Specific Yield as Determined by Type‐Curve Analysis of Aquifer‐Test Data

Abstract

AbstractType‐curve analysis of water‐table aquifer pumping‐test data has often resulted in values of specific yield that are unrealistically low when compared with values obtained by volume‐balance calculations. It has been suggested that such values are the result of inadequate representation of drainage processes in the unsaturated zone. The commonly used analytical solution developed by Neuman for a homogeneous, water‐table aquifer assumes that the drainage of pores in the zone above the water table due to lowering of the water table occurs instantaneously. Published pumping‐test data from three relatively homogeneous, unconsolidated, granular aquifers with significantly different hydraulic conductivities are used to show that type‐curve analysis based on the Neuman model will result in estimates of specific yield that agree with volume‐balance calculations, provided that established procedures are followed. These involve the use of composite plots of drawdown observed at more than one location and inclusion of effects of partial penetration. Noninstantaneous drainage of pores in the unsaturated zone accounts for the finding that both type‐curve analysis and volume‐balance calculations yield values of specific yield that are slightly less than those obtained from laboratory column‐drainage experiments. It may also account for a slight underestimation of vertical hydraulic conductivity as obtained by type‐curve analysis.