Satellite Hydrology and Hydrodynamics
Riverine deltas are landforms created where the river drains into an ocean, estuary or lake. The sediment deposition over long periods of time makes deltas hydrologically one of the most active terrestrial bodies. Some of the unique hydrologic features are: 1) intricate network of rivers resulting in high drainage density; 2) low spatial gradients of stream flow that gives rivers a tendency to overflow into floodplains during the wet season; 3) highly unsteady water regime in the delta created by fast flowing upstream boundary conditions and tidal changes in downstream estuary. The easy availability of fresh water and fertile soils has resulted in most of the world’s deltas hosting large population centers, complex irrigation systems and a water sensitive eco-system. This is especially true for Ganges-Brahmaputra-Meghna (GBM), Mississippi, Niger, Senegal, Okovango and Mekong deltas. Today, deltas provide livelihood to about half-billion people around the world. More than 200 million people live inside the humid deltas where many of the world’s mega cities (e.g., Dhaka, Bangkok) continue to withdraw water at an unsustainable rate.
Given how intimately water supports large population centers, agricultural productivity and the fragile eco-systems, an accurate understanding of the terrestrial hydrology is key to achieving sustainable water resources development in deltas. However, three specific issues make this understanding of hydrology very difficult: 1) because most riverine deltas are located at the downstream most end of international river basins, these deltas require basin-wide hydrologic measurements from upstream nations that are often unavailable; 2) the extremely low spatial gradients demand detailed two-dimensional knowledge of river structure for hydrodynamic modeling of the low-energy stream flow; and 3) increasing human impoundment of upstream rivers makes prediction inside the downstream deltas by stand-alone hydrologic models difficult.
The use of hydrologic remote sensing data from satellites has promise in overcoming these intractable issues. For example, for issue #1, given that most of the surface flow draining into humid deltas of the world is transboundary (i.e., arriving from other nations), the vantage of space in estimating rainfall and discharge has already been demonstrated by SASWE group as a feasible way to overcome the non-existing in-situ ground measurement network. For issue #2, the use of wide-swath interferometry (such as the Shuttle Radar Topography Mission, SRTM) and satellite radar altimetry (such as JASON-1/2, Envisat) hold promise for modeling the low spatial gradient rivers. The use of such data in hydrodynamic models can reveal a more detailed structure of the flow in the floodplains that hydrologic models alone cannot elucidate. For issue #3, it is currently estimated that half of the world’s rivers have at least one impoundment somewhere along their reaches. With a changing climate and increasing water scarcity, more river impoundments are likely to be built or maintained (rather than removed) in this century for securing a steady supply of fresh water. Examples of regulated flow in humid deltas are the Ganges & Meghna rivers (due to Farakka and Tipaimukh dams in India), Niger river, Nile river (due to the Aswan High Dam), Ebro river in Spain and Thailand’s Chao Phraya river. Thus, understanding the delta hydrology also requires knowledge of the human regulation of upstream flow. Given the progress made on satellite altimetry and interferometric Synthetic Aperture Radar (InSAR), it is reasonable now to expect that the information from satellite data on water levels and storage changes upstream of impounded rivers can estimate the discharge released from the spillways. Consequently, this can improve the predictability of hydrologic models inside the deltas that have regulated rivers upstream. The objectives of research in this area are:
1) To advance the hydrologic predictability (overland), especially of riverine deltas using a combination of satellite data and a hydrologic-hydraulic modeling scalable approach that is generic enough for implementation in the world’s humid deltas.
2) To investigate the optimum space-time scale of application and constellation configuration of satellite rainfall products (for GPM), altimetry and InSAR (for SWOT) to resolve the sub-daily hydrologic-hydraulic variability overland (and in humid deltas).