Comparisons of Independent Evaporation Measurements from Small Watershed Water Balances and Eddy Covariance

Eddy covariance measurements of evaporation are used to determine local, regional and global water budgets, calibrate and validate land surface models and acquire understanding of ecosystem processes. At the same time, there is a nearly universal non-closure of surface energy balance with the sum of the turbulent heat fluxes measured by eddy covariance often less than available energy at sites around the world, adding considerable uncertainty to the use of these measurements for numerous applications. In this presentation, I assess the accuracy of eddy covariance evaporation measurements by comparing them with those derived from small watershed water balances derived from precipitation, runoff, and soil moisture measurements. Previous comparisons published ten years ago with thirteen years of data from shrubland, grassland and savanna sites in southern Arizona USA reveal that the two independent measures of evaporation agreed to within an average of 3% annually and differed from -10 to +17% in any given year. The agreement between the two measures was generally better in drier years and at less topographically complex sites. In this talk, I will update these comparisons with many additional years of measurements, and I will also compare various corrections that have been proposed to address the apparent systematic underestimation of eddy-covariance evaporation.