Ganga‐Brahmaputra river discharge from Jason‐2 radar altimetry: An update to the long‐term satellite‐derived estimates of continental freshwater forcing flux into the Bay of Bengal

Abstract

[1] This paper discusses the use of Jason-2 radar altimeter measurements to estimate the Ganga-Brahmaputra surface freshwater flux into the Bay of Bengal for the period mid-2008 to December 2011. A previous estimate was generated for 1993-2008 using TOPEX-Poseidon, ERS-2 and ENVISAT, and is now extended using Jason-2. To take full advantages of the new availability of in situ rating curves, the processing scheme is adapted and the adjustments of the methodology are discussed here. First, using a large sample of in situ river height measurements, we estimate the standard error of Jason-2-derived water levels over the Ganga and the Brahmaputra to be respectively of 0.28 m and 0.19 m, or less than $4% of the annual peak-to-peak variations of these two rivers. Using the in situ rating curves between water levels and river discharges, we show that Jason-2 accurately infers Ganga and Brahmaputra instantaneous discharges for 2008-2011 with mean errors ranging from $2180 m 3 /s (6.5%) over the Brahmaputra to $1458 m 3

A review of applications of satellite SAR, optical, altimetry and DEM data for surface water modelling, mapping and parameter estimation

“…When compared with gauge data, RMSEs of altimetry data measured over the Amazon have ranges from 30 to 70 cm using data from EN-VISAT, ERS2, and GeoSaT (Tarpanelli et al, 2013); however, at cross track situations where altimetry measurements are taken at the same location with a gauging station, the difference can be < 20 cm (Seyler et al, 2009). The accuracy of altimeter measurements over rivers is also affected by the river width and the morphology of the river banks so that data on narrow rivers and vegetated banks have lower accuracy (Papa et al, 2012). Furthermore, the specifications of the altimetry system itself can affect the quality of measurements; for example, ENVISAT data have been shown to have lower RMSE compared to ERS2 data due to ENVISAT's ability to switch frequency modes in response to changes in terrain and its smaller bin width (Tarpanelli et al, 2013).…”

“…Due to this detailed resolution, optical satellite imagery is used for inundation mapping, drainage mapping, disaster monitoring, land-use/land-cover change analysis, etc. (Owe et al, 2001).…”

“…A review by Smith (1997) shows that the earliest hydrological applications were in water body and flood mapping; the review includes many examples of inundation maps developed from satellite imagery. Owe et al (2001) also compiled papers presented at a conference on applications of remote sensing in many aspects of hydrology. Beyond mapping, satellite data in the form of imagery, a digital eleva- tion model (DEM), altimetry data, etc., can be used as hydraulic model input forcing factors or to constrain model data during calibration/validation/verification (Pereira-Cardenal et al, 2011).…”

“…Radar can penetrate cloud cover and can be acquired at any time, independent of availability of sunlight. The penetration characteristic of the SAR satellites enables measurement of soil moisture in bare areas, making it useful for land-use and land-cover studies as well as Earth observation and monitoring (Owe et al, 2001). SAR is a side-looking instrument that sends out signals inclined at an angle.…”

“…The calibration results were affected by rainfall data spatial distribution. Although use of satellite altimetry for river stage monitoring is usually applied to large rivers with a few kilometres' width (Papa et al, 2012), altimetry data were used to estimate discharge in an ungauged part of the Po River basin (width: 200-300 m) using cross-sectional data (Tarpanelli et al, 2013). They used a simplified routing model (RCM) based on upstream data, wave travel time and hydraulic conditions on two river sections to get the flow in the second river section.…”