Explore the vast range of titles available.

Summer streamflow variations strongly affect water supply reliability and ecological functioning of western U.S. (WUS) streams. Traditional snow‐based forecasts of summer streamflow are becoming less accurate with warming‐induced reductions in winter snow accumulation. This reflects a rising importance of competing runoff‐generating processes in controlling summer streamflow variations, primarily an increasing role of rainfall in contrast to snowmelt. Here, based on a snowmelt‐rainfall tracking algorithm applied to two hydrological models, we show that cool‐season rainfall provides an important volumetric contribution to summer streamflow for many WUS streams in the current climate, and this contribution will increase under climate warming, especially in years with warm snow droughts and abnormally dry summers. We also show that seasonal rainfall (warm‐/cool‐seasons) dominates the variability of summer streamflow across ∼70% area of WUS. We show that an increasing warm‐season rainfall contribution to summer streamflow (largely replacing snowmelt) results in reduced summer streamflow predictability. Plain Language Summary Summer streamflow is a critical water resource in the generally dry summers of the western U.S. (WUS), and is routinely forecasted using spring snowpack and/or winter total precipitation as primary predictors. However, climate warming leads to reduced snowpacks, exacerbates summer low flows, and reduces the accuracy of snow‐based summer streamflow forecasts. On the other hand, the role of winter rainfall as a control on summer streamflow increases in a warmer climate. Here, we explicitly quantify the contributions from cool‐season rainfall, warm‐season rainfall, and snowmelt to summer streamflow across the WUS, and how they change under a uniformly 1°C warmer climate. We show that the cool‐season rainfall contribution to summer streamflow increases under warming across WUS, especially in streams that currently have low‐to‐moderate snow contributions to runoff, and in years with anomalously warm winters and/or dry summers. We also show that the warm‐season rainfall contribution to summer streamflow increases widely, especially in the southern interior of WUS in a warmer climate, and that increasing warm‐season rainfall contribution to summer streamflow (largely replacing snowmelt) results in reduced summer streamflow predictability. Key Points The cool‐season rainfall contribution to summer streamflow is greatest in low‐elevation coastal streams with dry summers Climate warming leads to an increased contribution of seasonal rainfall to summer streamflow as spring snowmelt contributions decline Summer streamflow predictability declines with reduced snowmelt and increased warm‐season rainfall contribution in a warmer climate

Ongoing runoff declines in the Colorado River Basin have been shown to be predominately driven by decreasing albedo from warming‐driven snow‐cover loss, especially in late‐spring (hereafter snowmelt‐radiation feedback). Here, we explore the feedback's impact on annual runoff sensitivity to warming across the western U.S. (WUS) using hydrologic model simulations. For 1°C uniform warming, we show that runoff is most sensitive to warming in modestly snow‐covered, interior mountain headwaters, especially the Rocky Mountains. Runoff sensitivities are most associated with the snowmelt‐radiation feedback in basins with runoff coefficients between 0.2 and 0.6, where runoff sensitivity increases with more snow and lower winter temperature. In aggregate, ∼48% of WUS runoff sensitivity is attributable to the snowmelt‐radiation feedback and is especially pronounced in the warming‐sensitive river basins (annual runoff decreases >5%/°C). We also show that the feedback's impact decreases with increasing temperature, which has unresolved implications for streamflow declines in a less‐snow future. Plain Language Summary Regional climate warming is driving strong runoff changes in the western U.S. (WUS), especially the Upper Colorado River Basin (UCRB). Previous work showed that warming‐related snow cover reductions lead to more solar radiation absorption and evapotranspiration, which largely explain ongoing runoff declines in UCRB. Here, we assess the impact of this snowmelt‐radiation feedback on warming‐induced runoff changes across WUS. In a warmer world, we find that the largest annual runoff sensitivities are in the interior mountainous WUS with modest snow cover. The snowmelt‐radiation feedback explains over half of the warming‐induced runoff changes in warming‐sensitive WUS basins and about half of WUS' overall runoff sensitivity. In areas influenced by the snowmelt‐radiation feedback, both runoff sensitivity and the feedback's contribution become smaller with higher temperatures, suggesting a potentially slower rate of streamflow decline as temperatures rise in a warmer future. Key Points Snowmelt‐radiation feedback accounts for ∼1/2 of warming‐driven runoff decline across the Western U.S. (WUS) Runoff sensitivities are most linked to snowmelt‐radiation feedback in river basins with runoff coefficients in the range 0.2–0.6 Runoff sensitivities to warming are largest in modestly snow‐covered, interior mountainous parts of WUS, especially the Rocky Mountains

Over 26% of the world's land area and ~8% of its population depend on snowmelt as the primary water source. A typical example is the western U.S. (WUS), where snowmelt has been the center of streamflow research for the past two decades. Under climate warming, snow's contribution to streamflow will decrease, increasing the uncertainty of streamflow fluctuations and challenging streamflow forecasting and management that are based on traditional snow-based methods. Addressing these challenges requires a deep understanding of the linkages among warming, snowmelt, and other runoff-generating processes and how they affect future streamflow. In my dissertation, I take the WUS as an example, using hydrologic modeling and in-situ data to explore the following questions: 1. How does seasonal warming affect annual streamflow in different watersheds, and why? 2. What composes runoff during water-scarce seasons, and how does the composition change under warming? 3. What mechanism dominates annual runoff decline under warming in snow-affected areas, and how do its dynamics affect the evolution of runoff? In 1, I found and explained an asymmetrical pattern that controls annual streamflow response to seasonal warming, which indicates the cooler, inland region streamflow is more sensitive to warm season warming, whereas warmer, coastal region streamflow is more sensitive to cool season warming. I found this asymmetry is explained by the bell-shape variation of evapotranspiration-temperature sensitivity as a function of increasing temperature, filling a long-lasting gap in the sub-annual linkages between water and climate in WUS. In 2, I developed a custom-period water source partition algorithm that uses a handful of output variables from land surface models to quantify the fractional contribution of custom-period rainfall to custom-period streamflow. Using this algorithm, I detected an increasing contribution of seasonal rainfall to summer streamflow under warming, which shed light on the evolution of a new dynamic for summer streamflow generation in the water-short WUS. In 3, I explored the ongoing runoff decline under warming across major WUS snow-covered regions and explored the linkage between the evolving trend dynamics with changing snow. In three major snow-covered basins, I discovered a smaller warming-sensitivity of runoff under a warmer climate with fixed precipitation, which could break the long-lasting static mechanism paradigm that governs hydrologic dynamics and a more rigorous assessment of future streamflow. In summary, the dissertation provides important contributions to understanding ongoing and future runoff evolution in WUS snow-affected regions under a warmer future.

Climate models project stronger warming in the warm season than in the cool season over much of the Western U.S. Across much of this region, the widely-accepted hydrologic signature of climate change is reduced winter snow accumulation and earlier streamflow peaks, which are mainly caused by cool season (October-March) warming. However, the warm season (April-September) warming effect receives far less attention. One of the few studies that investigated the warm season warming using a single land surface model showed that while runoff timing shifts consistently under cool season warming, runoff volume changes are mostly attributable to warm season warming in the western U.S. However, what remains unclear are the mechanisms controlling runoff responses to asymmetrical (warm/cool season) warming, as well as the extent to which the previous results are model-dependent. To answer the questions, we expand the earlier work to include experiments with four land surface models over the four major river basins of the West, with a focus on investigating the reasons causing differences in annual and seasonal streamflow response to asymmetric seasonal warming. Our results show that: (i) the general features of seasonal and annual streamflow responses to asymmetric warming are consistent across models, although the magnitudes vary; (ii) basins with higher ratio of warm to cool season gross incoming water, cooler summer, and colder winters have the strongest relative annual streamflow decreases for warm season warming than cool season warming, as a consequence of strongly influenced evapotranspiration-temperature sensitivity.