Establishment of Virtual Station based on Multi-mission Satellites for Near-daily River Discharge Observation

The implementation of action plans for sustainable water resources management requires daily river discharge time series at gauging stations, which are already decreasing in number worldwide. Although the development of remote sensing-based methods for river discharge estimation has proven its effectiveness worldwide, the temporal frequency especially at the daily scale for river discharge estimation is the most important research question to be explored. In this context, the study proposed a methodological framework to establish a virtual station (VS) where the information retrieved from the multi-mission satellites was merged using the non-parametric copula function for river discharge estimation. Here, in the first step, both passive (C/M) and active (altimeter) remote sensing signals can be integrated by deriving the joint probability distribution using the copula functions of the Archimedean family. Subsequently, the Frank copula was evaluated as the best-fit copula function as measured by the goodness-of-fit-test and subsequently selected for establishing the VS by merging the information. The proposed framework was tested on more than 10 rivers around the world. Here, MODIS from Aqua and Terra, Landsat series, and MSI from Sentinel-2 images were used for the C/M approach, whereas SARAL AltiKa, Sentinel-3 A and B, and Cryosat-2 mission altimeters were considered for water level retrieval. The established VSs along the river can be able to derive long near daily discharge time series while evaluating against the in situ discharge with reasonable accuracy measured by Nash-Sutcliffe efficiency, Root Mean Square Error, and Kling-Gupta efficiency. Conclusively, the establishment of this kind of VSs along the river can be able to derive missing discharge data records and long near-daily discharge time series along any world river which is one of the key variables for hydro climatological studies.   Keywords: Remote Sensing, Virtual Station, Copula, Satellite merging, River Discharge, Altimeters