Algorithm for Flow Direction Enforcement Using Subgrid-Scale Stream Location Data

Producing realistic surface water flow patterns can be difficult for hydrologic models when there is insufficient grid resolution as a result of computational constraints or when available digital elevation model (DEM) data are relatively coarse. This technical note describes an algorithm that allows for more realistic overland flow by incorporating subgrid-scale stream location data without increasing grid resolution. The algorithm takes stream location data from the National Hydrography Dataset (NHD), maps it to the hydrologic model horizontal grid coordinates, and produces a list of ordered points along the stream. Because the algorithm uses indexes to efficiently order points along the stream, large-scale meanders require special treatment, whereas small (grid) scale meanders are explicitly included in the algorithm logic. Slopes are ensured to be continuous along the stream’s path as defined on the model grid, distinguishing this approach from traditional “stream burning” algorithms. Stream coordinates on the model grid are calculated along with corresponding elevation and slope values so that the stream can then be integrated into the DEM if desired. The algorithm’s flow routing capabilities are demonstrated by using an integrated surface water-groundwater model, ParFlow, under rain and recession conditions. This case study is performed by using the real topography of Owens Valley, California, and NHD flowline data for the Owens River. An approach using a DEM that has undergone standard processing to fill sinks and a typical “stream burning” approach [similar to those often used in geographic information system (GIS) applications] fail to route flow out of the flood plain in ParFlow’s overland flow model that partitions and routes topography-driven flow along adjacent cells in one of four cardinal directions. In contrast, this approach, with the river elevations and continuous slopes integrated into the DEM, routes water to the river and out of the catchment, creating more realistic surface flow patterns in the region. Although the method is applied here to address problems associated with a flat flood plain, it may also be applied to any area in which stream flow is discontinuous because of insufficient resolution of topography on a model grid.