Karst Hydrologic Memory Supplements Streamflow During Dry Periods in Snow-Dominated, Mountainous Watersheds

Abstract

Analysis of PRISM and SNOTEL station data paired with USGS streamflow gage data in the western United States shows that, in snow-dominated mountainous watersheds, streamflow regimes differ between watersheds with karst geology and their non-karst neighbours. These carbonate aquifers exhibit a spectrum of flow paths encompassing karst conduits, including large fractures or voids that transmit water readily to springs and other surface waters, and matrix flow paths through soils, highly fractured bedrock, or porous media bedrock grains. A well-connected karst aquifer will discharge a large portion of its accumulated precipitation to surface water via springs and other groundwater flow paths on an annual scale, exhibiting a lagged response to precipitation presenting as a “memory effect” in hydrograph time series. These patterns were observed in the hydrologic records of gaged watersheds with exposed or near-surface carbonate layers accounting for > 30% of their drainage area. In western snow-dominated watersheds, where paired streamflow and SNOTEL data are available, analysis of the precipitation and flow time series shows low-flow volume is strongly related to karst aquifer conditions and winter precipitation when compared to low-flow volumes present in non-karst watersheds, which have a complex relationship to multiple driving metrics. Analysis of normalised streamflow and cumulative precipitation in karst watersheds show that low-flow conditions are highly dependent on the preceding winter precipitation and streamflow in both wet and dry periods. In non-karst watersheds, increased precipitation primarily impacts high-flow, spring runoff volumes with no clear relationship to low-flow periods. When comparing cumulative streamflow and precipitation volumes within each water year and over longer timescales, karst watersheds show the potential filling and draining of large amounts of karst storage, whereas non-karst watersheds demonstrate a more stable storage regime. Communities in many western US watersheds are dependent on snow-dominated karst watersheds for their water supply. This analysis, using widely available hydrologic data, can provide insight into the recharge and storage processes within these watersheds, improve our ability to assess current flow regimes, anticipate the impacts of climate change on water availability, and help manage water supplies.