New processing approaches on the retrieval of water levels in Envisat and SARAL radar altimetry over rivers: A case study of the Sao Francisco River, Brazil

Radar-based satellite altimetry is well recognized for oceanographic applications. For continental hydrology, its use is complicated by a number of environmental factors such as river width and shape, land cover type in the vicinity of the river banks, and the topography of the relief. These factors make precision vary significantly. Locations where the satellites cross the river can be used as "virtual gauging stations" that can complement the existing network of in situ stations. This article describes processing techniques that take some of these environmental factors (river shape and width) into account to improve the precision of altimetry measurements of the water level. These techniques are based on some a priori information about the river banks and on modeling a phenomenon called "off-nadir hooking". This approach is tested on the Sao Francisco River in Brazil, which for most of its path is considered narrow for satellite altimetry applied to hydrology. Data from Envisat cover the period 2002-2010 while the recently launched SARAL satellite provided data for 2013. The results show that the accuracy varies significantly depending on a number of environmental factors some of which are discussed in depth. In about one-half of the 16 satellite water gauging stations, the RMS errors are lower than 60 cm and in some cases better than 30 cm. These variations could not be directly related to the river width, but appear to be mostly related to the land cover and to the processing chain that often extracts altimetry points from an off-nadir location. All processing is fully described and the results are presented for both the Envisat/RA-2 and SARAL/Altika altimeters. (C) 2014 Elsevier Inc. All rights reserved.