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. Due to the low mobility of solution phosphorus, surface runoff will only partially interact with the solution P stored in the top 10 mm of soil. The amount of solution P transported in surface runoff is:
4:2.3.1
where is the amount of soluble phosphorus lost in surface runoff (kg P/ha), is the amount of phosphorus in solution in the top 10 mm (kg P/ha), is the amount of surface runoff on a given day (mm HO), is the bulk density of the top 10 mm (Mg/m) (assumed to be equivalent to bulk density of first soil layer), is the depth of the “surface” layer (10 mm), and is the phosphorus soil partitioning coefficient (m/Mg). The phosphorus soil partitioning coefficient is the ratio of the soluble phosphorus concentration in the surface 10 mm of soil to the concentration of soluble phosphorus in surface runoff.
Table 4:2-3: SWAT+ input variables that pertain to soluble P runoff.
Variable Name | Definition | Input File |
---|---|---|
SOL_BD
: Bulk density(Mg/m)
.sol
PHOSKD
: Phosphorus soil partitioning coefficient (m/Mg)
.bsn