Abstract
The main goal of this study is to evaluate the potential of the FAO-56 dual technique for the estimation of regional evapotranspiration (ET) and its constituent components (crop transpiration and soil evaporation), for two classes of vegetation (olives trees and cereals) in the semi-arid region of the Kairouan plain in central Tunisia. The proposed approach combines the FAO-56 technique with remote sensing (optical and microwave), not only for vegetation characterization, as proposed in other studies but also for the estimation of soil evaporation, through the use of satellite moisture products. Since it is difficult to use ground flux measurements to validate remotely sensed data at regional scales, comparisons were made with the land surface model ISBA-A-gs which is a physical SVAT (Soil–Vegetation–Atmosphere Transfer) model, an operational tool developed by Météo-France. It is thus shown that good results can be obtained with this relatively simple approach, based on the FAO-56 technique combined with remote sensing, to retrieve temporal variations of ET. The approach proposed for the daily mapping of evapotranspiration at 1 km resolution is approved in two steps, for the period between 1991 and 2007. In an initial step, the ISBA-A-gs soil moisture outputs are compared with ERS/WSC products. Then, the output of the FAO-56 technique is compared with the output generated by the SVAT ISBA-A-gs model.
Highlights
In semi-arid regions, and the Mediterranean basin in particular, agricultural productivity and water resources regularly suffer from serious crises, as a consequence of limited levels of precipitation, combined with the occurrence of long periods of drought, which are typical features of the Mediterranean climate [1]
The FAO-56 dual approach, which is commonly used for irrigation management, is applied to the simulation of evapotranspiration at the regional scale
The vegetation fractions represented by these two classes of vegetation are retrieved by means of a multi-temporal classification of SPOT-VGT time series images
Summary
In semi-arid regions, and the Mediterranean basin in particular, agricultural productivity and water resources regularly suffer from serious crises, as a consequence of limited levels of precipitation, combined with the occurrence of long periods of drought, which are typical features of the Mediterranean climate [1]. Various attempts have been made to combine the latter model with remote sensing data for operational applications [9,10,11,12,13,14] These studies were motivated, in particular, by the need to retrieve vegetation cover dynamics from vegetation indices derived from optical satellite observations [9,10,15,16]. A large number of studies have demonstrated the potential of low-resolution spaceborne (active microwave) scatterometers for land surface characterization, in particular for the estimation of soil moisture [32,33,34,35,36,37] In this context, the aim of the present study is to illustrate the ability of the FAO-56 dual approach to estimate evapotranspiration at regional scales, without making use of complex physical surface models requiring large quantities of input data.
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