THE SYNERGISTIC USE OF MULTIPLE SENSORS FOR HYDROLOGICAL PURPOSES

In recent years, the succession of the different satellite missions and the extensive availability of the remote sensing data facilitate their temporally continuative use for hydrological purposes. Nevertheless, the observation of the Earth is never complete with a single instrument and integrating multi-sensor measurements provides significant advantages over single source data and becomes a real possibility to study in depth the physical phenomena. Indeed, even if the use of a single sensor can support the modern hydrology, the synergy between complementary satellite missions contributes to improve the quality of the single application. As reported in the review paper by Pohl and Van Genderen (1998) on the image fusion, integrating different data represents an added value to the information than can be derived from data by single sensor (`1+1=3'). The availability of a large number of different sensors with low to very high resolution over the same area widens the possibilities to have more accurate information. Even though it is nearly impossible to acquire multi-sensor data simultaneously, this does not devalue the jointly exploitation of satellite observations because they increase the temporal information by observing the changes that might have occurred in a certain area during different times of day, month or year. As stated by Llinas and Hall (1998) and then by See and Abrahart (2001), the general concept is analogous to multiple senses, experience and the ability to reason used by humans and animals in order to improve their chances of survival. Although the large provision of satellite sensors and the advances in computing, new methods and techniques are necessary to make fusion of data increasingly efficient. Many studies have investigated the benefits and advantages of using the interaction of multiple sensors. Among them, the flooded areas assessment after an extreme event, the wetland storage analysis for ecosystem sustainability as well as the discharge assessment at ungauged river sites are matters of paramount interest for satellite applications. Specifically, the delineation of flooded areas and/or wetland is commonly represented by the fusion of Synthetic Aperture Radar (SAR) and Optical imaging sensors. The weakness represented by the optical sensor that are not able to see during the night-time and with bad weather is compensated by the use of space-borne microwave sensors that are widely used because of their all-weather and day-night capabilities. By contrast, optical band satellite imagery provide more reliable and easy to interpret results compared with SAR imagery. If these two observations are correctly associated then the combination of the two sensors data provides an improved determination of the contour of the flooded areas that could be obtained by either of the two independent sensors. Broadening the concept of fusion, also the integration of imaging information, derived by SAR or multispectral imagery, with profiling measurements by radar altimetry provide a valuable tool for the estimation of the river flow or for the evaluation of lake volume variation. In this case the high temporal resolution of the multispectral sensor (as for example Moderate Resolution Imaging SpectroRadiometer, MODIS or Medium Resolution Imaging Spectrometer, MERIS) is used for the monitoring of the temporal variation of the hydrological variables (flow velocity of the river or surface areas of the lakes), whereas the radar altimetry provides the estimates of the water surface level. Based on the above matters, this chapter presents an overview of integration, combination and fusion of satellite data for hydrological applications by focusing on 1) the assessment of the flooded areas, particularly interesting for civil defense and for the evaluation of the damage caused by disaster events, 2) evaluation of wetland extent and storage, often necessary for ecosystem services and 3) estimation of river discharge, essential for scientific and operational applications related to water resource management and flood risk prevention. This chapter is organized as follows: Section 2 provides an overview on the use of satellite products for flooding extension analysis. Section 3 contains the description of methods to estimate water storage and level in the wetland areas through remote satellite data. Section 4 a review of the river hydraulic characteristics obtained by remote sensors for the flow discharge assessment is presented. Section 5 discusses conclusions and future perspectives.