## 2 Water Resources Monitoring in Semi-Arid Regions: The WARM-SAR Project
### Land Cover Map Production  
The availability of repeated acquisitions offered by new-generation sensors opens new perspectives for remote-sensing applications in semi-arid environments through the definition of new products which fully exploit the particular Sahelian climate [23]. In fact, as discussed in [19], the occurrence of a condition of aridity of the scene at the peak of the dry season makes it possible to establish a reference scenario for the other available images. The procedure for building the Level-1\(\alpha\) products is presented in [19] and an example relevant to a small reservoir of the Yatenga district is depicted in Figure 1. The reference image, loaded on the blue band, was acquired at the end of April, _i.e._, at the peak of the dry season. The test images and the interferometric coherence are assigned to the green and red band, respectively.  
This combination of bands makes easy the users' pre-attentive processing, _i.e._, the unconscious accumulation of information from the environment [26], since it allows for an association between relevant physical features of the scene and the colors restituted by the RGB map, which is consistent with human expectation. With reference to Figure 1, this happens, as an example, for seasonal water, and for seasonal vegetation, which are displayed in blue and green, respectively.  
As for the seasonal water, the blue color results from the prevalence of the backscattering from rough soil emerging in the dry season image (blue band) when there is no water in the intake, with respect to the backscattering from flat water surface occurring in wet season images (green band).  
As for the seasonal vegetation, the green color results from the prevalence of the backscattering from the vegetation emerging in the wet season images (green band) with respect to the backscattering from rough surfaces occurring in the dry season image (blue band), in absence of vegetation [27].  
In both the cases, the coherence (red band) is almost null.  
When the electromagnetic response of the reference image and of the test image is comparable, the map exhibits a cyan tonality. Where both the reference and test image are covered by surface water, due to the weak electromagnetic response in these two bands, the composition restitutes the black color (see the dark area in the immediate proximity of the dam in Figure 1).  
The main characteristics of the Level-1\(\alpha\) products are the congruence with human vision, which allows for an immediate understanding of the more relevant natural cycles of a semi-arid environment (_i.e._, those of water and of vegetation), and the class detachability. The latter, in particular, makes these products very attractive for supervised classification procedures, even using extremely simple algorithms.  
The aforementioned characteristics of Level-1\(\alpha\) imagery provide a tool for a quick estimation of the reservoirs' surface areas for several basins, from which it is possible to retrieve the retained water volume, as is explained in Section 5. Thus, these products can be managed by a large variety of scientists and researchers, even non-experts in SAR issues, and could, therefore, potentially increase the use of such data, thereby bringing benefits to water-related research activities.