield depends on the distribution of pores, their shape, and grain size. The larger the surface area of the rocks or sediment particles per unit volume (“specific surface area”), the more water adheres to these surfaces. This decreases the specific yield in favor of specific retention. Fine‐grained sediments have the largest specific surface area, the highest specific retention, and the lowest specific yield. Coarse‐grained sediments have a very small specific surface area; hence, the specific retention is small, and the specific yield is large. Specific yield typically decreases with depth due to compaction of sediments.