Surveying and Monitoring River Systems Using Satellite Remote Sensing: A theoretical basis for the instrument requirements with special focus on the emerging wide-swath altimeters

Monitoring river dynamics via remote sensing instrument is a challenging task since the primary characteristics of interest (the river discharge) cannot be measured directly and has to be inferred from observable dynamic states such as water level or extent. While traditional ground measurements of discharge combines the continuous monitoring of the tracking variable (typically stage height) with regular detailed ground surveys to establish the relationship between the tracking variable and the desired discharge estimate, remote sensing techniques need to substitute both. Further difficulty is to specify the instrument requirements and assess the potential monitoring accuracy as a function of instrument characteristics. Based on observed rating curves (relating stage height to discharge) from ground measurements, we demonstrate the impact of sensor uncertainties on discharge estimates that can be used to define the sensor requirements for river monitoring missions. We also provide statistics derived from high frequency discharge records that can be used to establish the observation frequency requirements. We also demonstrate the development idealized cross-section geometry that can provide the basis for establishing the rating relationships to substitute ground surveys for calibrating remote sensing observations. We use the established riverbed geometry on high resolution (6 minute) gridded network to provide statistics of the global distribution of river system at various sizes and offer guidance about the minimum water surface requirement that the instruments have to able to monitor to provide meaningful discharge information. Overlaying the gridded network with different fictitious orbital configuration (with varying return frequency at 1, 4, 10 and 20 days) we demonstrate the trade-off between spatial coverage and temporal frequency.