Assessing uncertainties in estimating surface energy fluxes from remote sensing over natural grasslands in Brazil

Pâmela Suélen Käfer,Savannah Tâmara Lemos Da Costa,Lucas Ribeiro Diaz,Silvia Beatriz Alves Rolim,Débora Regina Roberti,Eduardo André Kaiser,Nájila Souza Da Rocha,Gustavo Pujol Veeck

doi:10.1007/s00704-021-03869-6

Abstract

Evapotranspiration (ET) is one of the main fluxes in the global water cycle. As the Brazilian Pampa biome carries a rich biodiversity, accurate information on the ET dynamics is essential to support its proper monitoring and establish conservation strategies. In this context, we assessed an operational methodology based on the Simplified Surface Energy Balance Index (S-SEBI) model to estimate energy fluxes over the natural grasslands of the Pampa between 2014 and 2019. The S-SEBI is an ET model that requires a minimum of meteorological inputs and has demonstrated reasonable accuracy worldwide. Therefore, we investigated the model performance considering radiation data from both ERA5 reanalysis and Eddy Covariance measurements from a flux tower. Furthermore, comparisons from satellite-based estimates with in situ measurements were performed with and without energy balance closure (EBC). Results indicated that the meteorological inputs have low sensitivity on daily ET estimates from the S-SEBI model. In contrast, the instantaneous energy balance components are more affected. The strong seasonality impacts the evaporative fraction, which is more evident in late summer and autumn and may compromise the performance of the model in the biome. The effects in the daily ET are lower when in situ data without EBC are considered as ground truth. However, they are less correlated with the remote sensing-based estimates. These insights are useful to monitor water and energy fluxes from local to regional scale and provide the opportunity to capture ET trends over the natural grasslands of the Pampa.

Highlights

Evapotranspiration (ET) is widely used to measure the amounts of total water loss through several key processes between land and the atmosphere (Wang and Dickinson 2012; Dou and Yang 2018)
We evaluated the agreement of ERA5 meteorological reanalysis with the in situ Rs and Rld measured at the flux tower at the Landsat 8 overpass time (Figure 3)
The errors have a similar range during all seasons in which it was found that ERA5 product underestimated the Rs, with Root Mean Square Error (RMSE) of 69.80 W m-2 and mean absolute error (MAE) of 62.37 W m-2 (Figure 3a)

Summary

Introduction

Evapotranspiration (ET) is widely used to measure the amounts of total water loss through several key processes between land and the atmosphere (Wang and Dickinson 2012; Dou and Yang 2018). ET can be directly measured by using either lysimeter or water balance approach (Kumar et al 2011). These methodologies do not allow estimating the land surface fluxes when dealing with large spatial scales (Chen et al 2005; Courault et al 2005). Satellite observations have been used for monitoring surface conditions over the last few decades because they provide the potential to bridge the gap between point measurements and larger scale surface processes (Ma et al 2015), supplying the need for accessible data for the regions lacking land measurements (Zhang et al 2018). Accurate land use maps, and abundant meteorological reference data have considerably improved the ability to obtain remotely sensed ET and its flux components (Webster et al 2016)

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Conclusion