Initialization of Soil Nitrogen Levels
Last updated
Last updated
Users may define the amount of nitrate and organic nitrogen contained in humic substances for all soil layers at the beginning of the simulation. If the user does not specify initial nitrogen concentrations, SWAT+ will initialize levels of nitrogen in the different pools.
Initial nitrate levels in the soil are varied by depth using the relationship:
3:1.1.1
where is the concentration of nitrate in the soil at depth (mg/kg or ppm), and is the depth from the soil surface (mm). The nitrate concentration with depth calculated from equation 3:1.1.1 is displayed in Figure 3:1-3. The nitrate concentration for a layer is calculated by solving equation 3:1.1.1 for the horizon’s lower boundary depth.
Organic nitrogen levels are assigned assuming that the C:N ratio for humic materials is 14:1. The concentration of humic organic nitrogen in a soil layer is calculated:
Nitrogen in the fresh organic pool is set to zero in all layers except the top 10 mm of soil. In the top 10 mm, the fresh organic nitrogen pool is set to 0.15% of the initial amount of residue on the soil surface.
While SWAT+ allows nutrient levels to be input as concentrations, it performs all calculations on a mass basis. To convert a concentration to a mass, the concentration is multiplied by the bulk density and depth of the layer and divided by 100:
Table 3:1-1: SWAT+ input variables that pertain to nitrogen pools.
3:1.1.2
where is the concentration of humic organic nitrogen in the layer (mg/kg or ppm), and is the amount of organic carbon in the layer (%). The humic organic is partitioned between the active and stable pools using the following equations:
3.1.1.3
3:1.1.4
where is the concentration of nitrogen in the active organic pool (mg/kg), is the concentration of humic organic nitrogen in the layer (mg/kg), is the fraction of humic nitrogen in the active pool, and is the concentration of nitrogen in the stable organic pool (mg/kg). The fraction of humic nitrogen in the active pool, , is set to 0.02.
3:1.1.5
where is the nitrogen in the fresh organic pool in the top 10 mm (kg N/ha), and is material in the residue pool for the top 10 mm of soil (kg/ha).
The ammonium pool for soil nitrogen, , is initialized to 0 ppm.
3:1.1.6
where is the concentration of nitrogen in a layer (mg/kg or ppm), is the bulk density of the layer (Mg/m), and is the depth of the layer (mm).
Variable Name | Definition | Input File |
---|---|---|
SOL_NO3
: Initial NO3 concentration in soil layer (mg/kg or ppm)
.chm
SOL_ORGN
: Initial humic organic nitrogen in soil layer (mg/kg or ppm)
.chm
RSDIN
: Material in the residue pool for the top 10mm of soil (kg ha-1)
.hru
SOL_BD
: Bulk density of the layer (Mg/m)
.sol
SOL_CBN
: Amount of organic carbon in the layer (%)
.sol