PREDICTING RIPARIAN VEGETATION RESPONSE TO GROUNDWATER WITHDRAWALS; AN INTERDISCIPLINARY MODELING APPROACH TO A REGIONAL SPRING SYSTEM, GRAND CANYON, AZ

Since 1989, increasing growth and water demand on the Coconino Plateau Sub-Basin has led to the development of the Redwall-Muav aquifer of the South Rim of the Grand Canyon as a water supply, with potential abstraction rates up to 1.1 x 10⁶ m³/yr (900 ac-ft/yr). This abstraction has the potential to result in decreased discharges at the approximately twenty springs with measurable discharge issuing from the South Rim. Such decreases will likely result in reduced base flow conditions in the spring-fed channels and larger depths to groundwater beneath the associated floodplains. These changes would alter the timing and magnitude of water available to riparian vegetation, potentially leading to mortality and/or reduced recruitment of cottonwoods (Populus) and willows (Salix). Damage to native riparian vegetation could in turn lead to negative impacts to both aquatic and terrestrial organisms. The goal of this pilot study is to develop the techniques and methods needed to quantitatively predict the impacts of groundwater pumping on the ecosystems associated with all of the South Rim springs. This study focuses on the development of a coupled local groundwater and surface-water flow model for the channel and floodplain associated with one of the smaller of the South Rim springs, Cottonwood Spring. Repeated field observations of the extent and magnitudes of surface flows in conjunction with existing USGS gauge data, allowed us to characterize spatial and temporal water availability, and provided data with which to calibrate the model to steady-state and transient conditions. The local flow model was coupled with an existing regional groundwater flow model to predict the changes in surface and groundwater availability expected for various groundwater pumping scenarios. Observed spatial variations in vegetation characteristics were related to water availability parameters in a vegetation water-use model. The vegetation model shows the decrease in vegetative area in response to the decreases in water availability caused by groundwater pumping.