Abstract
Abstract. The WAter Cycle Multi-mission Observation Strategy – EvapoTranspiration (WACMOS-ET) project aims to advance the development of land evaporation estimates on global and regional scales. Its main objective is the derivation, validation, and intercomparison of a group of existing evaporation retrieval algorithms driven by a common forcing data set. Three commonly used process-based evaporation methodologies are evaluated: the Penman–Monteith algorithm behind the official Moderate Resolution Imaging Spectroradiometer (MODIS) evaporation product (PM-MOD), the Global Land Evaporation Amsterdam Model (GLEAM), and the Priestley–Taylor Jet Propulsion Laboratory model (PT-JPL). The resulting global spatiotemporal variability of evaporation, the closure of regional water budgets, and the discrete estimation of land evaporation components or sources (i.e. transpiration, interception loss, and direct soil evaporation) are investigated using river discharge data, independent global evaporation data sets and results from previous studies. In a companion article (Part 1), Michel et al. (2016) inspect the performance of these three models at local scales using measurements from eddy-covariance towers and include in the assessment the Surface Energy Balance System (SEBS) model. In agreement with Part 1, our results indicate that the Priestley and Taylor products (PT-JPL and GLEAM) perform best overall for most ecosystems and climate regimes. While all three evaporation products adequately represent the expected average geographical patterns and seasonality, there is a tendency in PM-MOD to underestimate the flux in the tropics and subtropics. Overall, results from GLEAM and PT-JPL appear more realistic when compared to surface water balances from 837 globally distributed catchments and to separate evaporation estimates from ERA-Interim and the model tree ensemble (MTE). Nonetheless, all products show large dissimilarities during conditions of water stress and drought and deficiencies in the way evaporation is partitioned into its different components. This observed inter-product variability, even when common forcing is used, suggests that caution is necessary in applying a single data set for large-scale studies in isolation. A general finding that different models perform better under different conditions highlights the potential for considering biome- or climate-specific composites of models. Nevertheless, the generation of a multi-product ensemble, with weighting based on validation analyses and uncertainty assessments, is proposed as the best way forward in our long-term goal to develop a robust observational benchmark data set of continental evaporation.
Highlights
The importance of terrestrial evaporation for hydrology, agriculture, and meteorology has long been recognized
The global mean annual volume of evaporation has been intensively debated in recent years, with the range of reported global averages in current Coupled Model Intercomparison Project Phase 5 (CMIP5) models being large (Wild et al, 2014) and observational benchmark data sets differing significantly (Mueller et al, 2013)
The total annual magnitude of evaporation estimated by the WACMOS-ET models amounts to 54.9 × 103 km3 for PM-MOD, 72.9 × 103 km3 for Global Land Evaporation Amsterdam Model (GLEAM), and 72.5 × 103 km3 for Priestley–Taylor Jet Propulsion Laboratory model (PT-JPL)
Summary
The importance of terrestrial evaporation (or “evapotranspiration”) for hydrology, agriculture, and meteorology has long been recognized. Dalton, 1802; Horton, 1919; Penman, 1948) It has been during the last decade that the interest of the scientific community in land evaporation has increased more dramatically, following the recognition of the key role it plays in climate (Wang and Dickinson, 2012; Dolman et al, 2014). Due to the link between transpiration and photosynthesis, atmospheric carbon concentrations and carbon cycle feedbacks are closely linked to terrestrial evaporation (Reichstein et al, 2013). When these factors are taken together, evaporation represents a crucial nexus of processes and cycles in the climate system
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