Recent Advances in Modeling Earth Deformation from Monsoonal Flooding in Bangladesh using Hydrographic, GPS and GRACE Data

Abstract:

The Ganges, Brahmaputra and Meghna Rivers converge in Bangladesh with annual discharge second only to the Amazon. Most of the flow occurs during the summer monsoon causing widespread flooding. The impounded water represents a large surface load that is the second largest seasonal anomaly registered in the GRACE gravity field. Continuous GPS stations in Bangladesh record seasonal vertical motions up to 6 cm due to the monsoonal water load. We have used GRACE water mass estimates and surface water monitoring to calculate the seasonal load together with GPS observations of seasonal deformation due to this load in order to invert for lithospheric properties.

To estimate the water load in Bangladesh, we use >300 daily river gage measurements of water level and >1200 weekly groundwater level measurements from wells for the period 2003-2010. The total impounded water mass is partitioned between surface water and groundwater by using the SRTM DEM. The seasonal water loads calculated from the surface data and are in excellent agreement with GRACE estimates and are used to validate them. They show that seasonally ~100GT of water are stored in Bangladesh (7.5% of annual discharge), but can reach 150GT during extreme events. To calculate the water loads beyond Bangladesh, we project GRACE solutions using mascons to estimate the water mass in irregular blocks that represent the major areas of flooding and groundwater storage in the surrounding regions. These water loads cause elastic deformation with a large lateral extent. Therefore, deformation from these water loads are now calculated on a spherical earth in order to estimate deflections at 18 continuous GPS sites distributed throughout Bangladesh. The region of study and the GPS stations span different tectonic regions from the strong Indian craton to the weak, deep Bengal Basin. We vary Young’s modulus (E) and hence the strength of the lithosphere across these regions to estimate values of E corresponding to the different tectonic regions.