Winter Inputs Buffer Streamflow Sensitivity to Snowpack Losses in the Salt River Watershed in the Lower Colorado River Basin

Abstract

Recent streamflow declines in the Upper Colorado River Basin raise concerns about the sensitivity of water supply for 40 million people to rising temperatures. Yet, other studies in western US river basins present a paradox: streamflow has not consistently declined with warming and snow loss. A potential explanation for this lack of consistency is warming-induced production of winter runoff when potential evaporative losses are low. This mechanism is more likely in basins at lower elevations or latitudes with relatively warm winter temperatures and intermittent snowpacks. We test whether this accounts for streamflow patterns in nine gaged basins of the Salt River and its tributaries, which is a sub-basin in the Lower Colorado River Basin (LCRB). We develop a basin-scale model that separates snow and rainfall inputs and simulates snow accumulation and melt using temperature, precipitation, and relative humidity. Despite significant warming from 1968–2011 and snow loss in many of the basins, annual and seasonal streamflow did not decline. Between 25% and 50% of annual streamflow is generated in winter (NDJF) when runoff ratios are generally higher and potential evapotranspiration losses are one-third of potential losses in spring (MAMJ). Sub-annual streamflow responses to winter inputs were larger and more efficient than spring and summer responses and their frequencies and magnitudes increased in 1968–2011 compared to 1929–1967. In total, 75% of the largest winter events were associated with atmospheric rivers, which can produce large cool-season streamflow peaks. We conclude that temperature-induced snow loss in this LCRB sub-basin was moderated by enhanced winter hydrological inputs and streamflow production.

Five Decades of Observed Daily Precipitation Reveal Longer and More Variable Drought Events Across Much of the Western United States

“…Against this warming and drying backdrop, our daily‐scale analysis reveals novel aspects of hydroclimate change including large increases in dry interval and precipitation variability at both annual and season scales (Figures 2 and 3), portending potential large‐scale, detrimental socioecological consequences for the Desert Southwest. Changes to the form of precipitation are also likely, such as a transition from snow to rain as the climate warms (Figure 1b), and these changes would also likely have significant effects on the fate of precipitation (Robles et al., 2021).…”

Streamflow Response to Wildfire Differs With Season and Elevation in Adjacent Headwaters of the Lower Colorado River Basin

“…A characteristic feature of the LCRB, typified by the lower‐elevation Mogollon Rim headwaters, is the decoupling of energy and water availability because shallower snowpack melts early in the year, well before peak evaporative demand and vegetation activity (Barnhart et al., 2020; Robles et al., 2021). We suggest this may diminish the importance of reduced transpiration to the hydrologic disturbance response in warm, semiarid watersheds such as the Mogollon Rim (Knighton et al., 2020; Robles et al., 2021), explaining the lack of increased streamflow (Figure 7; Adams et al., 2012; Guardiola‐Claramonte et al., 2011). Meanwhile, the snowmelt pulse in the higher, colder White Mountains headwaters typically occurs between April and May (Figure 2), close to the summer peak in ET (Dore et al., 2010; Knowles et al., 2020).…”

“…The timing of snowmelt in a given watershed may also play a critical role in regulating the streamflow response to disturbance. In warm watersheds with earlier snowmelt, streamflow may be decoupled from the vegetation activity and energy availability of the growing season (Knighton et al., 2020; Robles et al., 2021), reducing the water “savings” to be realized from lower transpiration. Because of the differing biophysical controls regulating streamflow response to disturbance in forested mountain watersheds, here we separately consider streamflow arising from winter precipitation (streamflow occurring November–June) and summer precipitation (July–October).…”

Variable Streamflow Response to Forest Disturbance in the Western US: A Large‐Sample Hydrology Approach

“…In contrast, Hammond and Kampf (2020) observed both increased and decreased streamflow following shifts from snow to mixed rain and snow. Streamflow response to snow‐to‐rain transitions appear to be more strongly associated with the seasonal timing, particularly relative to the seasonal timing of maximum annual evapotranspiration, than the type of precipitation (de Lavenne & Andréassian, 2018; Knighton et al., 2020; Robles et al., 2021). In our study, increasing trends in Q / P and simultaneous increases in tree growth occurred in a wide variety of environments (Figure 3e), including the temperate Pacific Northwest, where snow fraction may be less than 0.15, as well as high‐elevation forested watersheds across the western U.S. where winter precipitation phase change may translate to more rain‐on‐snow events that produce rapid winter runoff.…”