A MODIS-Based Energy Balance to Estimate Evapotranspiration for Clear-Sky Days in Brazilian Tropical Savannas

Anderson L Ruhoff,Adriano R Paz,Walter Collischonn,Humberto R Rocha,Luiz E.O.C Aragao,Yadvinder S Malhi

doi:10.3390/rs4030703

Abstract

Evapotranspiration (ET) plays an important role in global climate dynamics and in primary production of terrestrial ecosystems; it represents the mass and energy transfer from the land to atmosphere. Limitations to measuring ET at large scales using ground-based methods have motivated the development of satellite remote sensing techniques. The purpose of this work is to evaluate the accuracy of the SEBAL algorithm for estimating surface turbulent heat fluxes at regional scale, using 28 images from MODIS. SEBAL estimates are compared with eddy-covariance (EC) measurements and results from the hydrological model MGB-IPH. SEBAL instantaneous estimates of latent heat flux (LE) yielded r 2= 0.64 and r2 = 0.62 over sugarcane croplands and savannas when compared against in situ EC estimates. At the same sites, daily aggregated estimates of LE were r 2 = 0.76 and r2 = 0.66, respectively. Energy balance closure showed that turbulent fluxes over sugarcane croplands were underestimated by 7% and 9% over savannas. Average daily ET from SEBAL is in close agreement with estimates from the hydrological model for an overlay of 38,100 km2 (r2 = 0.88). Inputs to which the algorithm is most sensitive are vegetation index (NDVI), gradient of temperature (dT) to compute sensible heat flux (H) and net radiation (Rn). It was verified that SEBAL has a tendency to overestimate results both at local and regional scales probably because of low sensitivity to soil moisture and water stress. Nevertheless the results confirm the potential of the SEBAL algorithm, when used with MODIS images for estimating instantaneous LE and daily ET from large areas.

Highlights

Evapotranspiration (ET) is one of the most important regulating factors of climate, at both local and global scales, linking energy, climate and hydrology
Rn derived from SEBAL near mid-day (Terra MODIS has a ~11:30 am local over-passing time) significantly explained over 80% of the variation of measured values at the USE (r2 = 0.88, p < 0.05)
Because of the large quantities of surface information obtained by MODIS sensor and its available products, estimates of energy fluxes of the Rio Grande basin given by SEBAL algorithm can be regarded as consistent

Summary

Introduction

Evapotranspiration (ET) is one of the most important regulating factors of climate, at both local and global scales, linking energy, climate and hydrology. Such methods are limited in that they provide values of ET at specific sites and not at a regional or larger scale. Because the costs of installing complex measurement systems are high, and human and economic resources scarce, methods have been developed in recent years which provide rapidly available hydrological data over large areas based on remote sensing. Index) [4], SEBAL (Surface Energy Balance Algorithm for Land) [5,6],T-SEB(Two-Source Energy. Balance) [7], R-SEB(Aerodynamic Resistance Surface Energy Balance) [8], among others. The aim of such algorithms is to estimate latent heat fluxes (LE) and ET at both local and regional scales, using remote sensing with complementary meteorological data. Other research has reported results that are less clear [16,17]

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Conclusion