Synthesis of Hydrological Science in the East River from Evapotranspiration Estimates to Long-Term Climate Dynamics


Yuxin Wu1* (, Bhavna Arora1, Max Berkelhammer2, Rosemary Carroll3, Jiancong Chen4, Chunwei Chou1, Baptiste Dafflon1, Brian Enquist5, Boris Faybishenko1, Cynthia Gerlein-Safdi1,4, Lara Kueppers4, Michelle Newcomer1, Thomas Powell1, Matthias Sprenger1, Tetsu Tokunaga1, Kenneth Williams1, Erica Woodburn1, Eoin Brodie1


1Lawrence Berkeley National Laboratory, Berkeley, CA; 2University of Illinois, Chicago, IL; 3Desert Research Institute, Reno, NV; 4University of California–Berkeley, Berkeley, CA; 5University of Arizona, Tucson, AZ



Watershed hydrological behaviors are fundamental to its functions. Quantifying the water fluxes moving across interfaces of compartments of the hydrological cycle is key to understanding and predicting its functions. Evapotranspiration (ET) moves a large quantity of water across the land–atmosphere interface and often contributes most to uncertainties of hydrological behavior. Accurate quantification of ET is a fundamental challenge in predicting watershed hydrological function. Two aspects of this ET research are highlighted: ET uncertainty quantification across different methods and a historic trend of ET behaviors.

Two major factors contribute to ET uncertainties and difficulties among different methods: unknown true ET values for benchmarking and differences in meteorological inputs, ET formulations, and parameterization. Understanding sources of uncertainty and developing gold standard benchmarking platforms and datasets are key to accurately predicting ET at watershed scales and beyond. For ET synthesis, a concerted effort was conducted to synthesize ET-related research across the Watershed Function SFA team. Some key progresses from this effort are highlighted, including the development of ET benchmarking platforms and datasets using a controlled lysimeter setup and its use to improve ET model parameterization, as well as the comparison of various ET methods at selected benchmark locations and time periods to identify sources of uncertainties. Results from this effort suggest (1) up to >50% variations of ET quantity across the different methods, particularly in the summer growth season; (2) non-linear and scattered correlations of time-stamped ET across different approaches suggesting fundamental deviations among these approaches, and (3) meteorological forcing (e.g., radiation and wind) is a significant contributor to variations across the approaches. A few key improvement needs are also identified to improve ET quantification at the East River watershed.

For historic ET analysis, a statistical time series analysis (1966 to 2021) was applied to 17 locations at the East River watershed. ET was calculated using the Budyko model, Thornthwaite and Hargreaves equations, and the Penman-Monteith equation. The results were used to calculate standard precipitation index and standard precipitation-evapotranspiration index. Analysis suggests shifting of more locations toward water limited scenarios, with these water-energy limitation zonation patterns driven by dynamic climatic processes. This provides a historic context for these observations of changes to ecosystem properties and function.