Long‐Term Trends in Streamflow and Precipitation in Northwest California and Southwest Oregon, 1953‐2012

Abstract

Using nonparametric Mann‐Kendall tests, we assessed long‐term (1953‐2012) trends in streamflow and precipitation in Northern California and Southern Oregon at 26 sites regulated by dams and 41 “unregulated” sites. Few (9%) sites had significant decreasing trends in annual precipitation, but September precipitation declined at 70% of sites. Site characteristics such as runoff type (groundwater, snow, or rain) and dam regulation influenced streamflow trends. Decreasing streamflow trends outnumbered increasing trends for most months except at regulated sites for May‐September. Summer (July‐September) streamflow declined at many sites, including 73% of unregulated sites in September. Applying a LOESS regression model of antecedent precipitation vs. average monthly streamflow, we evaluated the underlying streamflow trend caused by factors other than precipitation. Decreasing trends in precipitation‐adjusted streamflow substantially outnumbered increasing trends for most months. As with streamflow, groundwater‐dominated sites had a greater percent of declining trends in precipitation‐adjusted streamflow than other runoff types. The most pristine surface‐runoff‐dominated watersheds within the study area showed no decreases in precipitation‐adjusted streamflow during the summer months. These results suggest that streamflow decreases at other sites were likely due to more increased human withdrawals and vegetation changes than to climate factors other than precipitation quantity.

Characterizing Precipitation Variability and Trends in the World’s Mediterranean-Climate Areas

“…A finding of no significant trend is a common result among studies that use the Mann-Kendall test to identify trends among several sites [39,40], and is more common among precipitation than discharge [36]. This may be due to a break in classification.…”

“…Research has indicated that climate and associated hydrologic processes (such as streamflow production) have already begun to change in some parts of the world, including significant downward trends in precipitation in parts of India [34] and New Zealand [35], as well as downward trends in dry-season streamflow and upward trends in peak streamflow in parts of the United States [36][37][38][39][40]. If temperate dry-summer climate zones are undergoing changes in climate variables-such as increased or decreased annual precipitation-these changes could already be causing shifts in ecological community composition and structure.…”

Drainage from the Critical Zone: Lithologic Controls on the Persistence and Spatial Extent of Wetted Channels during the Summer Dry Season

“…Increasingly, there is a need to connect the fate of summer stream flow to land use practices (e.g., Price et al, ; Reed et al, ; Strauch et al, ; Sun et al, ; Weitzell et al, ), including water extraction (e.g., Arroita et al, ), as well as anticipated climate change (e.g., Asarian & Walker, ). In the Northern California Coast Ranges specifically, rapid expansion of cannabis cultivation is putting extreme pressure on summer water resources (e.g., Bauer et al, ).…”

Restoring Summer Base Flow under a Decentralized Water Management Regime: Constraints, Opportunities, and Outcomes in Mediterranean-Climate California

“…A number of factors in a watershed can contribute to low flow conditions in coastal California [43], many of them a result of legacy forest management issues having to do with fine sediment accumulation [44] and forest regeneration [45]. Other important lessons learned in Grape Creek were the importance of a healthy egosystem, where landowners and non-regulatory conservation partners could work together to solve problems without fear of regulatory overreach; and the importance of federal funding for dual purposes of supporting endangered species and enhancing individuals' water supply security.…”