Once the potential water uptake has been modified for soil water conditions, the actual amount of water uptake from the soil layer is calculated:

$w_{actualup,ly}=min\lfloor w''_{up,ly},(SW_{ly}-WP_{ly})\rfloor$ 5:2.2.7

where $w_{actualup,ly}$ is the actual water uptake for layer $ly$ (mm H$_2$O), $SW_{ly}$ is the amount of water in the soil layer on a given day (mm H$_2$O), and $WP_{ly}$ is the water content of layer $ly$ at wilting point (mm H$_2$O). The total water uptake for the day is calculated:

$w_{actualup}=\sum^n_{ly=1} w_{actualup,ly}$ 5:2.2.8

where $w_{actualup}$ is the total plant water uptake for the day (mm H$_2$O), $w_{actualup,ly}$ is the actual water uptake for layer $ly$ (mm H$_2$O), and n is the number of layers in the soil profile. The total plant water uptake for the day calculated with equation 5:2.2.8 is also the actual amount of transpiration that occurs on the day.

$E_{t,act}=w_{actualup}$ 5:2.2.9

where $E_{t,act}$ is the actual amount of transpiration on a given day (mm H$_2$O) and $w_{actualup}$ is the total plant water uptake for the day (mm H$_2$O).

Table 5:2-2: SWAT+ input variables that pertain to plant water uptake.

The potential water uptake from the soil surface to any depth in the root zone is estimated with the function:

$w_{up,z}=\frac{E_t}{[1-exp(-\beta_w)]}*[1-exp(-\beta_w*\frac{z}{z_{root}})]$ 5:2.2.1

where $w_{up,z}$ is the potential water uptake from the soil surface to a specified depth, $z$, on a given day (mm H$_2$O), $E_t$ is the maximum plant transpiration on a given day (mm H$_2$O), $\beta_w$ is the water-use distribution parameter, $z$ is the depth from the soil surface (mm), and $z_{root}$ is the depth of root development in the soil (mm). The potential water uptake from any soil layer can be calculated by solving equation 5:2.2.1 for the depth at the top and bottom of the soil layer and taking the difference.

$w_{up,ly}=w_{up,zl}-w_{up,zu}$ 5:2.2.2

where $w_{up,ly}$ is the potential water uptake for layer $ly$ (mm H$_2$O), $w_{up,zl}$ is the potential water uptake for the profile to the lower boundary of the soil layer (mm H$_2$O), and $w_{up,zu}$ is the potential water uptake for the profile to the upper boundary of the soil layer (mm H$_2$O).

Since root density is greatest near the soil surface and decreases with depth, the water uptake from the upper layers is assumed to be much greater than that in the lower layers. The water-use distribution parameter, $\beta_w$, is set to 10 in SWAT+. With this value, 50% of the water uptake will occur in the upper 6% of the root zone. Figure 5:2-3 graphically displays the uptake of water at different depths in the root zone.

The amount of water uptake that occurs on a given day is a function of the amount of water required by the plant for transpiration, $E_t$, and the amount of water available in the soil, $SW$. Equations 5:2.2.1 and 5:2.2.2 calculate potential water uptake solely as a function of water demand for transpiration and the depth distribution defined in equation 5:2.2.1. SWAT+ modifies the initial potential water uptake from a given soil layer to reflect soil water availability in the following ways.

If upper layers in the soil profile do not contain enough water to meet the potential water uptake calculated with equation 5:2.2.2, users may allow lower layers to compensate. The equation used to calculate the adjusted potential water uptake is:

$w'_{up,ly}=w_{up,ly}+w_{demand}*epco$ 5:2.2.3

where $w'_{up,ly}$is the adjusted potential water uptake for layer $ly$ (mm H$_2$O), $w_{up,ly}$ is the potential water uptake for layer $ly$ calculated with equation 5:2.2.2 (mm H$_2$O), $w_{demand}$ is the water uptake demand not met by overlying soil layers (mm H$_2$O), and $epco$ is the plant uptake compensation factor. The plant uptake compensation factor can range from 0.01 to 1.00 and is set by the user. As $epco$ approaches 1.0, the model allows more of the water uptake demand to be met by lower layers in the soil. As $epco$ approaches 0.0, the model allows less variation from the depth distribution described by equation 5:2.2.1 to take place.

As the water content of the soil decreases, the water in the soil is held more and more tightly by the soil particles and it becomes increasingly difficult for the plant to extract water from the soil. To reflect the decrease in the efficiency of the plant in extracting water from dryer soils, the potential water uptake is modified using the following equations:

$w''_{up,ly}=w'_{up,ly}*exp[5*(\frac{SW_{ly}}{(.25*AWC_{ly})}-1)]$ when $SW_{ly}< (.25*AWC_{ly})$ 5:2.2.4

$w''_{up,ly}=w'_{up,ly}$ when $SW_{ly} \ge (.25*AWC_{ly})$ 5:2.2.5

where $w''_{up,ly}$is the potential water uptake adjusted for initial soil water content(mm H$_2$O), $w'_{up,ly}$ is the adjusted potential water uptake for layer $ly$ (mm H$_2$O), $SW_{ly}$ is the amount of water in the soil layer on a given day (mm H$_2$O), and $AWC_{ly}$ is the available water capacity for layer $ly$ (mm H$_2$O). The available water capacity is calculated:

$AWC_{ly}=FC_{ly}-WP_{ly}$ 5:2.2.6

where $AWC_{ly}$ is the available water capacity for layer $ly$ (mm H$_2$O), $FC_{ly}$ is the water content of layer $ly$ at field capacity (mm H$_2$O), and $WP_{ly}$ is the water content of layer $ly$ at wilting point (mm H$_2$O).