Assessment of actual evapotranspiration over a semiarid heterogeneous land surface by means of coupled low-resolution remote sensing data with an energy balance model: comparison to extra-large aperture scintillometer measurements

Abstract

Abstract. In semiarid areas, agricultural production is restricted by water availability; hence, efficient agricultural water 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 surface temperature Tsurf), which are crucial factors controlling ET. In this study, spatially distributed estimates of ET (or its energy equivalent, the latent heat flux LE) in the Kairouan plain (central Tunisia) were computed by applying the Soil Plant Atmosphere and Remote Sensing Evapotranspiration (SPARSE) model fed by low-resolution remote sensing data (Terra and Aqua MODIS). The work's goal was to assess the operational use of the SPARSE model and the accuracy of the modeled (i) sensible heat flux (H) and (ii) daily ET over a heterogeneous semiarid landscape with complex land cover (i.e., trees, winter cereals, summer vegetables). SPARSE was run to compute instantaneous estimates of H and LE fluxes at the satellite overpass times. The good correspondence (R2 = 0.60 and 0.63 and RMSE = 57.89 and 53.85 W m−2 for Terra and Aqua, respectively) between instantaneous H estimates and large aperture scintillometer (XLAS) H measurements along a path length of 4 km over the study area showed that the SPARSE model presents satisfactory accuracy. Results showed that, despite the fairly large scatter, the instantaneous LE can be suitably estimated at large scales (RMSE = 47.20 and 43.20 W m−2 for Terra and Aqua, respectively, and R2 = 0.55 for both satellites). Additionally, water stress was investigated by comparing modeled (SPARSE) and observed (XLAS) 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 flux values to daily totals was less obvious, daily ET estimates are deemed acceptable.

Bridging Thermal Infrared Sensing and Physically‐Based Evapotranspiration Modeling: From Theoretical Implementation to Validation Across an Aridity Gradient in Australian Ecosystems

“…Using the two‐source energy balance model (TSEB) (Norman et al, ), some recent studies have reported RMSD to the order of 72–135 and 52–131 W m −2 in hourly λE and Η for a semi‐arid grassland in Spain (Kustas et al, ), 95–166 W m −2 in hourly λE (Song et al, ) to 45–50 W m −2 in daily λE for semi‐arid irrigated cotton in Texas and Arizona (Colaizzi et al, ; French et al, ), and 50–59 W m −2 in hourly λE for irrigated maize in China (Song et al, ). A variant of TSEB model (SPARSE model) is found to produce 43–47 W m −2 in instantaneous λE and 50–80 W m −2 in hourly λE in Tunisia and Morocco (Boulet et al, ; Saadi et al, ). Considering the error statistics of state‐of‐the‐art SEB models and their parameterization uncertainties (Timmermans et al, ); the performance of STIC1.2 indicates substantial potential of this model towards bridging thermal infrared sensing and physically‐based evapotranspiration modeling.…”

Integrating thermal stress indexes within Shuttleworth–Wallace model for evapotranspiration mapping over a complex surface

“…13 Comparison of estimated evapotranspiration from the modified SW model driven by Landsat-7 and Landsat-8 and measured by the eddy covariance system (a) and the scintillometer (b) between scintillometer measurement and ET estimated using the Two-source Energy Balance model (TSEB) over an heterogeneous area covered by Millet (58%), Fallow savannah (23%) and Degraded shrubs (19%) in the region of Niamey city, Niger. In the same vein, Saadi et al (2018) have used an extra-large scintillometer to validate ET values estimated using Soil Plant Atmosphere and Remote Sensing Evapotranspiration (SPARSE) model over a landscape dominated by agriculture products (cereals, olive groves, fruit trees, market gardening). They found a RMSE and R 2 of about 0.06 mm/h and 0.55, respectively.…”

“…By contrast, the scintillometer technique that was recently developed allows for measurements of sensible (optical LAS) and latent (microwave LAS) heat fluxes over transects up to 10 km (Ezzahar and Chehbouni 2009;Ezzahar et al 2009b, a;Zieliński et al 2013Zieliński et al , 2017Yee et al 2015). In addition, scintillometers are easy to install, to monitor and can provide measurements of the surface fluxes over complex surfaces which makes them suitable for validating remote sensing-based methods (Duchemin et al 2008;Tang and Li 2015;Huang et al 2016;Ait Hssaine et al 2018;Saadi et al 2018). The LAS applies the Monin-Obukhov Similarity Theory (MOST) to estimate the sensible heat flux.…”

Multi-Scale Evaluation of the TSEB Model over a Complex Agricultural Landscape in Morocco

“…By contrast, large or extra-large scintillometers (LAS or XLAS) provide spatially averaged turbulent fluxes along transects that can reach 10 km [16][17][18][19]. Success in validating remote sensing-based ET against LAS measurements has already been demonstrated in some other areas of the world [20][21][22][23][24][25].…”