The relationship between time of flow in Rillito Creek, an ephemeral stream near Tucson, Arizona, and water levels in nearby wells was defined and a mathematical model of the underlying aquifer was formulated. A digital computer program was used to determine model parameter values which most closely duplicated measured groundwater surface profiles. Values obtained were: effective channel width, 150 feet; permeability, 0.0022 feet per sencond; porosity, 0.20; and infiltration, 3.8 feet per day. Computer water levels agreed closely with measured values for another recharge period. Field results show that the displacement of chloride applied in irrigation water can be quantitatively predicted by linking the equations of solute and water movement through an unsaturated soil. Geophysical methods were examined for determining groundwater volumes in alluvial basins and coefficients of storage. Correlation of gravity changes with coefficient of storage indicates that the gravity method may be used for aquifers which have high coefficients of storage and in which the water table rises or declines 20 feet or more. (Author)