Assessing Climate Impacts Against Groundwater Pumping Impacts on Stream Flow with Statistical Analysis

Declining summer streamflow is observed in Pacific Northwest catchments, impacting endangered salmon species which need sufficient flow to reach their spawning grounds. Groundwater pumping for irrigation is generally considered the cause of low summer flow. However, it is unclear, how much water is lost due to water use or climatic factors, as there often is no data on pumping-volume. In this study we assess the lost amount of streamflow during summer low flows and quantify the shares attributable to climate change and agricultural water-consumption, only using streamflow data. As a case study we focused on the Scott River catchment, California, having 7% agricultural land use. We compared summer streamflow, snow water equivalent and precipitation between historic (1940-1976), intermediate (1977-1999) and modern (2000-2020) timeframes. Snow water equivalent showed negative significant trends at lower elevations (1600-1800 m). We also observed significant negative trends in mean and minimum streamflow as well as earlier starting and longer lasting low flow season. Using a paired-basin approach we were able to detect a mean 38.5% (37.5 +/- 3 Mm³) streamflow decrease from historic to modern timeframe years, where 14.6% (14.25 +/- 1.4 Mm³) were attributable to agricultural water consumption and 23.9% (23.2 +/- 1.4 Mm³) to climate change. These results demonstrate that agriculture substantially impacts streamflow; however, the influence of climate change dominates. Therefore, stopping water use in summer to increase low flows is insufficient. A possibility to ensure enough flow for endangered salmon could be artificial aquifer recharge during high flows to top of low flow season.