Assessing regional evapotranspiration and water balance across a Mediterranean montane climate gradient

Ray G Anderson,Yufang Jin,Michael L Goulden

doi:10.1016/j.agrformet.2012.07.004

Ray G Anderson, Yufang Jin + Show 1 more

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https://doi.org/10.1016/j.agrformet.2012.07.004

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Journal: Agricultural Meteorology	Publication Date: Aug 9, 2012
Citations: 71	License type: cc-by

Affiliation: University of California, Irvine

Abstract

We evaluate a new approach to estimate regional evapotranspiration (ET) across a montane, Mediterranean climate gradient in the San Jacinto and Santa Rosa Mountains of Southern California. Spatially distributed evaporative fraction (EF) measurements were made monthly from October 2008 to September 2009 at 54 locations across an elevational gradient using a mobile measurement platform, called the Regional Evaporative Fraction Energy Balance (REFEB) method. We used these measurements and the Enhanced Vegetation Index (EVI) from MODerate resolution Imaging Spectroradiometer (MODIS) observations to derive EF at a regional scale. We converted EF to monthly ET using remote-sensing based observations of available energy. We compared the REFEB ET estimates, along with modified Priestly–Taylor (PT) ET estimates driven by MODIS data against four eddy covariance (EC) towers and eight gauged catchments. Both of the satellite-based ET estimates were highly correlated with tower ET observations (r2=0.66 for REFEB and 0.95 for PT). The PT MODIS approach overestimated ET compared to precipitation estimates and stream gauge measurements, while REFEB ET was moderately lower than PT ET. The annual regional REFEB ET (193mm) was 87mm less than precipitation (280mm). REFEB ET underestimated EC tower ET (regression slope=0.78, p<0.001). Regional PT ET (288mm) exceeded precipitation by 8mm and significantly overestimated EC tower ET (regression slope=1.43, p<0.001). The relationship between precipitation and ET is not linear, with a break around 290mm/year, at which point ET becomes nearly constant at 200–300mm/year with increasing precipitation. This causes a break in the precipitation–runoff relationship, with a disproportionate increase in runoff when precipitation exceeds 290mm/year. REFEB provides a viable method to estimate regional ET, which is applicable to areas that are poorly constrained by other remote sensing approaches.

Highlights

Regional (102–105 km2) evapotranspiration (ET) and storage are critical, but poorly constrained, elements of the hydrologic cycle (Roads et al, 1994)
evaporative fraction (EF) values calculated from eddy covariance (EC) and the Regional Evaporative Fraction Energy Balance (REFEB)/T–q regression algorithms were similar for all the tower sites with the slopes from the linear regression of EC EF against T–q EF ranging between 0.93 at the Oak/Pine tower and 1.03 at the two Pinyon/Juniper towers
We developed a method for assessing regional evapotranspiration (ET) that combined a mobile evaporative fraction (EF) measurement technique, the Regional Evaporative Fraction Energy Balance (REFEB) mobile platform with vegetation and radiation data derived from the MODerate resolution Imaging Spectroradiometer (MODIS) satellite instrument and other sources

Summary

Introduction

Regional (102–105 km2) evapotranspiration (ET) and storage are critical, but poorly constrained, elements of the hydrologic cycle (Roads et al, 1994). Estimating and understanding regional ET and water balance are important to multiple environmental disciplines, including water resources planning and environmental modeling There are few large-scale observation networks for ET or storage at the regional scale (Rodell et al, 2004; Swenson and Wahr, 2006). Regional estimates of ET and water balance (runoff + terrestrial water storage anomalies) generally rely on assessing one component and inferring the other as a residual (Yeh and Famiglietti, 2008, 2009). Changes in the Earth’s gravity field as measured by missions such as the Gravity Recovery and Climate Experiment can be used to determine storage variations at larger scales, but cannot evaluate regions smaller than 105 km (Swenson and Wahr, 2002, 2006). Regional ET can be assessed through a number of satellite remote sensing approaches that combine radiometric temperatures, vegetation cover indices, and/or ancillary surface measurements, but these approaches require variations in these parameters that are related to ET and free of confounding effects (Li et al, 2009)

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