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:

Pperc=Psolution,surfwperc,surf10ρbdepthsurfkd,percP_{perc}=\frac{P_{solution,surf}*w_{perc,surf}}{10*\rho_b*depth_{surf}*k_{d,perc}} 3:2.4.1

where PpercP_{perc} is the amount of phosphorus moving from the top 10 mm into the first soil layer (kg P/ha), Psolution,surfP_{solution,surf} is the amount of phosphorus in solution in the top 10 mm (kg P/ha), wperc,surfw_{perc,surf} is the amount of water percolating to the first soil layer from the top 10 mm on a given day (mm H2_2O), ρb\rho_{b} is the bulk density of the top 10 mm (Mg/m3^3) (assumed to be equivalent to bulk density of first soil layer), depthsurfdepth_{surf} is the depth of the “surface” layer (10 mm), and kd,perck_{d,perc} is the phosphorus percolation coefficient (m3^3/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 NameDefinitionInput File

SOL_BD

ρb\rho_b: Bulk density of the layer (Mg/m3^3)

.sol

PPERCO

kd,perck_{d,perc}: Phosphorus percolation coefficient (m3^3/Mg)

.bsn

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