UCLA

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.