Suitability of Earth Engine Evaporation Flux (EEFlux) Estimation of Evapotranspiration in Rainfed Crops

Sunil A Kadam,Zhongming Gao,Eric S Russell,Mingliang Liu,Claudio O Stöckle

doi:10.3390/rs13193884

Abstract

This study evaluated evapotranspiration (ET) estimated using the Earth Engine Evapotranspiration Flux (EEFlux), an automated version of the widely used Mapping Evapotranspiration at High Spatial Resolution with Internalized Calibration (METRIC) model, via comparison with ET measured using eddy covariance flux towers at two U.S. sites (St. John, WA, USA and Genesee, ID, USA) and for two years (2018 and 2019). Crops included spring wheat, winter pea, and winter wheat, all grown under rainfed conditions. The performance indices for daily EEFlux ET estimations combined for all sites and years dramatically improved when the cold pixel alfalfa reference ET fraction (ETrF) in METRIC was reduced from 1.05 (typically used for irrigated crops) to 0.85, with further improvement when the periods of early growth and canopy senescence were excluded. Large EEFlux ET overestimation during crop senescence was consistent in all sites and years. The seasonal absolute departure error was 51% (cold pixel ETrF = 1.05) and 23% (cold pixel ETrF = 0.85), the latter reduced to 12% when the early growth and canopy senescence periods were excluded. Departures of 10% are a reasonable expectation for methods of ET estimation, which EEFlux could achieve with more frequent satellite images, better daily weather data sources, automated adjustment of daily ETrF values during crop senescence, and a better understanding of the selection of adequate cold pixel ETrF values for rainfed crops.

Highlights

IntroductionET can be estimated with lysimeters, Bowen ratio, eddy covariance, and scintillometer systems [1,2,3,4]
We evaluated the performance of Engine Evapotranspiration Flux (EEFlux) in the estimation of daily ET of rainfed crops using eddy covariance (EC) ET as a reference, but as shown in Figure 3, one difficulty is that EC ET is not a perfect reference due to lack of closure of the energy balance (Figure 4) and uncertainties in the estimation of the footprint or ground area represented by the measured water vapor exchange
The cold pixel ETrF is typically set at a value of 1.05 in irrigated agriculture, we found that a value of 0.85 resulted in better agreement and avoided large ET

Summary

Introduction

ET can be estimated with lysimeters, Bowen ratio, eddy covariance, and scintillometer systems [1,2,3,4]. These methods measure ET over relatively small areas and are difficult to extrapolate in time and space given heterogeneous land surfaces [5] and crop, soil and weather variations [1]. These limitations can be overcome by using methods based on remote sensing data

Objectives

Methods

Discussion

Conclusion