Leaching

The primary mechanism of phosphorus movement in the soil is by diffusion. Diffusion is the migration of ions over small distances (1-2 mm) in the soil solution in response to a concentration gradient. The concentration gradient is created when plant roots remove soluble phosphorus from soil solution, depleting solution P in the root zone.

Due to the low mobility of phosphorus, SWAT+ allows soluble P to leach only from the top 10 mm of soil into the first soil layer. The amount of solution P moving from the top 10 mm into the first soil layer is:

$P_{perc}=\frac{P_{solution,surf}*w_{perc,surf}}{10*\rho_b*depth_{surf}*k_{d,perc}}$ 3:2.4.1

where $P_{perc}$ is the amount of phosphorus moving from the top 10 mm into the first soil layer (kg P/ha), $P_{solution,surf}$ is the amount of phosphorus in solution in the top 10 mm (kg P/ha), $w_{perc,surf}$ is the amount of water percolating to the first soil layer from the top 10 mm on a given day (mm H$_2$O), $\rho_{b}$ is the bulk density of the top 10 mm (Mg/m$^3$) (assumed to be equivalent to bulk density of first soil layer), $depth_{surf}$ is the depth of the “surface” layer (10 mm), and $k_{d,perc}$ is the phosphorus percolation coefficient (m$^3$/Mg). The phosphorus percolation coefficient is the ratio of the phosphorus concentration in the surface 10 mm of soil to the concentration of phosphorus in percolate.

Table 3:2-4: SWAT+ input variables that pertain to phosphorus leaching.

Variable Name	Definition	Input File
SOL_BD	$\rho_b$: Bulk density of the layer (Mg/m$^3$)	.sol
PPERCO	$k_{d,perc}$: Phosphorus percolation coefficient (m$^3$/Mg)	.bsn