Spatial Modeling of Nitrogen Leaching
39
The major components of EPIC (Fig. 5) include weather simulation, hydrology,
soil temperature, erosion-sedimentation, nutrient cycling, tillage, crop
management and growth, pesticide and nutrient movement with water and
sediments, and field-scale costs and returns.
The EPIC water quality model can be used to simulate water and nitrogen
transport in unsaturated soils. In order to operate the program for estimating
nitrogen leaching and percolation, various input parameters like climate, soil data,
plant parameters, crop rotation, and tillage are necessary.
The model for the given project area was verified by field measurements
(Cepuder et al. 1998).
The following sections briefly describe some basic components of the EPIC
model which are used for the purpose of estimating nitrogen leaching (Sharpley
and Williams 1990):
2.2.1 Percolation
The EPIC percolation component uses a storage routing technique to simulate
flow through soil layers. Flow from a soil layer occurs when soil water content
exceeds field capacity (FC [mm]). Water drains from the layer until the storage
returns to field capacity. The reduction in soil water is simulated with the routing
equation
(1)
where SW and SWo are the soil water contents at the end and the start of time
interval t (24 h) and IT is travel time through layer (h).
Thus, daily percolation can be computed by taking the difference between SW
andSWo·
(2)
where a is the percolation rate for layer (mm d-I).
Travel time through a layer is computed with the linear storage equation
IT; = (Pat -FCJ / SCt ,
(3)
where PO is the porosity (mm), FC is field capacity (mm), and SC is saturated
conductivity (mm h-I).
The routing process is applied from the soil surface layer by layer through the
deepest layer. Since the saturated conductivity of some layers may be much lower
than that of others, the routing scheme can lead to an impossible situation
(porosity of low saturated conductivity layers may be exceeded). For this reason, a
back pass is executed from the bottom layer to the surface. If the porosity of a
layer is exceeded, the excess water is transferred to the layer above. This process
continues through the top layer.
Percolation is also affected by freezing temperature. Water can flow into a
frozen layer but is not allowed to percolate from the layer.
39
The major components of EPIC (Fig. 5) include weather simulation, hydrology,
soil temperature, erosion-sedimentation, nutrient cycling, tillage, crop
management and growth, pesticide and nutrient movement with water and
sediments, and field-scale costs and returns.
The EPIC water quality model can be used to simulate water and nitrogen
transport in unsaturated soils. In order to operate the program for estimating
nitrogen leaching and percolation, various input parameters like climate, soil data,
plant parameters, crop rotation, and tillage are necessary.
The model for the given project area was verified by field measurements
(Cepuder et al. 1998).
The following sections briefly describe some basic components of the EPIC
model which are used for the purpose of estimating nitrogen leaching (Sharpley
and Williams 1990):
2.2.1 Percolation
The EPIC percolation component uses a storage routing technique to simulate
flow through soil layers. Flow from a soil layer occurs when soil water content
exceeds field capacity (FC [mm]). Water drains from the layer until the storage
returns to field capacity. The reduction in soil water is simulated with the routing
equation
(1)
where SW and SWo are the soil water contents at the end and the start of time
interval t (24 h) and IT is travel time through layer (h).
Thus, daily percolation can be computed by taking the difference between SW
andSWo·
(2)
where a is the percolation rate for layer (mm d-I).
Travel time through a layer is computed with the linear storage equation
IT; = (Pat -FCJ / SCt ,
(3)
where PO is the porosity (mm), FC is field capacity (mm), and SC is saturated
conductivity (mm h-I).
The routing process is applied from the soil surface layer by layer through the
deepest layer. Since the saturated conductivity of some layers may be much lower
than that of others, the routing scheme can lead to an impossible situation
(porosity of low saturated conductivity layers may be exceeded). For this reason, a
back pass is executed from the bottom layer to the surface. If the porosity of a
layer is exceeded, the excess water is transferred to the layer above. This process
continues through the top layer.
Percolation is also affected by freezing temperature. Water can flow into a
frozen layer but is not allowed to percolate from the layer.