freezes. Figure 3 exemplarily shows the dependence of maximum liquid water content on soil temperature for some selected soil-types. Considering the differences in volumes taken by water and ice, the volumetric ice content n =(ntotal -nmax)— (9)
P w is proportional to the difference of the total water (liquid, solid, gaseous) within the soil layer minus the maximum liquid water content for temperatures below freezing point
Water extraction by roots and the following transpiration act as a soil-water sink. Soil-water uptake by roots, among other things, depends on vegetation-type, soil-physical and geologic characteristics, plant available soil-water, soil-temperature, aeration, competition or interaction with roots of other species, fertilizer, biologic and soil-chemical processes and transpiration. Various parameterizations have been developed with varying complexity (e.g., Gardner 1960, Cowan 1965, Federer 1979, Sellers et al. 1986, Martin 1990, Molders et al., 2003 a). The main differences between the various approaches are the assumptions on water-uptake restrictions, root-length, vertical distribution, whether or not root distribution varies with time, soil and/or vegetation type. Most recent LSMs used in atmospheric models assume equal distribution of roots in the root zone or only distinguish between the upper and lower root space (cf. Table 1). In the latter case, it is further assumed that the boundary between the two root spaces falls together with a soil-layer boundary; the same is true for maximum root length (e.g., Wilson et al. 1986, Martin 1990).
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