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Land use and crop evapotranspiration in Tensift/Marrakech plain: inter -annual analysis based on MODIS satellite data

Congress: 2008
Author(s): I. Benhadj, B. Duchemin,S. Er-Raki, R. Hadria, P. Maisongrande,V. Simonneaux, B. Mougenot, S. Khabba, M.H. Kharrou, A. Chehbouni
I. Benhadj(1), B. Duchemin(1), Er-Raki S.(2), Hadria R.(1), P.Maisongrande(1), V. Simonneaux (1,2), B. Mougenot(1), S. Khabba (2), A. Chehbouni (1) (1) Centre d’Etudes Spatiales de la BIOsphère (CESBIO), Toulouse Fr

Keyword(s): Evapotranspiration, irrigation management, semi arid , Land use, MODIS , NDVI
AbstractAgricultural water management is a key issue to ensure sustainable development of semi-arid regions. In southern Mediterranean countries, irrigation water represents up to 90% of total water use (FAO 2005. Irrigation in Africa in figures. AQUASTAT Survey). These regions are characterized by a strong increasing demand in contrast of the scarcity of available water resources. There is thus a crucial need to develop tools for quantifying agricultural water use at a regional scale. This is one of the primary objectives of the SudMed project (Chehbouni et al., International Journal of Remote Sensing, in press), which is the frame of this study. In this study, we aim at monitoring the water balance over the semi-arid plain of Tensift/Marrakech, a 2000 km² intensively cropped area in center of Morocco. This requires firstly to map the land use, secondly to monitor the vegetation dynamics, and thirdly to evaluate evapotranspiration, which is the key-variable of water balance in semi-arid plains. In this context, we investigate the potential offered by Terra-MODIS satellite, which provide a costless daily global coverage of the Earth. We use a six-year archive of 16-day composite NDVI images from 2000-2001 to 2005-2006 agricultural seasons. However, the use of medium (250 m) spatial resolution data makes difficult to directly monitor land surfaces. Indeed, each pixel (mixed pixel) generally includes different types of surface, and consequently its spectral response results from the contribution of each land classes. In a first phase, the land use and the vegetation dynamics are retrieved using linear unmixing model applied on NDVI time series. Identification of end-members, i.e. the specific NDVI time course of individual land classes, is based on the assumption that pure pixels can be identified directly from MODIS NDVI images. The approach is set up to map the land use fractions of the three classes that are the most important for agricultural water management in the study area (non cultivated areas, orchards, annual crops). It is evaluated on two particular years for which reference land use maps are derived from high spatial resolution images. The inter-annual variability of land use estimates and vegetation seasonality is also discussed according to water availability (irrigation and rainfall). In a second phase, the information on land use and vegetation dynamics is used to estimate evapotranspiration. The method is adapted from the FAO-56 algorithm (Allen R.G. 2000. Journal of Hydrology 229:27–41), which computes crop water needs from a reference evapotranspiration (ETo) and cultural crops coefficients (Kc). ETo is calculated by applying spatial interpolation of the meterorological data available in the study area. The crop coefficients, which vary according to the crop type, phenological stage and soil water content, are retrieved for each land classes according to their NDVI time courses using various scenarii of irrigation. The spatio-temporal patterns of evapotranspiration maps are evaluated against ground measurements and analysed at a regional scale together with the main driving variables (climate and water availability). Finally, the perspectives offered by the approach for the monitoring of irrigation are discussed.
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