A Seasonal Groundwater Flow Model of the Upper San Pedro River Basin, Cochise County, Arizona

Abstract

The hydrologic system of the Upper San Pedro River Basin is threatened with the depletion of surface water flow in the San Pedro River. The decrease in agricultural pumping in the San Pedro National Conservation Area (SPRNCA) from the late 1980s to mid 1990s did not increase baseflow in the San Pedro significantly. A four-season model with seasonal variations in agricultural pumping, inflow from Government Draw, and ET was developed for two purposes: 1) to better understand the hydrologic system and provide potential explanations for observed baseflow trends and 2) to assist with future management of the SPRNCA. The steady oscillatory model reasonably simulated heads in baseflows observed in the late 1930s to early 1940s. The conceptual model showed higher stream leakages during season 3 than in season 2, but the model simulated greater leakage in season 2. The variations in flows from season to season may be better reproduced with varying channel geometry for each season. The transient model covered the 1941 to 1995 period and was calibrated with respect to ET from 1987 to 1995. Calculations ofET and groundwater discharge along the San Pedro revealed some interesting results. Both ET and groundwater discharge near Palominas decreased significantly from the late 1980s. Near Charleston, groundwater discharge shows a decreasing trend but ET shows an increasing trend. Near Tombstone, baseflow shows a slight decrease and relatively constant ET. Though the model predicts head distributions relatively well, baseflows are overpredicted during periods of low flows. The total modeled inflows into the basin are contributing to the inability of the model to simulate baseflows, and the model needs to be calibrated with respect to baseflows between Palominas and Lewis Springs.