Evaluating Difference between Measured and Modeled Potential Evapotranspiration in the State of Colorado

Potential evapotranspiration (PET) is a critical variable to measure and constrain accurately in order to model the hydrologic cycle, monitor drought, and track trends in aridity in a changing climate. In Colorado, potential evapotranspiration is monitored using both weather station measurements, and gridded reanalysis datasets. In this study, PET measurements and anomalies from two unlinked data sources are compared for recent years. Measured PET data from 18 Colorado Agricultural Meteorological Network (CoAgMET) weather stations are compared and contrasted with overlaying gridded data from the North American Land Data Assimilation System (NLDAS) Phase II. Of the 18 CoAgMET sites used, six are on fully irrigated lands, six are on partially irrigated lands, and six are on non-irrigated lands. NLDAS data used in this study will not include an irrigation parameterization. Findings from this study explore the role of irrigation, and the role of complex topography in generating differences between measured and modeled seasonal evaporative demand measurements and anomalies. Findings from this study include the following:

1. Seasonal PET accumulations as measured by CoAgMET stations and estimated by overlaying NLDAS grid cells compare favorably. The two data sources show clearly similar interannual variability patterns.
2. On average, NLDAS-estimated PET is slightly higher than what was measured at CoAgMET sites.
3. CoAgMET sites on or near irrigated land showed significantly less seasonal PET than overlaying NLDAS grid cells. The two data sources align most closely for CoAgMET stations over unirrigated land.
4. The actual topography within a single NDLAS grid cell can be complex, but this did not result in significant differences in PET accumulations between CoAgMET sites and their overlaying NLDAS grid cells.
5. NLDAS and CoAgMET data, while similar, are sometimes different enough to appraise drought conditions at different levels of severity.