Modeling analysis of ground water recharge potential on alluvial fans using limited data

A modeling approach is developed to evaluate the potential for artificial r echarge on alluvial fans in the Salinas Valley, California, using limited d ata of soil texture, soil hydraulic properties, and interwell stratigraphy. Promising areas for surface recharge are identified and mapped on a broad- scale using soil surveys, geologic investigations, permeability tests, and seasonal ground water response to rainfall and runoff. Two-dimensional repr esentations of the vadose zone at selected sites are then constructed from drillers' logs and soil material types are estimated. Next, hydraulic prope rties are assigned to each soil material type by comparing them to laborato ry-tested cores of similar soils taken from one site. Finally, water flow t hrough the vadose zone is modeled in two dimensions at seven sites using a transient, finite-difference, variably saturated flow model. Average infilt ration rates range from 0.84 to 1.54 cm/hr and recharge efficiency, the per centage of infiltrated water that reaches the water table, varies from 51% to 79%. Infiltration rates and recharge efficiency are found to be relative ly insensitive to recharge basin pending depth due to the thickness of the vadose zones modeled (31 to 84 m). The impact of artificial recharge on the Salinas Valley ground water basin is investigated by simulating the region al ground water response to surface spreading and streamflow augmentation w ith a recently calibrated, finite-element, ground water-surface water model for the basin. It was determined that a combined approach of surface recha rge and streamflow augmentation significantly reduces the state of ground w ater overdraft and, to a lesser extent, reduces the rate of sea water intru sion.