Extending Height Above Nearest Drainage to Model Multiple Fluvial Sources in Flood Inundation Mapping Applications for the U.S. National Water Model.

Height Above Nearest Drainage (HAND), a drainage normalizing terrain index, is a means able of producing flood inundation maps (FIMs) from the National Water Model (NWM) at large scales and high resolutions using reach‐averaged synthetic rating curves. We highlight here that HAND is limited to producing inundation only when sourced from its nearest flowpath, thus lacks the ability to source inundation from multiple fluvial sources. A version of HAND, known as Generalized Mainstems (GMS), is proposed that discretizes a target stream network into segments of unit Horton‐Strahler stream order known as level paths (LPs). The FIMs associated with each independent LP are then mosaiced together, effectively turning the stream network into discrete groups of homogeneous unit stream order by removing the influence of neighboring tributaries. Improvement in mapping skill is observed by significantly reducing false negatives at river junctions when the inundation extents are compared to FIMs from that of benchmarks. A more marginal reduction in the false alarm rate is also observed due to a shift introduced in the stage‐discharge relationship by increasing the size of the catchments. We observe that the improvement of this method applied at 4%–5% of the entire stream network to 100% of the network is about the same magnitude improvement as going from no drainage order reduction to 4%–5% of the network. This novel contribution is framed in a new open‐source implementation that utilizes the latest combination of hydro‐conditioning techniques to enforce drainage and counter limitations in the input data. Plain Language Summary: Flooding is one of the most impactful natural disasters on life and property. The United States National Water Model (NWM) provides flood forecasts to adequately warn people for safe evacuations and protective measures across the entire country. To convert streamflow from the NWM to flood inundation maps (FIM), a model, Height Above Nearest Drainage (HAND), is used that translates elevation data from height above mean sea‐level to height above the nearest river. This model suffers from issues in mapping performance because inundation sourced from rivers is only considered from the nearest river. We developed a technique that mitigates these errors by removing consideration for neighboring tributaries in the relative elevation computation process. This is done by splitting the stream network into continuous river segments known as level paths (LPs) which removes the effects of tributaries. HAND is computed independently for each LP and the resulting FIMs are mosaiced together to form one seamless map. By computing HAND and catchments on an LP scale, catchments and FIMs are allowed to overlap which can account for multiple river sources of inundation especially at river confluences. We compared these HAND derived FIMs to maps from physically‐based models and found improvement in mapping performance. Key Points: Flood maps derived from Height Above Nearest Drainage (HAND) are subject to nearest flowpath limitations that affect inundation skillA means of resolving this limitation is provided by reducing HAND processing units to level paths with effective unit stream orderDiscretizing the stream network for HAND computation affects the stage‐discharge relationship and leads to higher skill inundation [ABSTRACT FROM AUTHOR]