Principal Investigator (PI): Dennis Lettenmaier, University of Washington

Co-Investigators (Co-PI): Charon Birkett, University of Maryland; Eric Wood, Princeton University; and C.K. Shum and Mike Durand, Ohio State University

The scarcity of global-scale water use data limits the ability of the land surface schemes used in earth system models to represent the effects of water management impacts on the land surface water and energy balances. As a result, many such models ignore human impacts on the water cycle altogether. In addition, river flows are poorly monitored in many regions of the world. Furthermore, the amount of water stored in liquid phase at the land surface and its space-time variability, which have implications for sea level rise, are poorly known. Satellite altimeters provide one source of data that has the potential to these effects.

Although the current generation of nadir-pointing track altimeters was intended primarily for ocean problems, impressive progress has been made in estimation of the relative variations in height of large lakes and reservoirs globally, and the heights of some rivers. The constraints imposed by track altimeters on the availability of altimeter-derived surface water data will be resolved with the launch of NASA's Surface Water and Ocean Topography (SWOT) mission, expected in 2019. While current altimeters are constrained for the reasons noted above, there nonetheless will be great value in producing quality controlled records of those elements of the surface water cycle that SWOT will measure, and for which records can be established back to the beginning of the satellite era, preparatory to the SWOT mission.

We propose here to extend the existing inland water altimetry-derived datasets (mostly of surface heights) in several ways. First, we will utilize all available altimetry data (especially including Jason-1 and 2 during the post-TOPEX era). Second, we will extend recent work by the PIs group to estimate storage change in large natural lakes and river channels as well as reservoirs using altimetry-based heights and surface extent from visible sensors (primarily Moderate Resolution Imaging Spectroradiometer (MODIS). Third, we will develop altimetry-based records of surface slope, width, and elevation for a set of large global rivers, based on multiple track altimeter crossings and from MODIS-based (and possibly other visible band) surface width estimates. We will also extend the coverage of river information by retracking altimetry measurements to maximize accuracy of river heights for rivers of moderate width. In addition, we intend to produce model-based estimates of surface water storage (in lakes, reservoirs, and river channels) and fluxes (runoff and streamflow) from the mid-20th century to present.

Our work will draw from three current inland water altimetry data sets (the US Department of Agriculture's Global Reservoirs and Lakes Monitor (G-REALM), Centre national d'études spatiales (CNES), and European Space Agency (ESA)). For this, reason, we have direct links to all three on the project team. This proposal is directly relevant to the call's request for projects that provide precursor products (to planned missions, specifically SWOT) that begin to baseline geophysical parameters that will be produced from these satellites, and develop long-term, consistent, and calibrated data and products that are valid across multiple missions and satellite sensors.

The project supports the Group on Earth Observations (GEO) Integrated Global Water Cycle Observations theme objectives 1 and 2, and the needs of the GEO Global Climate Observing System and the Global Terrestrial Network which list lake level and river discharge as Essential Climate Variables. The project team consists of PI Dennis Lettenmaier and Co-I Bart Nijssen (University of Washington), Co-PIs Charon Birkett (University of Maryland), Eric Wood (Princeton University), and C.K. Shum and Mike Durand (Ohio State University), and unfunded collaborators Sylvain Biancamaria (CNES/Laboratoire d’Etudes en Géophysique et Océanographie Spatiales (LEGOS)) and Jerome Benveniste (ESA), all of whom have extensive previous experience with the processing and development and/or application of satellite altimetry products to inland waters.