Is evapotranspiration more supplied by summer precipitation or winter precipitation: understanding precipitation sources of terrestrial water use and their variations across wetter and drier years and distinct eco-climatological regions
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Authors
Kesting, Helen Marie
Issue Date
2023
Type
Thesis
Language
Keywords
Alternative Title
Abstract
Abstract 1:The impacts of changing precipitation regimes on the contributions of different seasons’ precipitation to evapotranspiration (ET) versus runoff is not well known because empirical insights are scarce. However, such insights are important for improving model predictions of future water-resource availability and ecosystem responses to summer and winter droughts. Here we use using long-term measurements of fluxes and isotope ratios (δ18O) in runoff and precipitation to calculate the partitioning of seasonal precipitation between ET and runoff, using an end-member mixing and splitting framework and focusing on effects of wetter versus drier summers and winters. We hypothesized that drier summers would involve carryover of winter precipitation to mitigate shortages, but related findings were partially indeterminate. However, we did find increases in winter precipitation across a 500-mm range involved the fraction of ET from summer precipitation decreasing from 100% (within error) and the fraction of winter precipitation contributing to ET increasing from 0% (within error) to 21%. Although there were substantial uncertainties in the trends we identified, we expect that the novel approach used here could be a useful framework for understanding the sensitivity of ET partitioning to climatic change, especially where precipitation δ18O varies substantially between seasons.Abstract 2: To understand how changing patterns of seasonal precipitation inputs may affect ecosystems and water resources, we need to know how precipitation from different seasons travels through landscapes and contributes to runoff versus evapotranspiration (ET). In this study, we use stable isotope data measured in the National Ecology Observation Network (NEON) aquatic sites to quantify the partitioning of winter and summer precipitation into runoff and ET. Across the 23 watersheds, ranging in size from 1.1 to 47,000 km2, we found the fraction of summer precipitation routed to ET ranged from 0.13 to 1.00, which coincided with the fraction of ET composed of summer precipitation ranging from 0.04 to 0.76 (readers should note these ranges do not include implausible values generated for 4 sites where the available data were insufficient to constrain uncertainties). To identify factors that influence seasonal precipitation partitioning, a stepwise regression was used with a set of potential predictor variables related to topography, climate, and vegetation. Although many individual variables proved to be significant correlates, 83 % of the variation in the fraction of summer precipitation routed to ET was estimated by a 4-term model using chlorophyll carotenoid index (CCI) variability, mean annual precipitation, and enhanced vegetation indices (EVI) metrics. The fraction of ET sourced from summer precipitation was estimated by the ratio of summer precipitation to annual precipitation and minimum EVI. This is the first cross-site study on seasonal precipitation partitioning, and thus the findings here advance our fundamental understanding of how precipitation is routed to ET versus runoff in differing landscapes.