Mostafavi M, Roohi Sh, Emadi R and Torabi Azad M
The “blue marble” planet called Earth is 70% covered by water. Fresh water holds about 2.5% of the Earth’s surface water and only 0.26% of the fresh water can be found in rivers and lakes. Rivers are the most important freshwater resource for human so monitoring the quantity and quality of water in rivers requires a reliable system. Water level and discharge are two essential parameters in such a monitoring. Satellite altimetry measurements enable hydrologists to measure basin-wide of discharge and storage, which are much easier than monitoring changes from in situ gauge networks. This study was conducted in the Amazon (the largest river basin in the world) and Danube (second largest river basin in Europe) rivers. The altimetric data used for this study are produced by ESA (18 Hz Envisat) and by CNES (40 Hz SARAL). To obtain water level variation, 12 possible scenarios (Ocean, Ice-1, Ice-2 and Sea-Ice retrackers using ALL, MEDIAN and MEAN values) were processed. After removing outliers, water level from each scenario was validated against available in situ gauge, to find the most robust water level estimator. Then the discharge of the rivers in different segment has been estimated from the best scenario, i.e. a scenario which leads to minimum RMS for water level. To check the performance of the estimates, we used the root mean square (RMS) and Nash–Sutcliffe coefficient (NS) for water level and discharge. With one exception, the RMS of the water level is between 37 and 72 cm. A good agreement between altimetry and in situ data observed for station Jatuarana at the Amazon River. Mainly the water level from MEDIAN values follows the in situ gauge water level better than that the MEAN and ALL values. The errors of the derived water level time series yield about 55 cm on average for Amazon and 62 cm for Danube river basin with best results below 40 cm at Jatuarana station using SARAL data. Also best results of discharge were obtained for Budapest (SARAL data) and Baja (Envisat data) stations considering RMS and Jatuarana (SARAL data) considering NS.
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