The WACMOS-ET project – Part 2: Evaluation of global terrestrial evaporation data sets

Abstract

Abstract. The WACMOS-ET project aims to advance the development of land evaporation estimates at global and regional scales. Its main objective is the derivation, validation and inter-comparison 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 and 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. (2015) inspect the performance of these three models at local scales using measurements from eddy-covariance towers, and include the assessment the Surface Energy Balance System (SEBS) model. In agreement with Part 1, our results here indicate that the Priestley and Taylor based products (PT-JPL and GLEAM) perform overall best for most ecosystems and climate regimes. While all three products adequately represent the expected average geographical patterns and seasonality, there is a tendency from PM-MOD to underestimate the flux in the tropics and subtropics. Overall, results from GLEAM and PT-JPL appear more realistic when compared against surface water balances from 837 globally-distributed catchments, and against separate evaporation estimates from ERA-Interim and the Model Tree Ensemble (MTE). Nonetheless, all products manifest 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, implies caution 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. Yet, 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.

Local tower-based merging of two land evaporation products

“…Compared to Miralles et al (2016), the GLEAM and PT-JPL contributions of each component to the total ET are comparable, with the small differences attributable to the above-described changes in the models and/or inputs (e.g., 25 new precipitation product for GLEAM, or the revised use of the LAI/FAPAR for PT-JPL). Re- Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | call that, as discussed in Section 2.4, only the sum of the soil evaporation and transpiration is validated against tower fluxes.…”

“…The GLEAM and PT-JPL models, and the inputs required to run them at the global scale are described in and Miralles et al (2016). Only the main differences with respect to the original WACMOS-ET runs are fully described here.…”

“…The GLEAM and PT-JPL required global inputs remain unchanged with respect to Miralles et al (2016), apart from the precipitation product, and are applied at the same resolution of 25 km. Common inputs to GLEAM and PT-JPL are the surface net radiation, coming from the NASA and GEWEX Surface Radiation Budget (SRB, Release 3.1 Stackhouse et al,10 2004), and the near-surface air temperature, sourced from the ERA-Interim atmospheric reanalysis (Dee et al, 2011).…”

“…The recent GLEAM v3 model of Martens et al (2016) is adopted for this study and re-20 places the model of Miralles et al (2011b) previously applied for the WACMOS-ET runs.…”

“…The FLUXNET 2015 synthesis data set (http://fluxnet.fluxdata.org/) is used to obtain in situ measurements of evaporation (referred to as tower ET), and it is processed as in Martens et al (2016) to retain only high-quality data appropriate to evaluate the evaporation models. Starting from the original time resolution (generally 30 minutes or 1 hour), the processing 25 involves: (1) masking measurements using the provided quality flags; (2) masking measure-7…”