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#### 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: