Highly water-soluble pesticides can be transported with percolation deep into the soil profile and potentially pollute shallow groundwater systems. The algorithms used by SWAT+ to calculated pesticide leaching simultaneously solve for loss of pesticide in surface runoff and lateral flow also. These algorithms are reviewed in Chapter 4:3.

Degradation is the conversion of a compound into less complex forms. A compound in the soil may degrade upon exposure to light (photo degradation), reaction with chemicals present in the soil (chemical degradation) or through use as a substrate for organisms (biodegradation).

The majority of pesticides in use today are organic compounds. Because organic compounds contain carbon, which is used by microbes in biological reactions to produce energy, organic pesticides may be susceptible to microbial degradation. In contrast, pesticides that are inorganic are not susceptible to microbial degradation. Examples of pesticides that will not degrade are lead arsenate, a metallic salt commonly applied in orchards before DDT was invented, and arsenic acid, a compound formerly used to defoliate cotton.

Pesticides vary in their susceptibility to degradation. Compounds with chain structures are easier to break apart than compounds containing aromatic rings or other complex structures. The susceptibility of a pesticide to degradation is quantified by the pesticide’s half-life.

The half-life for a pesticide defines the number of days required for a given pesticide concentration to be reduced by one-half. The soil half-life entered for a pesticide is a lumped parameter that includes the net effect of volatilization, photolysis, hydrolysis, biological degradation and chemical reactions in the soil. Because pesticide on foliage degrades more rapidly than pesticide in the soil, SWAT+ allows a different half-life to be defined for foliar degradation.

Pesticide degradation or removal in all soil layers is governed by first-order kinetics:

$pst_{s,ly,t}=pst_{s,ly,o}*exp\lfloor-k_{p,soil}*t\rfloor$ 3:3.2.1

where $pst_{s,ly,t}$ is the amount of pesticide in the soil layer at time $t$ (kg pst/ha), $pst_{s,ly,o}$ is the initial amount of pesticide in the soil layer (kg pst/ha), $k_{p,soil}$ is the rate constant for degradation or removal of the pesticide in soil (1/day), and $t$ is the time elapsed since the initial pesticide amount was determined (days). The rate constant is related to the soil half-life as follows:

$t_{1/2,s}=\frac{0.693}{k_{p,soil}}$ 3:3.2.2

where $t_{1/2,s}$ is the half-life of the pesticide in the soil (days).

The equation governing pesticide degradation on foliage is:

$pst_{f,t}=pst_{f,o}*exp\lfloor-k_{p,foliar}*t\rfloor$ 3:3.2.3

where $pst_{f,t}$ is the amount of pesticide on the foliage at time $t$ (kg pst/ha), $pst_{f,o}$ is the initial amount of pesticide on the foliage (kg pst/ha), $k_{p,foliar}$ is the rate constant for degradation or removal of the pesticide on foliage (1/day), and $t$ is the time elapsed since the initial pesticide amount was determined (days). The rate constant is related to the foliar half-life as follows:

$t_{1/2,f}=\frac{0.693}{k_{p,foliar}}$ 3:3.2.4

where $t_{1/2,f}$ is the half-life of the pesticide on foliage (days).

Table 3:3-2: SWAT+ input variables that pertain to pesticide degradation.

A portion of the pesticide on plant foliage may be washed off during rain events. The fraction washed off is a function of plant morphology, pesticide solubility, and the timing and intensity of the rainfall event. Wash-off will occur when the amount of precipitation on a given day exceeds 2.54 mm.

The amount of pesticide washing off plant foliage during a precipitation event on a given day is calculated:

3:3.1.1

where is the amount of pesticide on foliage that is washed off the plant and onto the soil surface on a given day (kg pst/ha), is the wash-off fraction for the pesticide, and is the amount of pesticide on the foliage (kg pst/ha). The wash-off fraction represents the portion of the pesticide on the foliage that is dislodgable.

Table 3:3-1: SWAT+ input variables that pertain to pesticide wash-off.

One of the primary purposes of tillage and harvesting practices in early farming systems was to remove as much plant residue from the field as possible so that pests had no food source to sustain them until the next growing season. As research linked erosion to lack of soil cover, farmers began to perform fewer tillage operations and altered harvesting methods to leave more residue. As mechanical methods of pest control were minimized or eliminated, chemical methods of pest control began to assume a key role in the management of unwanted organisms.

Pesticides are toxic by design, and there is a natural concern about the impact of their presence in the environment on human health and environmental quality. The fate and transport of a pesticide are governed by properties such as solubility in water, volatility and ease of degradation. The algorithms in SWAT+ used to model pesticide movement and fate are adapted from GLEAMS (Leonard et al., 1987).

Pesticide may be aerially applied to an HRU with some fraction intercepted by plant foliage and some fraction reaching the soil. Pesticide may also be incorporated into the soil through tillage. SWAT+ monitors pesticide amounts on foliage and in all soil layers. Figure 3:3-1 shows the potential pathways and processes simulated in SWAT+.