Dynamics of terrestrial water budget over Amazon and Mississippi basins using satellite data

The components of the water budget and their spatio-temporal variability are diagnosed using monthly-averaged remote sensing-based data products over the Amazon and Mississippi basins. These two large basins are divided into 14 and 12 smaller sub-basins (SB) respectively, and for each of these SBs, fresh water discharge is estimated from the water balance equation using satellite data products. The purpose of this study is to learn how to apply satellite data with global coverage over the large tropical and mid-latitude regions; therefore several combinations of remote sensing estimates including total water storage changes, precipitation and evapotranspiration. The results are compared to gauge-based measurements and the best spatio-temporal agreement between estimated and observed runoff is within 1 mm/d for the combination of precipitation from the GPCP and the Montana evapotranspiration product. Mean annual precipitation, evapotranspiration and runoff for the whole basin are estimated to be 6.1, 2.2 and 3.0 mm/d respectively but also show large spatial and temporal variations at SB scale. Using the most consistent data combination, the seasonal dynamics of the water budget within the Amazon system are examined. Agreement between satellite based and in-situ runoff is improved when lag-times between SBs are accounted (RMSE from 0.98 to 0.61 mm/d). We estimate these lag times based on satellite inferred inundation extents. The results reveal not only variations of the basin forcing but also the complex response of the inter-connected SB water budgets. Inter-annual and inter-sub basin variation of the water components are investigated and show large anomalies in north-western and eastern downstream SBs; aggregate behavior of the whole Amazon is more complex than can be represented by a simple integral of the forcing over the whole river system. Moreover, the same approach proposed for Amazon for estimating the runoff is applied to large Mississippi basin. The results show that applying the proposed method can improve the estimation of runoff using the satellite information. Some limitations exist in this basin that will decrease the reliability of the results, as the uncertainty of estimated runoff is greater that the magnitude of runoff due to existence of dams, and smaller precipitation rate compared to Amazon basin.