Command line/ Scripting
A sample usage of the Flow Accumulation tool with the {in_weight_raster} might be to determine how much rain has fallen within a given watershed. In such a case, the {in_weight_raster} may be a continuous raster representing average rainfall during a given storm. The output of Flow Accumulation would then represent the amount of rain that would flow through each cell, assuming that all rain became runoff and there was no interception, evapotranspiration, or loss to groundwater. This could also be viewed as the amount of rain that fell on the surface, upslope from each cell.
The results of Flow Accumulation can be used to create a stream network by applying a threshold value to select cells with a high accumulated flow. For example, the procedure to create a raster where the value one represents the stream network on a background of NoData could use one of the following:
- Perform a Con operation in which the input conditional raster is "Flowacc", the input true raster a constant "1", and the expression "> 100".
- Alternatively, perform a Set Null in which the input conditional raster is "Flowacc", the input true raster a constant "1", and the expression "< 100".
In both examples, all cells that have more than 100 cells flowing into them are assigned one; all other cells are assigned NoData. For future processing, it is important that the stream network, a set of raster linear features, be represented as values on a background of NoData.
The resulting stream network can be used as input to the Stream Order, Stream Line, and Stream Link tools.
This method of deriving accumulated flow from a DEM is presented in Jenson and Domingue (1988). An analytical method for determining an appropriate threshold value for stream network delineation is presented in Tarboton et al. (1991).
Map Algebra/ ArcObjects
A sample usage of FlowAccumulation with the {weight_grid} might determine how much rain has fallen within a given watershed. In such a case, the {weight_grid} may be a continuous grid representing average rainfall during a given storm. The output of FlowAccumulation would then represent the amount of rain that would flow through each cell, assuming that all rain became runoff and there was no interception, evapotranspiration, or loss to groundwater. This could also be viewed as the amount of rain that fell on the surface, upslope from each cell.
The results of FlowAccumulation can be used to create a stream network by applying a threshold value to select cells with a high accumulated flow. For example, the expression to create a grid where the value one represents the stream network on a background of NoData could be:
con (flowacc > 100, 1)
or
setnull (flowacc < 100, 1)
In both examples, all cells that have more than 100 cells flowing into them are assigned 1; all other cells are assigned NoData. For future processing, it is important that the stream network, a set of raster linear features, be represented as values on a background of NoData.
The resulting stream network can be used as input to the StreamOrder, StreamLine, and StreamLink functions.
This method of deriving accumulated flow from a DEM is presented in Jenson and Domingue (1988). An analytical method for determining an appropriate threshold value for stream network delineation is presented in Tarboton et al. (1991).
References
Jenson S. K. and J. O. Domingue. 1988. Extracting Topographic Structure from Digital Elevation Data for Geographic Information System Analysis.
Photogrammetric Engineering and Remote Sensing 54 (11): 1593–1600.
Tarboton, D. G., R. L. Bras, and I. Rodriguez–Iturbe. 1991. On the Extraction of Channel Networks from Digital Elevation Data.
Hydrological Processes. 5: 81–100.