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The interception of rainfall by the vegetation is modeled in GSSHA using the two parameter method published by Gray (1970). An initial quantity of rainfall (mm), entirely intercepted by foliage, is specified with the STORAGE_CAPACITY project card. The fraction of rainfall retained as intercepted water after satisfying the storage capacity is specified with the INTERCEPTION_COEFF card. These two cards are used to specify GRASS ASCII maps of the required parameters. Alternatively, both storage capacity and the interception coefficient may be assigned use the Mapping Table and an index map based on vegetation.

In GSSHA, the interception rate (i) is expressed as:

i(t)=r(t) while I < a
i(t)=b*r(t) while I > a


  • r (t) denotes rainfall intensity at time t,
  • a is the storage capacity (mm),
  • b is the interception coefficient (unitless, between 0.0 and 1.0),
  • and I is the cumulative interception depth.

Storage capacity and interception values are usually inferred from vegetation or land cover. Values of storage capacity and interception coefficient values can be found in Gray (1970, section 4.6) or Bras (1990) p. 233.

To reflect the seasonal nature of interception in temperate regions, the storage capacity variable can be seasonally adjusted with the SEASONAL_RS card. This card is specified in the project file. If this card is specified, the storage capacity is varied based on the month. This option is only valid if LONG_TERM simulations are being conducted. If using this option, peak growing season values should be specified for the storage capacity. These values will be modified inversely with the canopy resistance values, as described in Section 9 Continuous Simulations. While this method has not been independently validated, it is based on the reasonable assumption that the storage capacity for interception is dependent on the available leaf area. Canopy resistance is the inverse of the leaf area index, thus storage capacity is inversely related to the canopy resistance. Factors were developed for two regions in the northern hemisphere, basically north and south, north is specified as latitudes above 37.0 degrees. The storage capacity is divided by the following factors for each month. The methods were employed in the calibration and validation of flow and sediment data in the Eau Galle Model (Downer, 2008) Eau Galle TN.

Month South Factor North Factor
1 4.0 4.0
2 4.0 4.0
3 4.0 4.0
4 2.5 4.0
5 1.0 3.0
6 1.0 2.0
7 1.0 1.0
8 1.0 1.0
9 1.0 1.0
10 2.5 2.5
11 4.0 4.0
12 4.0 4.0

Table - Seasonal storage capacity adjustment factors

GSSHA User's Manual

6 Precipitation
6.1     Spatially and Temporally Uniform Precipitation
6.2     Spatially and Temporally Varied Precipitation
6.3     Interpolation Between Gages
6.4     Interception