Attributing summer streamflow decline to climate vs. agriculture

Abstract. Climate warming is reducing the mountain snowpacks that sustain summer streamflow, yet in agricultural and urban catchments this climatic decline is often superimposed on the effects of local water use, and separating the two is difficult. We investigate how much of the continuing dry season-flow decline in a snow-reliant, anthropogenically influenced intermontane catchment with a distinct summer dry season is driven by climate change rather than human water use, and whether the roughly even contributions reported in another study for the last quarter of the 20th century still hold under U.S. Southwest megadrought conditions of the first quarter of the 21st century. Comparing three multi-decadal timeframes across nine catchments in Northern California, we find no significant difference in annual precipitation but a widespread rise in vapor pressure deficit and declining snowpack at lower elevations. Summer flows declined significantly in the catchment affected by human use of groundwater and surface water compared with the near-natural reference catchment. However, unchanged annual total flows indicate seasonal redistribution rather than an overall loss of water. Using a paired-catchment attribution and the mid-20th century record as pre-development and pre-climate change reference, we decompose the late 20th century and early 21st century decline in the managed catchment into climatic and anthropogenic components. The total summer-flow loss nearly doubled between the two climate change periods, from about 21 to 40 Mm³: During late 20th century, climate change and anthropogenic water use contributed almost equally. But under 21st century megadrought conditions the climatic loss (26 Mm³) had grown to roughly twice the anthropogenic impact (14 Mm³), accounting for about two-thirds of the decline. The even contribution no longer holds: climate change now governs the decline. The results suggest that curtailing withdrawals might not sufficiently sustain ecologically relevant late-summer flow and fall pulse flows and additional measures might require managed aquifer recharge, adaptation, and climate mitigation alongside local water management.