General Considerations:Watershed Mask

The required WATERSHED_MASK project file card is used to input the name of the file containing the mask map. The mask map is used to define the watershed boundaries within the rectangular grid and reduces memory requirements by an amount directly proportional to the ratio of mask area over the bounding rectangle area. The watershed mask is a map containing only 0s and 1s, where 0 and 1 represent grid cells that are outside and within the watershed, respectively. For instance, a watershed that is approximately the shape of Texas would have the following a mask map file:

north: yyyyyyy.y1

south: yyyyyyy.y2

east: xxxxxx.x1

west: xxxxxx.x2

rows: 15

cols: 12

0 0 0 0 1 1 1 0 0 0 0 0

0 0 0 0 1 1 1 1 1 1 0 0

0 0 0 0 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1

0 1 1 1 1 1 1 1 1 1 1 1

0 0 0 1 1 1 1 1 1 1 1 0

0 0 0 0 1 0 1 1 1 1 0 0

0 0 0 0 0 0 1 1 1 0 0 0

0 0 0 0 0 0 0 1 0 0 0 0

If the watershed mask contains a 0 in a particular grid cell, GSSHA ignores data in all input maps for that particular grid cell. In the above map, there are a total of 15*12=180 grid cells. There are 85 grid cells with a 1 in the watershed mask. In this example, GSSHA would only allocate memory for 85 of the 180 grid cells for each map, representing a decrease of 53% of memory required to store the data for each map. Errors in watershed delineation (hence mask creation) will propagate through all data sets input to and output from GSSHA. The watershed should be delineated with care. WMS creates the watershed mask as part of the watershed delineation processes. GRASS users can use the r.watershed command to create the mask. During each simulation, the GSSHA model writes out the watershed mask with the internal number assigned to each grid cell in GRASS ASCII format in a file called “maskmap”. This file is useful for deciphering error messages from GSSHA that refer to the grid cell number where the problem occurred.