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

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 (193 mm) was 87 mm less than precipitation (280 mm). REFEB ET underestimated EC tower ET (regression slope = 0.78, p < 0.001). Regional PT ET (288 mm) exceeded precipitation by 8 mm 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 290 mm/year, at which point ET becomes nearly constant at 200–300 mm/year with increasing precipitation. This causes a break in the precipitation–runoff relationship, with a disproportionate increase in runoff when precipitation exceeds 290 mm/year. REFEB provides a viable method to estimate regional ET, which is applicable to areas that are poorly constrained by other remote sensing approaches.

► Mountains are critical water sources in Mediterranean regions, but ET is poorly constrained.
► We integrate a new measurement platform with satellite observations to assess regional ET.
► Our approach provides realistic spatial observations of ET and runoff.
► Relationship between P and ET is non-linear.
► Small, projected changes in P or ET (±10%) could cause large (>30%) changes in runoff.