1. Satellite-Based Estimation of Water Discharge and Runoff in the Magdalena River, Northern Andes of Colombia

The Magdalena River and its most important tributary, the Cauca, drain the northern Andes of Colombia. During the wet season, flood events affect the whole region and cause huge damage in low-income communities. Mitigation of such natural disasters in Colombia lacks science-supported tools for evaluating river response to extreme climate events. Here we introduce near-real-time estimations of river discharge toward technical capacity building for evaluation of flood magnitudes and variability along the Magdalena and Cauca. We use the River Watch version 3 system of the Dartmouth Flood Observatory (DFO) at five selected measurement sites on the two rivers. For each site, two different rating curves were constructed to transform microwave signal from TRMM, AMSR-E, AMRS-2, and GPM satellites into river discharge. The first rating curves were based on numerical discharge estimates from a global Water Balance Model (WBM); the second were obtained from the relationship between satellite signal and measured river discharge at ground gauging stations at nearby locations. Determination coefficients (R ²) between observed versus satellite-derived daily discharge data range from 0.38 to 0.57 in the upper basin, whereas in the middle of the basin R ² values vary between 0.47 and 0.64. In the lower basin, observed R ² values are lower and range from 0.32 to 0.4. Once time lags between the microwave satellite signal and river discharge from either WBM estimates or ground-based gauging stations are taken into account, the R ² values increase considerably.

The time series of satellite-based river discharge during the 1998–2016 period show high interannual variability as well as strong pulses associated with the ENSO (La Niña/El Niño) cycle. Numerical runoff magnitude estimates at peaks of extreme climatic anomalies are more correlated than stream flows measured at ground-based gauging stations. In fluvial systems such as the Magdalena, characterized by high spatial variability in climate, morphology, and human-induced changes (e.g., deforestation and related erosion and sedimentation), satellite-based observation of water discharge is useful for flood hazard planning and mitigation.