Influence of the Amazon Floodwave on the Intra-Basin Variability of GRACE Water Storage Estimates

The water mass in the Amazon Basin consists largely of precipitation, which averages well over 2 m/yr and water stored on floodplains and in wetlands, which cover ~15% to 20% of the Basin. Our knowledge of this water mass is limited by a low spatial sampling and availability of stream flow and precipitation measurements. For example, published estimates of annual Amazon River discharge vary from ~150,000 to ~200,000 cubic-meters per second: a difference greater than twice the annual flow of the Mississippi. GRACE measurements are anticipated to help overcome this problem. While monthly GRACE gravity solutions are typically used to estimate the basin-wide average of the water storage variation, we analyze the spatial and temporal variability within the Amazon Basin by estimating the water storage directly from the raw satellite-tracking data. The spatial and temporal resolutions with this method are improved to 3x3 degrees and 15 days, respectively, allowing better quantification of the intra-basin variability. We analyzed equally spaced 3x3 degree regions along the mainstem Amazon River and along major tributaries flowing from the south and from the north. Our GRACE estimates of storage change are compared to precipitation from the Global Precipitation Climatology Project (GPCP), in-situ stream-gauge discharge, evaporation estimates, and remotely sensed measurements of flooded areas. We find that GRACE amplitudes compare well to GPCP based precipitation (P) minus model based evaporation (E), but precede the peak and trough P-E by a month. Changes in river channel discharge, measured as Qin minus Qout in each of the regions along the mainstem, account for this timing offset. The storage capacities of the floodplains and wetlands in those regions are sufficient to hold and release the water mass as estimated by GRACE.