If the Penman-Monteith equation is selected as the potential evapotranspiration method, transpiration is also calculated with the equations summarized in Section 2:2.2.1. For the other potential evapotranspiration methods, transpiration is calculated as:

$E_t=\frac{E'_o*LAI}{3.0}$ when $0 \le LAI \le 3.0$ 2:2.3.5

$E_t=E'_o$ when $LAI>3.0$ 2:2.3.6

where $E_t$ is the maximum transpiration on a given day (mm H$_2$O), $E'_o$ is the potential evapotranspiration adjusted for evaporation of free water in the canopy (mm H$_2$O), and $LAI$ is the leaf area index. The value for transpiration calculated by equations 2:2.3.5 and 2:2.3.6 is the amount of transpiration that will occur on a given day when the plant is growing under ideal conditions. The actual amount of transpiration may be less than this due to lack of available water in the soil profile. Calculation of actual plant water uptake and transpiration is reviewed in Chapters 5:2 and 5:3.

Evapotranspiration is a collective term that includes all processes by which water at the earth’s surface is converted to water vapor. It includes evaporation from the plant canopy, transpiration, sublimation and evaporation from the soil.

Evapotranspiration is the primary mechanism by which water is removed from a watershed. Roughly 62% of the precipitation that falls on the continents is evapotranspired. Evapotranspiration exceeds runoff in most river basins and on all continents except Antarctica (Dingman, 1994).

The difference between precipitation and evapotranspiration is the water available for human use and management. An accurate estimation of evapotranspiration is critical in the assessment of water resources and the impact of climate and land use change on those resources.

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 is the maximum amount of water that can be trapped in the canopy on a given day (mm HO), is the maximum amount of water that can be trapped in the canopy when the canopy is fully developed (mm HO), is the leaf area index for a given day, and 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:

and

when 2:2.1.2

and

when 2:2.1.3

where is the initial amount of free water held in the canopy on a given day (mm HO), is the final amount of free water held in the canopy on a given day (mm HO), is the amount of precipitation on a given day before canopy interception is removed (mm HO), is the amount of precipitation on a given day that reaches the soil surface (mm HO), and is the maximum amount of water that can be trapped in the canopy on a given day (mm HO).

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