The accumulation of surface and soil organic matter affect several critical soil properties and processes including bulk density, water holding capacities, soil evaporation rates, surface water runoff rates and water infiltration rates. Tillage events also affect soil properties (described here). These dynamics are modeled in CSM.
Soil bulk density
As organic matter is added to the soil, bulk density is decreased and the soil layer thickness is increased. CSM computes these soil dynamics on a daily time step. The modified bulk density and soil layer thickness generate base values onto which tillage effects are superimposed. After a tillage event, rainfall kinetic energy settles the soil to the bulk density and layer thicknesses that have been updated based on organic matter content at each time step.
Soil water holding capacity
Addition of organic matter to the soil increases the capacity of the soil to retain moisture. CSM quantifies changes to the drained upper limit and the lower limit of a soil based on changes these changes in organic matter.
Surface organic matter effects on soil evaporation occur by two mechanisms in CSM. First, surface organic matter acts as a sponge, absorbing rainfall and irrigation water and losing water through evaporation. Evaporative demand in CSM is assumed to be preferentially met by the mulch layer, then by the soil. Thus, the mulch layer has the effect of buffering the water content in the upper soil layers, by preventing rapid depletion due to evaporation.
The second mechanism affecting soil evaporation is by shading of the soil and modifications to surface albedo and therefore potential evapotranspiration.
Surface water runoff and infiltration rates
Bare soils tend to form a crust due to the impact of raindrops on the surface of the soil, thereby reducing the infiltration capacity of the soil. This raindrop action is avoided where there is a protective cover of crop foliage, residues, mulches or even weeds at or over the soil surface. In this manner a mulch layer will increase the infiltration rate thereby reducing surface water runoff for most storms. In addition to the effects of a higher infiltration rate, a mulch cover can allow increased temporary ponding of water on the surface due to the increased surface roughness thereby allowing more time for infiltration to occur. CSM captures these effects by modifying the SCS runoff coefficient and the amount of initial abstraction (the amount of rainfall absorbed before runoff occurs).
Details of these effects and how they are computed in CSM are desribed in Porter, et al., 2010.
Porter, C.H., J.W. Jones, S. Adiku, A.J. Gijsman, O. Gargiulo, J.B. Naab. 2010. Modeling organic carbon and carbon-mediated soil processes in DSSAT v4.5. Oper. Res. Int. J. 10(3):247-278. doi:10.1007/s12351-009-0059-1. http://link.springer.com/article/10.1007%2Fs12351-009-0059-1