Error Assessment Of National Water Model Analysis Assimilation And Short Range Forecasts

Flooding is the costliest natural disaster in the United States and tragically often leads to loss of life. Flood prediction, response and mitigation are therefore critical areas of research and have been for many decades. Hydrologic and hydraulic models are a key component of flood prediction methods and highly detailed models have been implemented in many areas of high risk which often correspond to areas with high population. However, the high cost and complexity of highly detailed models means that many areas of the US are not covered by flood prediction early warning systems. Recent increases in computational power and increased resolution and coverage of remotely sensed data have allowed for the development of a continental scale streamflow prediction system known as the National Water Model which is currently forecasting streamflow values for over 2.7 million stream reaches across the US. Flood inundation predictions can be derived from the National Water Model using digital elevation data to extract reach-scale rating curves and therefore river stage height. Using the height above nearest drainage method, flood inundation maps can be created from the stage height at relatively low computational cost at continental scale. The National Water Model is currently operating as a deterministic model for short-term predictions and does not currently include an estimate of the uncertainty in these predictions. The final streamflow values are at the end of a chain of models which originate from precipitation forecasts and go through rainfall-runoff and finally routing modules. The total uncertainty in the streamflow predictions is therefore a function of the uncertainty in each step. Uncertainty analysis commonly relies on an assessment of uncertainty in model parameters and boundary conditions, the use of perturbed inputs or through comparison of several different models of the same systems. Estimated uncertainty from the first model in a chain can then be propagated to the next model and so on until a final estimate is achieved. Unfortunately, the National Water Model is operated on a super computer and the details of the model are not available for perturbation analysis. One step in the National Water Model hourly cycle is the assimilation of USGS gage data which allows for corrections to the model state before the forecast simulation is made. This excludes USGS gage data from being used as a verification dataset. Even so, it is still an informative exercise to compare NWM predictions at these sites. There are numerous local and regional gaging stations which are not assimilated into the National Water Model and can be used as an independent check on the model output. Recent flooding in the Llano River basin in central Texas provides an opportunity to compare National Water Model predictions to both USGS and non-USGS gage readings. This thesis presents an assessment of the error in National Water Model predictions in the Llano River basin