Modeling Earth deformation from monsoonal flooding in Bangladesh using hydrographic, GPS, and Gravity Recovery and Climate Experiment (GRACE) data

The Ganges, Brahmaputra, and Meghna rivers converge in Bangladesh with an annual discharge second to the Amazon. Most of the flow occurs during the summer monsoon causing widespread flooding. The impounded water represents a large surface load whose effects can be observed in Gravity Recovery and Climate Experiment (GRACE) and GPS data. Bangladesh is at the center of the second largest seasonal anomaly in the GRACE gravity field, reflecting water storage in Southeast Asia. Eighteen continuous GPS stations in Bangladesh record seasonal vertical motions up to 6 cm that inversely correlate to river level. We use 304 river gages to compute water height surfaces with a digital elevation model to separate surface water from groundwater. Porosity of 20% was used to estimate groundwater mass and calculate the water load. Results show similar to 100 GT of water are stored in Bangladesh (7.5% of annual discharge) but can reach 150 GT during extreme events. The calculated water mass agrees with monthly GRACE water mass equivalents from Bangladesh within statistical limits. We compute the deformation due to this water load on an elastic half-space, and we vary Young's modulus to fit GPS data from our two most continuous records. The water loading can account for >50% of the variance in the GPS data. The best fitting Young's modulus is 117-124 GPa for DHAK and 133-135 GPa for SUST, although the upper bound is not well constrained. These estimates lie between sediment (30-75 GPa) and mantle (190 GPa) values, indicating that response to loading is sensitive to structure throughout the lithosphere and is not absorbed by the weak sediments.