UCLA

Droughts and floods are among the most catastrophic yet least understood weather and climate threats. Accurate forecasting of droughts and floods is crucial due to their significant financial and human impacts. Concurrently, precise streamflow simulation is critical for effective water management and disaster prevention. The subseasonal drought forecast, vital for water management and disaster mitigation, has been under-studied due to a lack of appropriate meteorological forecast databases until recently. The NOAA's National Water Model (NWM), anchored by its hydrological core Noah Multi-parameterization (Noah-MP), needs a comprehensive comparison with SAC-SMA model-based River Forecast Center (RFC) forecasts to evaluate its flood forecasting efficiency in the Western United States. Accurate daily streamflow predictions are crucial, and a comprehensive, calibrated Land Surface Model (LSM) parameter set is important for reliable streamflow predictions.This dissertation explores evaluations and enhancements of hydrological simulations and forecasts in the Western U.S., with a focus on three key aspects: a) subseasonal forecast accuracy for drought onset and termination using NOAA’s Climate Testbed Subseasonal Experiment (SubX) reforecasts, b) the flood forecasting capabilities of the Noah-MP in comparison to current RFC forecasts, and c) the development of high-resolution calibrated parameters for two notable LSMs, the Variable Infiltration Capacity (VIC) model and Noah-MP. For the first aspect, I employ SubX to drive Noah-MP and produce drought forecasts of different severity and for lead weeks 1-4. I find significant drought termination and onset forecast skill within the initial two weeks and limited skill or no skill at week 4 regardless of drought severity. I find that skill is generally higher for drought termination than for onset for all drought events and that drought prediction skill generally decreases from north to south for all drought events. For the second aspect, I start with selection of appropriate physics options for Noah-MP and calibration of parameters in seven watersheds that form a transect along the U.S. Pacific Coast. I find promising flood prediction capacities of Noah-MP in northern basins but requires refinement for southern basins both in terms of bias and variability. For the third aspect, I develop calibrated and regionalized hydrologic parameters for VIC and Noah-MP at a precision of 1/16� latitude-longitude resolution across 4816 HUC-10 basins in the Western U.S., aiming to enhance the accuracy of hydrological modeling and predictions. In summary, the dissertation provides important contributions to understanding and improving the hydrological simulation and forecast in the Western U.S.