The need for global, satellite-based observations of terrestrial surface waters



River discharge as well as lake and wetland storage of water are critical terms in the surface water balance, yet they are poorly observed globally and the prospects for improvement from in-situ networks are bleak [e.g.,Shiklomanov et al., 2002; IAHS, 2001;Stokstad, 1999]. Indeed, given our basic need for fresh water, perhaps the most important hydrologic observations that can be made in a basin are of the temporal and spatial variations in discharge. Gauges measuring discharge rely on flow converging from the upstream catchment to a singular in-channel cross section. This approach has successfully monitored many of the world's densely inhabited and typically heavily engineered basins for well over a century. However, much of the globally significant discharge occurs in sparsely gauged basins, many with vast wetlands that lack flow convergence (e.g., Figures 1 and 2); thus leading to poorly defined values of runoff at local, regional, and continental scales.