For a typical backwards analysis, particles are released at sinks, such as well, river or drain cells. On this screen, you can choose in which of those cells particles should be released and the distribution for each cell.
For Well cells, WEAP will try to guess which layers are pumped by examining the expression for Pump Layer for each demand site and catchment land use. However, in cases where the Pump Layer expression changes over time, WEAP will just select all layers. Therefore, you should review the layers selected and decide which to include. Even well or river cells from the original MODFLOW files that are not linked to WEAP groundwater nodes or river reaches will be listed and can be included for particle generation.
Click the Add button to create subregions for the selected cells. If multiple cells are selected, WEAP will try to group adjacent cells into rectangular subregions, so that the total number of subregions created is minimized (i.e., not one subregion for every cell).
You can release one or more particles per cell. Locations of particles for each cell can be generated either as a 3-dimension array of particles inside the cell ("within cell"), or as a 2-dimension array around one or more of the six faces of the cell ("on faces").
If you choose "within cell," specify the number of particles within each cell along the layer, row and column dimensions. The number of particles within the cell is the product of these three numbers.
If you choose "on faces," select which faces on which you would like to place particles. For each selected face (Face 1 = left face, Face 2 = right face, Face 3 = front face, Face 4 = back face, Face 5 = bottom face, and Face 6 = top face), specify the other two dimensions of particles. For example, for Face 1, a 2x3 array (2 layers, 3 rows) would place 6 particles on the left face of each cell in the subregion.
When backtracking from a cell to define a capture area, it is generally best to place particles on the cell faces rather than distribute them internally within the cell. For example, 16 particles on each of the faces 1-4.
The same distribution will apply to all cells added. If you want to use different distributions for different cells, first add the cells with the first distribution, then come back and add the cells with the other distribution.
The total number of particles released in each subregion is Number of Cells * Number of Particles per Cell, and is shown at the bottom of the window. Note: the more particles you have defined, the longer it will take for MODPATH to run and for WEAP to display results.