Satellite Measurements Of River Discharge And Runoff
Brakenridge, G. R. 1
1 University of Colorado
As noted in “Stream-Gaging Program of the U.S. Geological Survey, USGS Circular 1123", “Perhaps the biggest challenge that confronts the stream-gauging program is that of maintaining long-term and consistent nationwide data sets.” The USGS operates a ground-based discharge monitoring network of more than 6000 gages nationwide. Meanwhile, surface-water-sensing microwave satellites provide complete global coverage on a daily basis, and without significant interference from cloud cover. Using a recently developed processing approach, these sensors can monitor river discharge changes. Among many other applications, stream flow data are important for drought monitoring, flood hazard mapping, and validation of hydrologic runoff models (Hirpa et al, 2013).
As rivers rise and discharge increases, the “spatial ratio” processing algorithm allows passive microwave sensing to track channel and floodplain surface water flow-area variation, as change occurs within selected 10x10km pixels centered over river measurement sites. This information sensitively monitors discharge variation (Brakenridge et al., 2007; 2012). The experimental technology is already being used operationally for flood alerts by the European Commission (GDACS, Global Disaster Alert and Coordination System) and by the GeoSUR geospatial data consortium for Latin America.
The high sensitivity of the sensors allows detectable flow-area changes within the pixels to be quite small. The relatively large size of the pixels limits the distance between points along a river where discharges can be monitored, but does not itself limit measurement precision (a function of sensor dynamic range, calibration, and other factors). AMSR-2, TRMM, WindSat/Coriolis, and GPM are providing current information; similar channels aboard TRMM, AMSR-E, and the Nimbus-7 SMMR and the DMSP SSM/I series provide consistent daily records back to the late 1970s.
The satellite-based remote sensing method has been tested for locations worldwide and has the potential to provide important complementary data for the existing gauging network (when gauges are lost to damage, or are discontinued), and opportunities to monitor discharges between gauges and where no other measurements are available. As an ancillary product, removal of river ice cover can also be monitored.
At the time of this writing, otherwise unmeasured major flooding was occurring along the Mamore River in Bolivia. The microwave data indicate Log Pearson III return periods in excess of 30 years at several microwave measurement sites. This agrees with numerous media reports indicating the exceptional severity of the flood.
Brakenridge, G. R. et al., 2012, Calibration of orbital microwave measurements of river discharge using a global hydrology model: Journal of Hydrology, v. 475, p. 123-136.
Brakenridge, G. R., et al., 2007, Orbital microwave measurement of river discharge and ice status. Water Resources Research, doi:10.1029/2006WR005238.
De Groeve, T., 2010, Flood monitoring and mapping using passive microwave remote sensing in Namibia. Geomatics, Natural Hazards and Risk v. 1, 19-35.
Hirpa, et al, 2013, Upstream satellite remote sensing for river discharge forecasting: Application to major rivers in South Asia. Remote Sensing of the Environment , v 131, p. 140–151.