INTERCOMPARISON AND VALIDATION OF CONTINENTAL WATER LEVEL PRODUCTS DERIVED FROM SATELLITE RADAR ALTIMETRY, MODELING AND FORECASTING TROPICAL LAKE LEVELS

Abstract

This dissertation focuses on validating the use of satellite radar altimetry products to observe and forecast water level in lakes and reservoirs.

Satellite measurements of lake and reservoir water levels complement <italic>in situ</italic> observations by providing stage information for ungauged basins and by filling data gaps in gauge records. Yet different satellite radar altimeter-derived continental water level products may differ significantly due to choice of satellites, geophysical corrections, etc. To explore the impacts of these differences, in the first part of this dissertation a direct comparison between three different altimeter-based lake level estimates is presented and validated with lake level gauge time series for lakes of a variety of sizes and conditions (e.g. whether they freeze seasonally). This comparison provides quantitative estimates of the error in lake levels as well as advice on product choices to end users. The largest discrepancies among the altimeter products occur for the lakes that freeze.

In the second part of this dissertation a simple water balance model is developed relating net freshwater flux on a catchment basin to lake level. The model is constructed with two empirical parameters: effective catchment to lake area ratio and time delay between freshwater flux and lake level response. This model allows comparison of observed net freshwater flux with the lake level estimates from altimetry for a series of 12 tropical lakes distributed across three continents. The results show encouraging agreement between these independent datasets.

The third part of this dissertation uses the simple lake model, developed in the second part of this dissertation, and applies it to NOAA's Climate Forecast System (CFS) coupled model thus allowing us to produce seasonal lake level forecasts based on seasonal predictions of net freshwater flux. In the CFS net freshwater flux data bias with respect to the independent reanalysis is determined. One example of such a lake level model forecast is presented, showing promising significant results over most examined tropical lakes, but failing for reservoirs and smaller lakes. Model forecast bias with respect to altimeter observations is proposed to be further investigated for multiple lead times.