The Upper San Pedro River aquifer in Southern Arizona has been modeled using MODFLOW several times. The current model improves upon previous models by switching stream packages, adding a third season to represent the summer monsoon, and thereby creating a seasonalized steady-state oscillatory model. Recharge was seasonalized using a method to develop seasonal recharge estimates using ratios of seasonal precipitation to seasonal actual or potential evapotranspiration (ET). Maximum ET was seasonalized according to estimates of riparian groundwater consumption by vegetation in the study area. The model was run with inputs of 80%, 100% and 120% of base values for recharge and maximum ET rates to assess the sensitivity of the groundwater system and river to the seasonal timing and quantity of recharge and ET. The greatest amount of baseflow, 47%, occurred during the wet winter season, 35% occurred during the dry summer, and 18% during the wet summer (monsoon) season.