Remotely sensed terrestrial open water evaporation

Abstract

Terrestrial open water evaporation is difficult to measure both in situ and remotely yet is critical for understanding changes in reservoirs, lakes, and inland seas from human management and climatically altered hydrological cycling. Multiple satellite missions and data systems (e.g., ECOSTRESS, OpenET) now operationally produce evapotranspiration (ET), but the open water evaporation data produced over millions of water bodies are algorithmically produced differently than the main ET data and are often overlooked in evaluation. Here, we evaluated the open water evaporation algorithm, AquaSEBS, used by ECOSTRESS and OpenET against 19 in situ open water evaporation sites from around the world using MODIS and Landsat data, making this one of the largest open water evaporation validations to date. Overall, our remotely sensed open water evaporation retrieval captured some variability and magnitude in the in situ data when controlling for high wind events (instantaneous: r2 = 0.71; bias = 13% of mean; RMSE = 38% of mean). Much of the instantaneous uncertainty was due to high wind events (u > mean daily 7.5 m·s−1) when the open water evaporation process shifts from radiatively-controlled to atmospherically-controlled; not accounting for high wind events decreases instantaneous accuracy significantly (r2 = 0.47; bias = 36% of mean; RMSE = 62% of mean). However, this sensitivity minimizes with temporal integration (e.g., daily RMSE = 1.2–1.5 mm·day−1). To benchmark AquaSEBS, we ran a suite of 11 machine learning models, but found that they did not significantly improve on the process-based formulation of AquaSEBS suggesting that the remaining error is from a combination of the in situ evaporation measurements, forcing data, and/or scaling mismatch; the machine learning models were able to predict error well in and of itself (r2 = 0.74). Our results provide confidence in the remotely sensed open water evaporation data, though not without uncertainty, and a foundation by which current and future missions may build such operational data.

Calculation of Kinetic Fractionation of Open‐Water Evaporation Over an Upland Reservoir

“…If one has the land‐based evaporation data, he/she should use them as the first reference to any open‐water evaporation calculation. In this study, we consider that, while the open‐water evaporation flux value may be different from the land‐based record, its magnitude variability is likely similar to the land‐based record variability (Fisher et al., 2023). In other words, the ratio between the open‐water evaporation rate and the land‐based one is constant over time.…”

Current and Near-Term Earth-Observing Environmental Satellites, Their Missions, Characteristics, Instruments, and Applications

“…It has proven to be well suited for examining changes in water budgets at regional scales, for monitoring changes in soil moisture and vegetation moisture content, and for collecting data to estimate evapotranspiration [ 257 , 258 , 259 , 260 ]. Fisher et al [ 261 ] were able to separate and map open water evaporation from ET and mixed sites. Doughty et al [ 262 ] used ECOSTRESS to determine tropical forest stress levels and their sites were found to approach critical temperature thresholds.…”