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A key element in hydraulic and hydrologic modeling is the specification of representative channel geometry. For continental‐scale modeling, the large amount of high‐resolution data required, as well as the considerable computational effort needed to incorporate such data, has led to simplifying assumptions such as rectangular or trapezoidal channels for river reaches. The National Water Model (NWM) uses a trapezoidal channel representation for 2.7 million river reaches to forecast water discharge for the entire continental United States. This has created uncertainties in when to initiate hydraulic predictions. The aim of this study is to: (1) evaluate the impact of simplified (NWM trapezoidal) channel geometry representation on hydrological model predictions and (2) suggest an improved representation of channel geometry while maintaining parsimony in model input and runtime. The Hydrologic Engineering Center’s River Analysis System model was used to simulate hydraulic dynamics in three study sites under varying streamflow conditions (including flooding) with four different geometry representations at each site: NWM (trapezoidal), surveyed, and two proposed generalized geometries. Statistical analyses show that more realistic channel geometry improves simulated Muskingum‐Cunge routing parameters and stage/discharge predictions, indicating potential for geometric improvements to enhance hydrological models like the NWM.

The Floodwater Depth Estimation Tool (FwDET) calculates water depth from a remote sensing-based inundation extent layer and a Digital Elevation Model (DEM). FwDET’s low data requirement and high computational efficiency allow rapid and large-scale calculation of floodwater depth. Local biases in FwDET predictions, often manifested as sharp transitions or stripes in the water depth raster, can be attributed to spatial or resolution mismatches between the inundation map and the DEM. To alleviate these artifacts, we are introducing a boundary cell smoothing and slope filtering procedure in version 2.1 of FwDET (FwDET2.1). We present an optimization analysis that quantifies the effect of differing parameterization on the resulting water depth map. We then present an extensive intercomparison analysis in which 16 DEMs are used as input for FwDET Google Earth Engine (FwDET-GEE) implementation. We compare FwDET2.1 to FwDET2.0 using a simulated flood and a large remote sensing derived flood map (Irrawaddy River in Myanmar). The results show that FwDET2.1 results are sensitive to the smoothing and filtering values for medium and coarse resolution DEMs, but much less sensitive when using a finer resolution DEM (e.g., 10 m NED). A combination of ten smoothing iterations and a slope threshold of 0.5% was found to be optimal for most DEMs. The accuracy of FwDET2.1 improved when using finer resolution DEMs except for the MERIT DEM (90 m), which was found to be superior to all the 30 m global DEMs used.

The construction of dams and impoundments for hydropower generation, flood control, irrigation, and water supply is among the greatest stressors to the connectivity and functionality of world's rivers (