IWRA World Water Congress 2008 Montpellier France
1. Water availability, use and management
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),
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.