Pre-print -In review for publication in Water Resources Research Trends in Regional Evapotranspiration across the United States under the Complementary Relationship Hypothesis

Abstract The hypothesis of a complementary relationship in regional evapotranspiration allows for estimation of actual evapotranspiration on a regional scale by simple, physically based models that take into account feedbacks in land surface-atmosphere dynamics. A regional, seasonal Advection-Aridity model is used to create a spatially distributed, monthly time-series of actual evapotranspiration for a period of 27 years at a 5-km resolution over the conterminous United States. For the conterminous United States as a whole, a 4.3% increase in annual actual evapotranspiration over the period WY 1962WY -1988 was observed, a trend that was significant at the 90% confidence level according to the Mann-Kendall test. Trends in annual evapotranspiration are analyzed across the spatial scales of the continental United States, a Water Resources Region (WRR), a river basin of 16,000 km 2 , and an individual 5-km square cell. Reducing the spatial scale allowed for clearer identification of areas with significant trends. To establish a base-line for the study of climate change and/or variability, a methodology for rigorous examination of past trends in actual evapotranspiration is proposed, wherein such trends are broken down into the climatic components of actual evapotranspiration in the context of the complementary relationship, and no assumptions are made about the temporal stationarity of the net available energy. Trends in actual evapotranspiration can thus be determined to originate in either the energy budget or the water budget, or both

AGU hydrology days 2004

24th annual AGU hydrology days was held at Colorado State University on March 10-12, 2004.Includes bibliographical references.Our aim is to develop a long-term, high resolution net radiation data set that accounts for the effects of local topography that confound simpler analyses of the shortwave radiative balance in rugged terrain; such a dataset may then be used in direct observations of the effects of long-term change and variability in the solar radiation input to the land surface-atmosphere interface, particularly with reference to the estimation of evaporation. Direct and diffuse horizontal solar radiation data are gathered from all reporting stations across the conterminous United States for the period of 1950-1994 and integrated at a monthly time-step. Twelve years of diffuse horizontal radiation data missing from the data sources are replicated based on their historical relationships to coincident and contemporaneous observed direct normal and global radiation. A topographic correction factor is derived to account for the incidence of direct solar radiation on arbitrarily oriented surfaces at any latitude at any moment in any day of the year, and combined with slope and aspect surfaces for the conterminous U.S. derived from a 5-km digital elevation mode. This factor takes into account the solar geometry throughout the seasonal and diurnal cycles by incorporating an hourly weighting in proportion to the diffuse horizontal radiation recorded during the middle day of the month, and is then applied to spatially interpolated surfaces of direct solar radiation and combined with spatially interpolated surfaces of diffuse radiation. Summed, these provide the total incident solar radiation input to an existing energy budget analysis to yield the net surface radiation that may then be applied in models of evaporation. As preliminary uses of this dataset, mean annual and long-term trend surfaces of net surface radiation over the conterminous U.S. for the period WY 1953-1994 are presented

The energy balance of a US Class A evaporation pan

Concurrent with the trend of rising global average air temperature, there have been worldwide observations of a decline in pan evaporation over the last 30-50 years. This global phenomenon has since received much attention from the scientific community.

Anatomy of an interrupted irrigation season: Micro-drought at the Wind River Indian Reservation

Drought is a complex phenomenon manifested through interactions between biophysical and social factors. At the Wind River Indian Reservation (WRIR) in west-central Wyoming, water shortages have become increasingly common since the turn of the 21st century. Here we discuss the 2015 water year as an exemplar year, which was characterized by wetter-than-normal conditions across the reservation and, according to the U.S. Drought Monitor, remained drought-free throughout the year. Yet parts of the reservation experienced harmful water shortages, or “microdrought” conditions, during the growing season in 2015. In this assessment of the 2015 water year at the WRIR we: (1) describe the hydroclimatic and social processes under way that contributed to the 2015 water year micro-drought in the Little Wind Basin; (2) compare water availability conditions within and between other basins at the WRIR to illustrate how microdroughts can result from social and environmental features unique to local systems; and (3) describe how a collaborative project is supporting drought preparedness at the WRIR. We combine a social science assessment with an analysis of the hydroclimate to deconstruct how shortages manifest at the WRIR. We provide insights from this study to help guide drought assessments at local scales

Drought risk assessment under climate change is sensitive to methodological choices for the estimation of evaporative demand

<div><p>Several studies have projected increases in drought severity, extent and duration in many parts of the world under climate change. We examine sources of uncertainty arising from the methodological choices for the assessment of future drought risk in the continental US (CONUS). One such uncertainty is in the climate models’ expression of evaporative demand (E₀), which is not a direct climate model output but has been traditionally estimated using several different formulations. Here we analyze daily output from two CMIP5 GCMs 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. These methodological choices yield different assessments of spatio-temporal variability in E₀ and different trends in 21^st century drought risk. First, 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 21^st century, there are also large differences among the three formulations, particularly the Penman-Monteith relative to the other two formulations. The 21^st century E₀ trends, particularly in percent change and standardized anomalies of E₀, are found to be sensitive to the long-term mean value and the amplitude of interannual variability, i.e. if the magnitude of E₀ and its interannual variability are relatively low for a particular E₀ formulation, then the normalized or standardized 21^st century trend based on that formulation is amplified relative to other formulations. This is the case for the use of Hargreaves-Samani and Priestley-Taylor, where future E₀ trends are comparatively much larger than for Penman-Monteith. When comparing Penman-Monteith E₀ responses between different choices of input variables related to wind speed, surface roughness, and net radiation, we found differences in E₀ trends, although these choices had a much smaller influence on E₀ trends than did the E₀ formulation choices. These methodological choices and specific climate model selection, also have a large influence on the estimation of trends in standardized drought indices used for drought assessment operationally. 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 21^st century trends in the Standardized Precipitation Evapotranspiration Index (SPEI) and Evaporative Demand Drought Index (EDDI) over selected regions of the CONUS to demonstrate the practical implications of these methodological choices for the quantification of drought risk under climate change.</p></div

21^st century trends in EDDI and SPEI.

<p>Comparison of 12-week EDDI and SPEI computed with the three E₀ formulations for each day between 1950–2100 for GFDL-ESM2M and CanESM2 for the Northern Great Plains region. Daily EDDI or SPEI values are binned into specific percentile categories (spanning between driest and wettest categories) relative to the historical (1976–2005) distribution.</p

21^st century trends in E₀ across the different E₀ formulations.

<p>Trends in MJJAS E₀ projected from Penman-Monteith (green), Hargreaves-Samani (blue), and Priestley-Taylor (red) formulations driven by GFDL-ESM2M and CanESM2 data for the Northern Great Plains. The top row shows seasonal totals in mm, center row shows E₀ anomalies as % of the 1976–2005 mean, and bottom row shows standardized E₀ anomalies.</p

Analysis summary.

<p>Analysis summary.</p