Drought Risk Assessment Under Climate Change is Sensitive to Methodological Choices for the Estimation of Evaporative Demand

Candida Dewes

Tuesday, Apr 18, 2017, 2:00 pm
DSRC Room 2A305


Abstract

Several studies have projected increases in drought severity, extent and duration in many parts of the world under climate change. Many popular drought indices utilize potential evapotranspiration – or more broadly, evaporative demand (E0) – as a fundamental variable, but how it is estimated can be a significant source of uncertainty when future climate projections are considered. Here we analyze daily output from two CMIP5 GCMs over the Continental U.S. to evaluate how differences in E0 formulation, treatment of meteorological driving data, choice of GCM, and standardization of time series influence the estimation of E0.



We estimate E0 using three widely used E0 formulations: Penman-Monteith; Hargreaves-Samani; and Priestley-Taylor. Our analysis, which primarily focuses on the May-September warm-season period, shows that E0 climatology and its spatial pattern differ substantially between these three formulations. Overall, we find higher magnitudes of E0 and its interannual variability using Penman-Monteith, in particular for regions like the Great Plains and southwestern US where E0 is strongly influenced by variations in wind and relative humidity. When examining projected changes in E0 during the 21st century, there are also large differences among the three formulations, particularly the Penman-Monteith relative to the other two formulations. While most drought indices are standardized, we find that standardization tends to amplify divergences between the E0 trends calculated using different E0 formulations, because standardization is sensitive to both the climatology and amplitude of interannual variability of E0. For different methodological choices and GCM output considered in estimating E0, we examine potential sources of uncertainty in 21st century trends in the Standardized Precipitation Evapotranspiration Index (SPEI) and Evaporative Demand Drought Index (EDDI) over the Northern Great Plains to demonstrate the practical implications of these methodological choices for the quantification of drought risk under climate change.



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