A ground-water modelling study in the Tucson Basin

Abstract

A digital numerical model of the Cortaro area, which underlies about 150 square miles of the northwest section of the Tucson Basin, was constructed. The model is based on a novel, mixed explicit-implicit, finite-element scheme. All previous ground-water models for the Tucson Basin have been constructed with prescribed flux boundary conditions along stream courses and exterior boundaries; however, the magnitudes of these fluxes were generally unknown and had to be estimated using methods of uncertain validity. In contrast, the boundary conditions of the present model were prescribed hydraulic heads derived from direct measurements. The model was calibrated under the quasi-steady-state conditions prevailing in 1940, and verified for the period 1940-65 under transient conditions. The head distributions computed by the model for 1940 and 1965 are within a few feet of measured values over most of the region. At prescribed head boundaries, the model computed time-varying source terns representing subsurface lateral flow rates and recharge rates along stream courses. Model results show: 1) an increase in recharge along the Santa Cruz River between 1951 and 1965 (due to infiltration of sewage effluent) together with a decline in the overall rate of pumpage in the Cortaro Basin during that period caused the rate of lateral flow out of the basin at Rillito Narrows to increase gradually with time, and 2) the rate of lateral subsurface flow entering the Cortaro Basin from the valley of Canada del Oro is more than twice the rate entering from the Tucson Basin through the southeast boundary of the study area. These results differ from those given in the published literature.