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Estimation of Evapotranspiration Across the Conterminous United States Using a Regression with Climate and Land-Cover Data1

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Estimation of Evapotranspiration Across the Conterminous United States Using a Regression with Climate and Land-Cover Data1 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION Vol. 49, No. 1 AMERICAN WATER RESOURCES ASSOCIATION February 2013 ESTIMATION OF EVAPOTRANSPIRATION ACROSS THE CONTERMINOUS UNITED STATES USING A REGRESSION WITH CLIMATE AND LAND-COVER DATA1 Ward E. Sanford and David L. Selnick2 ABSTRACT: Evapotranspiration (ET) is an important quantity for water resource managers to know because it often represents the largest sink for precipitation (P) arriving at the land surface. In order to estimate actual ET across the conterminous United States (U.S.) in this study, a water-balance method was combined with a climate and land-cover regression equation. Precipitation and streamflow records were compiled for 838 watersheds for 1971-2000 across the U.S. to obtain long-term estimates of actual ET. A regression equation was developed that related the ratio ET ⁄ P to climate and land-cover variables within those watersheds. Precipitation and temperatures were used from the PRISM climate dataset, and land-cover data were used from the USGS National Land Cover Dataset. Results indicate that ET can be predicted relatively well at a watershed or county scale with readily available climate variables alone, and that land-cover data can also improve those predictions. Using the climate and land-cover data at an 800-m scale and then averaging to the county scale, maps were pro- duced showing estimates of ET and ET ⁄ P for the entire conterminous U.S. Using the regression equation, such maps could also be made for more detailed state coverages, or for other areas of the world where climate and land-cover data are plentiful. (KEY TERMS: evapotranspiration; hydrologic cycle; precipitation; streamflow.) Sanford, Ward E. and David L. Selnick, 2012. Estimation of Evapotranspiration Across the Conterminous Uni- ted States Using a Regression with Climate and Land-Cover Data. Journal of the American Water Resources Association (JAWRA) 49(1): 217-230. DOI: 10.1111 ⁄ jawr.12010 INTRODUCTION fraction of the former. Thus, quantifying ET is criti- cal to quantifying surface runoff to reservoirs or recharge to aquifers (Healy and Scanlon, 2010). Evapotranspiration (ET) is a major component of Quantifying ET is also critical for studies of ecosys- the hydrologic cycle, and as such, its quantity is of tem water balances (Sun et al., 2011a) and regional major concern to water resource planners around the carbon balances (Sun et al., 2011b). world. The long-term average quantity of water avail- In spite of the critical nature of this hydrologic able for human and ecological consumption in any component, its measurement on regional to continen- region is roughly the difference between the mean tal scales has been problematic. Measurement of ET, annual precipitation and mean annual ET (Postel although possible directly at the land surface (e.g., et al., 1996) with the latter frequently a majority Stannard, 1988), is usually made either indirectly by 1Paper No. JAWRA-11-0134-P of the Journal of the American Water Resources Association (JAWRA). Received December 16, 2011; accepted September 24, 2012. ª 2012 American Water Resources Association. This article is a U.S. Government work and is in the public domain in the USA. Discussions are open until six months from print publication. 2Respectively, Research Hydrologist and Hydrologist, 431 National Center, U.S. Geological Survey, Reston, Virginia 20192 (E-Mail ⁄ Sanford: [email protected]). JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 217 JAWRA SANFORD AND SELNICK quantifying the energy balance at a local land surface streamflow on a monthly or yearly time scale. The (Ward and Trimble, 2004) or by eddy covariance at water-balance approach has been used across the some distance above the land surface (Baldocchi regional and continental scale along with other et al., 2001; Mu et al., 2007). The energy-balance watershed factors to estimate relative contributions approach requires meteorological measurements on to ET (Zhang et al., 2004; Cheng et al., 2011; Wang an hourly period for each small-scale plot of land and Hejazi, 2011). Recently, the use of remotely because ET can vary in time and space in the range sensed data (radiation and land cover) has been com- of hours and meters. Thus, this approach has tradi- bined with theoretical forest canopy ET estimates to tionally been data and labor-intensive and not suit- estimate continental scale ET across Canada (Liu able for scaling up to regional estimates of ET. The et al., 2003); however, for this method, the radiation eddy covariance approach can be used at a single site measurements have been limited in time and there- to get averages over months or years, but still has fore the ET estimates have been made only for a sin- the disadvantage of measuring ET only within a lim- gle year. ited spatial extent. Another indirect measurement We propose here to combine watershed water- approach to estimate ET is by first estimating the balance data with now publically available meteoro- potential ET, where the latter is the actual ET that logical data across the United States (U.S.) to create were to occur if there were ample water available. a regression equation that can be used to estimate Potential ET has been traditionally estimated by the long-term ET at any location within the conterminous use of equations that contain terms for local meteoro- U.S. (CUS). Such a regression equation was devel- logical data such as relative humidity and wind oped recently for the state of Virginia (Sanford et al., speed. This type of data is more available than 2011). The difference between this study and other energy measurements for many regions, so this previous studies is that we rely on long-term (1971- approach has been employed in regional studies 2000) average streamflow conditions in order to mini- (Ward and Trimble, 2004). A relation between actual mize the relative size of the change in storage term, and potential ET, however, is still required, which is and thereby are able to use data from several hun- usually also a function of meteorological variables. dred watersheds across the CUS as a proxy for ET The term ‘‘ET’’ is often used to emphasize that what observations. The purpose of the current study is to is being considered is ET that has actually occurred, demonstrate the development and calibration of a and not the potential ET. In this article, we consider regression equation that would apply to the entire the terms ET and actual ET to be the same, and will CUS, and to make a map of estimated long-term use only the term ET for the remainder of the article mean annual ET for the CUS and an equivalent map except in the final figure, where the modifier ‘‘actual’’ of the ratio of ET to precipitation. The regression is used only for emphasis. equation is first developed using only climatic vari- A third approach for measuring ET indirectly is ables, but a second improved equation is also devel- the water-balance approach, usually conducted for a oped that included land-cover variables. The map of watershed where the other components (precipita- the long-term mean annual ET should prove to be of tion, change in storage, and stream discharge) are great value to water managers planning for long-term measured, and the remainder is attributed to ET sustainable regional use of the resource, and the (Ward and Trimble, 2004; Healy and Scanlon, 2010). equation should be useful for examining the variabil- This method is advantageous when the goal is to ity of ET at more local to state scales, or to other obtain a long-term average because when the period areas of the world where such climate and ⁄ or land- of record examined is long enough, the change in cover data are available. storage term can be neglected. The method has also been used in modeling of ET on monthly or yearly scales (Sun et al., 2011b) and compared with eddy covariance data, but this approach has three limita- METHODS tions: the shorter time step amplifies the importance of the change in storage that has high uncertainty, the eddy covariance instruments each sample a rela- The approach taken in this study is to obtain esti- tively small area, and eddy covariance data have mates of ET from watersheds across the U.S. and only been available for about the last decade or so relate these estimates to climate and land-cover data (Baldocchi et al., 2001). Remotely sensed soil-mois- such that a regression equation can be developed and ture data have been used as a proxy for change in applied to all counties of the CUS. A mean annual storage, but remote sensors measure shallow soil streamflow for the period 1971-2000 was obtained moisture only and not the change in storage in from at least one watershed in every state in the groundwater, which can be substantial relative to CUS based from the U.S. Geological National Water JAWRA 218 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION ESTIMATION OF EVAPOTRANSPIRATION ACROSS THE CONTERMINOUS UNITED STATES USING A REGRESSION WITH CLIMATE AND LAND-COVER DATA Information System (NWIS) database (http://waterdata. surface over this 30-year period is small relative to usgs.gov/nwis/rt). Selection criteria included a com- the amount of water that has exited by streamflow plete monthly flow record from 1971 to 2000, a during the same period. Sources of error in the esti- watershed area between 100 and 1,000 km2, and lack mates of ET, in addition to this change in storage, of any known water-impoundment features (e.g., res- could be error in the precipitation estimate, flow-rate ervoirs) or water exports or imports from the estimate, or area of the watershed estimate.
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