GIS, or geographic information systems, are computerized tools used to store, visualize, analyze, and interpret geographic data. Geographic data (also called spatial, or geospatial data) identifies the geographic location of features making these features "spatially-aware."

Spatial-awareness is not a requirement for solving problems in hydrology but since hydrology is inherently geographic, spatial-awareness can help solve some hydrologic problems more efficiently.

Spatial Reference Frameworks

All geospatial hydrologic algorithms fundamentally rely on the calculation of distance and area.  Therefore, geographic (unprojected) coordinate systems (e.g., Latitude/Longitude) cannot be used, as the distance represented by a degree changes depending on the location of the Earth's surface.  For distributed modeling, area-preserving projections such as the Albers Equal Area Conic are highly recommended to ensure that mass balance (precipitation volume = area x depth) is conserved evenly across all grid cells.

The Coordinate System can be used to set, view, or reproject the coordinate system for a Basin Model.  Available options to set the coordinate system include:

  1. Select a predefined coordinate system: Standard Hydrologic Grid (SHG) or Universal Transverse Mercator (UTM).
  2. Browse to GIS data with a coordinate system specification (e.g. shape file, raster data, .prj file, etc.).
  3. Enter the coordinate system specification in the test field using an OGC WKT format.

Terrain Conditioning and Sink Removal

Raw DEMs derived from LiDAR or radar often contain artificial depressions (sinks or pits) caused by data resolution limits, vegetation anomalies, or infrastructure (e.g., bridges acting as dams).  If left untreated, flow routing algorithms can trap water in these sinks, terminating the flow path prematurely.

While most cases can often skip the Terrain Reconditioning step entirely, specialized conditions may require targeted interventions to build walls and/or burn streams. 

Build Walls: Applied to raise elevation values along specified boundaries.  This is highly effective in extremely flat watersheds, or when forced alignment is required to match published datasets, such as the USGS Watershed Boundary Dataset (WBD) or subbasin boundaries from prior studies.

Burn Streams: Applied to impose a pre-defined stream network onto the DEM.  This step artificially incises the terrain to improve channel definition and ensure accurate flow routing.

Build Walls

Wall building is an optional first step in the Terrain Reconditioning wizard used to enforce watershed boundaries using an input shapefile.  The algorithm modifies the DEM using two primary methods:

  1. Smooth Raise: Linear interpolation of elevated cells within a user-defined Cell Buffer Width.  Cells that overlap the shapefile polylines will be raised to the target Smooth Raise Height and the surrounding cells within the buffer will tapering down to the native terrain elevation at the buffer limit, creating a gradual transition from the wall or boundary to the native terrain.
  2. Sharp Raise: A non-buffered operation that applies a uniform Sharp Raise Height exclusively to cells intersecting the shapefile polylines.
  3. Smooth Raise AND Sharp Raise: When both parameters are defined, the sharp height is additively applied to the peak of the smooth-raised boundary cells.

Notes

  • To force strict delineation alignment with the input shapefile, use a buffer of 1 or 2 cells and a height of 100 units.
  • Negative values in either parameter invert the operation, creating a drainage trench rather than a wall.
  • Large cell buffers may result in long runtimes.
  • Vertical units will automatically match the vertical units of the terrain data component assigned to the Basin Model.

Burn Streams

Stream burning is an optional second step in the Terrain Reconditioning wizard used to impose a stream network onto the terrain using an input shapefile.  This is especially useful when native terrain data poorly captures channel networks.  The algorithm modifies the DEM using two primary methods:

  1. Smooth Drop: Linear interpolation of lowered cells within a user-defined Cell Buffer Width.  Cells that overlap the shapefile polylines will be lowered to the target Smooth Drop Height and the surrounding cells within the buffer will taper upward to the native terrain elevation at the buffer limit, creating a gradual transition from the stream invert to the overbank. 
  2. Sharp Raise: A non-buffered operation that applies a uniform Sharp Drop Height exclusively to cells intersecting the stream shapefile polylines.
  3. Smooth Raise AND Sharp Raise: When both parameters are defined, the sharp drop height is additively applied to the deepest dells (the invert) of the smooth-dropped buffered channel.

Notes

  • If you built walls, it is highly recommended to burn streams.  The algorithm will use the streams to naturally breach the walls at intersections, allowing for proper hydrologic drainage.
  • Negative values in either parameter invert the operation, raising stream elevations instead of lowering them.
  • Large cell buffers may result in long runtimes.
  • Vertical units will automatically match the vertical units of the terrain data component assigned to the Basin Model.