Soil Physical Structure Submodel in the Century5 Model

A model of the physical details of the soil describes the texture, bulk density, water content, and bulk organic matter content. A soil is described by a set of layers, each layer having a value for thickness, sand, silt, and clay fractions, bulk density, wilting point, field capacity, and percent organic matter. New site parameters are provided for describing the initial soil configuration. A maximum of 10 layers can be described in the site parameters, though additional layers may be added using the new deposition event (see Erosion and Deposition Submodels ). Only the first NLAYER parameters are used at the start of a simulation.

The default values for the layer thickness are taken from the ADEP parameters in the fixed parameter set. A thickness parameter with a value of zero will be assigned a default thickness from the corresponding ADEP parameter. No other default values are used. If site parameters are imported from a Century 4 site file, which has only the first layer for SAND, SILT, CLAY, and BULKD, this single value is copied throughout the array. Use the Site Editor to modify the initial layer values for the soil layers.

Texture

The new site parameter names are SAND(1..10) , SILT(1..10) , CLAY(1..10) , BULKD(1..10) , and THICK(1..10) (where 1..10 specifies a range of array values), and replace SAND, SILT, CLAY, and BULKD, respectively, in the Century 4 site file.

In Century 4, calculations relating organic C to texture utilized only the surface soil, that is, the Century 4 site parameters, SAND, SILT, and CLAY. In version 5, the textural information is calculated as a weighted mean over the depth of the physical soil corresponding to the simulation layer depth. Using a layered soil may produce significantly different simulation results compared with Century 4.

Texture can change during a simulation due to erosion and deposition events, and cultivation which homogenizes the surface layers of the soil. After textural changes occur, OMPC, wilting point, field capacity are recalculated depending upon the value of the SWFLAG site parameter.

Water Content

Soil water content in Century is a function of infiltration field capacity. See the section Water Budget, Leaching and Soil Temperature for details on the water submodel. Field capacity and wilting point of a layer can change during a simulation depending upon the value of the SWFLAG site parameter. Calculation of these values uses the methods of Gupta and Larson (1979) and Rawls et al (1982). See the description of the SWFLAG site parameter for details on selecting these calculations.

Organic Matter Content

Modeling of the depth distribution of organic C is described in the section Depth Distribution of Soil Carbon.

Algorithms

The physical soil submodel includes algorithms for erosion, deposition, splitting of a layer, homogenization of layers, calculation of field capacity and wilting point, a bucket model for water flow, and calculation of organic matter content distribution.

Erosion removes soil from the top layer, effectively thinning the layer. Deposition adds soil to the top layer. Splitting a layer divides the layer into two new layers which identical properties.

Homogenization of the surface soil occurs with cultivation. The depth of homogenization is proportional to the degree of disturbance as defined by the mean cultivation factor for decomposition due to disturbance, CLTEFF(1..4). For example, if the mean of the parameters is 1.3 in a range of 1.0 to 1.6, then half of the simulation depth will be homogenized. During homogenization soil layers spanned by the depth range are combined; amounts are summed (such as soil water content) and properties are recalculated as a weighted average by layer thickness (such as bulk density).