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The transformation of dry lands into irrigated fields using groundwater resources has been the key to agricultural development in a large number of countries. Particularly in arid and semiarid areas, this transformation needs to take into account the limited water resources available.

It is therefore, necessary to increase water application efficiency by modernising irrigation systems as well as continuously monitoring the irrigated areas in order to regulate the volume of irrigation water used.

In this paper, we present tools based on Irrigation Advisory Service (IAS) and remotely sensed data combined with geographic information systems (GIS) which assist in this kind of monitoring, especially for large areas.

The identification of crops in areas with small fields requires high spatial resolution satellite images. The classification criterion is based on the different temporal evolution of different crops. The crop development is described by means of the normalised difference vegetation index (NDVI). Crops belonging to the same class are assumed to have similar water requirements.

We define a new concept, the hydrological management unit (HMU) which includes all the fields which use water jointly. Their spatial location is obtained from the digital rural cadaster (DRC), a vectorial map which delimits each field. The Irrigation Advisory Service of the area determines the water requirements of each crop and estimates the volume of irrigation water actually used. By introducing this information into a GIS, we can quantify the spatial and temporal distribution of water extractions. The regulation of these extractions is the key to sustainability in aquifer case studies. (C) 2002 Elsevier Science B.V All rights reserved.