2:2.1 Canopy Storage

The plant canopy can significantly affect infiltration, surface runoff and evapotranspiration. As rain falls, canopy interception reduces the erosive energy of droplets and traps a portion of the rainfall within the canopy. The influence the canopy exerts on these processes is a function of the density of plant cover and the morphology of the plant species.

When calculating surface runoff, the SCS curve number method lumps canopy interception in the term for initial abstractions. This variable also includes surface storage and infiltration prior to runoff and is estimated as 20% of the retention parameter value for a given day (see Chapter 2:1). When the Green and Ampt infiltration equation is used to calculate surface runoff and infiltration, the interception of rainfall by the canopy must be calculated separately.

SWAT+ allows the maximum amount of water that can be held in canopy storage to vary from day to day as a function of the leaf area index:

$can_{day}=can_{mx}*\frac{LAI}{LAI_{mx}}$ 2:2.1.1

where $can_{day}$ is the maximum amount of water that can be trapped in the canopy on a given day (mm H$_2$O), $can_{mx}$ is the maximum amount of water that can be trapped in the canopy when the canopy is fully developed (mm H$_2$O), $LAI$ is the leaf area index for a given day, and $LAI_{mx}$ is the maximum leaf area index for the plant.

When precipitation falls on any given day, the canopy storage is filled before any water is allowed to reach the ground:

$R_{INT(f)}=R_{INT(i)}+R'_{day}$ and $R_{day}=0$

when $R'_{day} \le can_{day} - R_{INT(i)}$ 2:2.1.2

$R_{INT(f)}=can_{day}$ and $R_{day}=R'_{day}-(can_{day}-R_{INT(i)})$

when $R'_{day}>can_{day}-R_{INT(i)}$ 2:2.1.3

where $R_{INT(i)}$ is the initial amount of free water held in the canopy on a given day (mm H$_2$O), $R_{INT(f)}$ is the final amount of free water held in the canopy on a given day (mm H$_2$O), is the amount of precipitation on a given day before canopy interception is removed (mm H$_2$O), $R_{day}$ is the amount of precipitation on a given day that reaches the soil surface (mm H2O), and $can_{day}$ is the maximum amount of water that can be trapped in the canopy on a given day (mm H$_2$O).

Table 2:2-1: SWAT+ input variables used in canopy storage calculations.