Distributed Precipitation

The VIC-nL model includes consideration for sub-grid scale variability in various properies, including precipitation. Precipitation is distributed throughout all or a portion of a grid cell, as a function of the intensity of the precipitation event.

The fractional coverage of a grid cell by a precipitation event is determined based on a relationship of the form:

where I represents the grid cell averaged precipitation intensity.

As the intensity of the precipitation event changes during a storm, this approach allows the adjustment of the fractional coverage accordingly. As storm intensity increases, the fractional coverage of the grid cell by the storm increases; with decreasing storm intensity the coverage decreases. Before a new storm event, the soil water content throughout the grid cell is set to the average value for the grid cell.

The "a" coefficient includes the effects of grid cell size, as well as geographic and other variations. Using values for I in millimeters per hour (mm/hr), the VIC model sets the value for "a" to 0.6. This value can be set to a specific value by the model user. Values for the coefficient "a" have been estimated by calibration in the southern U.S. plains for different grid cell sizes (Fan, et al. 1996). These results consider square grid cell sizes from 40 km to 500 km in size, with variation shown in the graph (click graph for more detail):

In the instance of precipitation occurring as snow, an exception is made to the above formulation. If there is snow on the ground in the grid cell, or if the precipitation for an event is snow, the fractional coverage for the grid cell is set to 1.0.

Reference:

Ying Fan, E. Wood, M.L. Baeck, and J. Smith. 1996. Fractional Coverage of Rainfall Over a Grid: Analyses of NEXRAD data over the southern Plains. Water Resources Research, vol. 32, no. 9, pp. 2787-2802, September 1996

See page with main references for further documentaiton.

return to the Distributed Precipitation Schematic