#### Module: intcp

##### Description

Interception of precipitation is computed as a function of the cover density (*covden_sum* and *covden_win*) and the storage available (*snow_intcp*, *srain_intcp*, and *wrain_intcp*) for the predominant vegetation on an HRU. HRU precipitation is obtained from a precipitation distribution module in the form of total precipitation (*hru_ppt*) and the amounts of *hru_ppt* that are in the form of rain (*hru_rain*) and snow (*hru_snow*). Net rain (*net_rain*) during the summer period is computed by:

\(net\_rain=[hru\_rain*(1.0−covden\_sum)]+(thrufall*covden\_sum)\)

where

- \(covden\) = the summer period cover density, and

thrufall is computed by:

\(thrufall=hru\_rain−(srain\_intcp−intcp\_stor)\)

where

- \(srain_intcp\) = rain interception storage capacity for the major vegetation type during the summer period (in.), and
- \(intcp_stor\) = current depth of interception storage (in.)

*net_rain* for the winter period is computed as above but with the winter cover density (*covden_win*) substituted for *covden_sum* and the winter interception storage capacity for rain (*wrain_intcp*) substituted for *srain_intcp*. *net_snow* is also computed in the same manner but with the substitution of *hru_snow* for *hru_rain*, winter cover density (*covden_win*) for *covden_sum*, and the interception storage capacity for snow (*snow_intcp*) for *srain_intcp*.

The existence of intercepted precipitation is denoted by setting *intcp_on* to a value of 1. A value of 0 indicates no intercepted precipitation. The form of the intercepted precipitation is denoted by *intcp_form* which is set to 0 for rain and 1 for snow. If precipitation is a mixture of rain and snow, rain is assumed to occur first and interception is computed for each precipitation form. *net_ppt* is the sum of *net_rain* plus *net_snow*. When snow falls on intercepted rain, *intcp_form* is changed to 1 and *net_snow* is computed as above. *snow_intcp* is assumed to always be greater than or equal to *wrain_intcp*.

The potential evaporation rate for intercepted precipitation is computed as a function of interception form. Intercepted rain is assumed to evaporate at a free-water surface rate. If pan-evaporation data are used, then the rain evaporation rate (*evrn*) equals the pan loss rate. If potential evapotranspiration (*potet*) is computed from meteorological variables, *evrn* is computed by:

\(evrn = \frac {potet} {epan_coef}\)

where

*epan_coef*= monthly evaporation-pan coefficient

Sublimation of intercepted snow (*evsn*) is assumed to occur at a rate proportional to *potet* and is computed by: