Assessment of actual evapotranspiration over a semi-arid heterogeneous land surface by means of coupled low resolution remote sensing data with energy balance model: comparison to extra Large Aperture Scintillometer measurements

Abstract

Abstract.In semi-arid areas, agricultural production is restricted by water availability; hence efficient agricultural water 20 management is a major issue. The design of tools providing regional estimates of evapotranspiration (ET), one of the most relevant water balance fluxes, may help the sustainable management of water resources. Remote sensing provides periodic data about actual vegetation temporal dynamics (through the Normalized Difference Vegetation Index NDVI) and water availability under water stress (through the land surface temperature LST) which are crucial factors controlling ET. 25In this study, spatially distributed estimates of ET (or its energy equivalent, the latent heat fluxes LE) in the Kairouan plain (Central Tunisia) were computed by applying the Soil Plant Atmosphere and Remote Sensing Evapotraspiration (SPARSE) model fed by low resolution remote sensing data (Terra and Aqua MODIS). The work goal was to assess the operational use of the SPARSE model and the accuracy of the modelled i) sensible heat flux (H) and ii) daily ET over a heterogeneous semi-arid landscape with a complex land cover (i.e. trees, 30 winter cereals, summer vegetables). The SPARSE's layer approach was run to compute instantaneous estimates of H and LE fluxes at the satellite overpass time ; for Terra and Aqua, respectively) between instantaneous H estimates and large aperture scintillometer (XLAS)'s H measurements along a pathlength of 4 km over the study area showed that the SPARSE model presents 35 satisfactory accuracy. Results showed that, despite the fairly large scatter, the instantaneous LE can be suitably ; for Terra and Aqua, respectively and R 2 = 0.55 for both satellites). Additionally, water stress was investigated by comparing modelled (SPARSE derived) to observed (XLAS derived) water stress values; we found that most points were located within a 0.2 confidence interval, thus the general tendencies are well reproduced. . Even though extrapolation of instantaneous latent heat 40 flux values to daily totals was less obvious, daily ET estimates are deemed acceptable.

Combining a Two Source Energy Balance Model Driven by MODIS and MSG-SEVIRI Products with an Aggregation Approach to Estimate Turbulent Fluxes over Sparse and Heterogeneous Vegetation in Sahel Region (Niger)

“…In this study, the Horst and Weil model, which has been adapted to scintillometer measurements, was used to evaluate the source surface. It has been used successfully in previous studies [27,35].…”

“…A spatial aggregation approach is conceived as a method which seeks to link the model parameters that control surface exchange (Ts, α, d, z_0, LAI, and hc) on a patch scale with the area-average value of equivalent model parameters applicable on a larger scale or grid-scale, assuming that the same equations are used to describe surface fluxes at both scales [27,[30][31][32][33][34]. In general, aggregation schemes can be applied on the input requirements [27] or directly to predicted fluxes [35]. Nevertheless, the efficiency of this approach lies on the evaluation of their outputs/performances against field observations, which require the development of a measurement network at the local scale such as Eddy covariance systems [36][37][38][39].…”