ASSESSING SATELLITE AND GROUND-BASED POTENTIAL EVAPOTRANSPIRATION FOR HYDROLOGIC APPLICATIONS IN THE COLORADO RIVER BASIN

This study evaluates a remotely sensed and two ground-based potential evapotranspiration (PET) products for hydrologic application in the Upper Colorado River Basin (UCRB). The remotely sensed Moderate Resolution Imaging Spectroradiometer product (MODIS-PET) is a continuous, daily time series with 250m resolution derived using the Priestley-Taylor (P-T) equation. The MODIS-PET is evaluated against regional flux tower data as well as a synthetic pan product (Epan; 0.125 degrees, daily) derived from the North American Land Data Assimilation System (NLDAS) and a Hargreaves PET derived from DAYMET variables (DAYMET-PET; 1km, daily). Compared to point-scale PET computed using regional flux tower data, the MODIS-PET had lower errors, with RMSE values ranging from 2.24 to 2.85mm/day. Epan RMSE values ranged from 3.70 to 3.76mm/day and DAYMET-PET RMSE values ranged from 3.55 to 4.58mm/day. Further investigation showed biases in temperature and radiation data contribute to uncertainty in the MODIS-PET values, while bias in NLDAS temperature, downward shortwave (SW), and downward longwave (LW) propagate in the Epan estimates. Larger discrepancies between methods were observed in the warmer, drier regions of the UCRB, however, the MODIS-PET was more responsive to landcover transitions and better captured basin heterogeneity. Results indicate the satellite-based MODIS product can serve as a viable option for obtaining spatial PET values across the UCRB.