WATER BUDGET AND TREND ANALYSES OF A DEEP KARST AQUIFER IN A SEMI-ARID ENVIRONMENT, GRAND CANYON NATIONAL PARK

Quantification of the current water supply for Grand Canyon National Park (GRCA) was improved through new hydrograph recession analyses, water budget, and trend analyses for two major springs discharging from the Redwall-Muav (R) aquifer on the north side of the canyon into Bright Angel Creek (BAC): Roaring and Emmett Springs. The characterization of these GRCA water sources is imperative to inform the park’s water management decisions regarding intake from BAC, for use in climate change models to forecast future water security for this system, and for determining possible impact on springs-dependent species in BAC. A modified Maillet equation for exponential curve fitting was used to calculate fitted curves and recession coefficients (a measure of change in discharge over time) for additional recession events at Roaring and Emmett Springs. Environmental flow statistics were calculated for these and other R-aquifer springs to analyze trends in flow characteristics for the aquifer system. An estimated catchment area for Roaring and Emmett Springs was delineated on the Kaibab Plateau using results of a previous dye trace study and watershed boundaries. The total annual discharge at these springs, estimated base-flow of three BAC sites, and total precipitation values for the estimated catchment area were used to estimate a water budget for the BAC system. Analyses indicate that peak snowmelt discharge from Roaring Springs is occurring later in the water year, and the annual minimum flows and total volume discharged from snowmelt recessions have linearly increased since 2015. Alternatively, there is some indication that minimum flow and total volume discharged from snowmelt recession events have decreased since 2013 at Emmett Spring. Analysis of trends indicates that flow rate through conduits has increased at Emmett Spring, while there is no similar trend for Roaring Springs. Continued trend analyses will be useful for future water quantification modeling and forecasting impacts on the BAC system, and these results may indicate a change in flow characteristics of this aquifer system.