Abstract
Abstract. Accurate global gridded estimates of evapotranspiration (ET) are key to understanding water and energy budgets, in addition to being required for model evaluation. Several gridded ET products have already been developed which differ in their data requirements, the approaches used to derive them and their estimates, yet it is not clear which provides the most reliable estimates. This paper presents a new global ET dataset and associated uncertainty with monthly temporal resolution for 2000–2009. Six existing gridded ET products are combined using a weighting approach trained by observational datasets from 159 FLUXNET sites. The weighting method is based on a technique that provides an analytically optimal linear combination of ET products compared to site data and accounts for both the performance differences and error covariance between the participating ET products. We examine the performance of the weighting approach in several in-sample and out-of-sample tests that confirm that point-based estimates of flux towers provide information on the grid scale of these products. We also provide evidence that the weighted product performs better than its six constituent ET product members in four common metrics. Uncertainty in the ET estimate is derived by rescaling the spread of participating ET products so that their spread reflects the ability of the weighted mean estimate to match flux tower data. While issues in observational data and any common biases in participating ET datasets are limitations to the success of this approach, future datasets can easily be incorporated and enhance the derived product.
Highlights
Improving the accuracy and understanding of uncertainties in the spatial and temporal variations of evapotranspiration (ET) globally is key to a number of endeavours in climate, hydrological and ecological research
The results indicated that PT–JPL achieved the highest statistical performance, followed closely by Global Land Evaporation Amsterdam Model (GLEAM), whereas Penman–Monteith scheme by Mu (PM-Mu) and surface energy balance system (SEBS) tended to under- and overestimate fluxes respectively
We use a modified relative standard deviation (SD) metric modified relative standard deviation (MRSD) that measures the variability of latent heat flux relative to the mean of the flux measured at each site
Summary
Improving the accuracy and understanding of uncertainties in the spatial and temporal variations of evapotranspiration (ET) globally is key to a number of endeavours in climate, hydrological and ecological research. FLUXNET (Baldocchi et al, 2001; Baldocchi, 2008), a global group of tower sites that measures the exchange of energy, water and carbon between the land surface and the atmosphere, provides direct observations of ET and most of the drivers and outputs of LSMs with soundly quantifiable uncertainty and sufficient accuracy to make diagnostic evaluation possible. This fact has led to a range of model evaluation, comparison and benchmarking studies using FLUXNET data
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