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Soils

A brief description of the soil processes in JULES

Soil Thermodynamics

Subsurface temperatures are updated using a discretized form of the heat diffusion equation, which is coupled to the soil hydrology module. It includes soil water phase changes and the associated latent heat, soil thermal characteristics which are dependent on soil moisture content (liquid water and ice).

The temperature of the nth soil layer is incremented by the diffusive heat fluxes into and out of the layer, and the net heat flux advected from the layer by the moisture flux. The local soil thermal conductivity (Cox et al, 1999) is modified in the presence of lying snow. The relationship between unfrozen water concentration and temperature can be derived by minimizing the Gibbs free energy.

Soil Hydrology

The soil hydrology component of JULES is based on a finite difference approximation to the Richards' equation (Richards, 1931), with the same vertical discretization as the soil thermodynamics module. The total soil moisture content within a soil layer is incremented by the diffusive water flux flowing in from the layer above, the diffusive flux owing out to the layer below, and the evaporation extracted directly from the layer by plant roots and soil evaporation which is calculated from the total evaporation, based on the profiles of soil moisture and root density. The water fluxes are given by the Darcy equation which depends on the hydraulic conductivity and is the soil water suction. To close the model it is necessary to assume forms for the hydraulic conductivity and the soil water suction as a function of the soil moisture concentration. Either of the dependencies suggested by Clapp and Hornberger (1978) and van Genuchten et al (1991) can be used in JULES.

Soil Carbon

Soil carbon storage is increased by the total litterfall and reduced by microbial soil respiration which occurs at a rate dependent on soil moisture, temperature and soil carbon content. The total litterfall is made-up of the area-weighted sum of the local litterfall from each PFT, along with terms due to the large-scale disturbance rate and PFT competition.

The competition is derived by imposing carbon conservation on the soil-vegetation system implying that the NPP of each PFT will be lost entirely as litter once the PFT occupies all of the space available to it.

The moisture dependence of the soil respiration is based on the model of McGuire et al (1992) in which the respiration rate increases with soil moisture content until an optimum value of moisture is reached. Thereafter the rate of respiration is reduced with further increases in soil moisture. The curves presented by McGuire et al (1992) were approximated by piecewise linear functions in order to minimise the number of additional soil variables required.

References

Clapp, R., and G. Hornberger, 1978: Empirical equations for some soil hydraulic properties. Water Resources Research, 14, 601{604.

Cox, P. M., R. A. Betts, C. B. Bunton, R. L. H. Essery, P. R. Rowntree, and J. Smith, 1999: The impact of new land surface physics on the GCM simulation of climate and climate sensitivity. Clim. Dyn., 15, 183-203

McGuire, A., J. Melillo, L. Joyce, D. Kicklighter, A. Grace, B.M. Ill and C. Vorosmarty, 1992. Interactions between carbon and nitrogen dynamics in estimating net primary productivity for potential vegetation in North America. Global Biogeochemical Cycles. 6 101-124.

Richards, L., 1931: Capilliary conduction of liquids through porous mediums. Physics, 1, 318-333.

van Genuchten, M., F. Leij, and S. Yates, 1991: The RETC code for quantifying the hydraulic functions of unsaturated soils. Technical Report EPA/600/2-91/065, U.S. Environmental Protection Agency.