Impact on simulated soil moisture

Since the water cycle is the main focus here, further analysis will concentrate on changes in the parameter wmax and the simulated plant-available soil water content. Changes in wnmx, the maximum plant-available water holding capacity of the soil, are documented in Fig. 3, showing the initial (a) and adjusted fields (b), and the difference between the two (c).

The initial fields, i.e. those assumed before assimilation and derived from the analysis of Patterson as used in the ECHAM-4 climate model, largely reflect changes in soil porosity, with relatively small changes in the assumed rooting depth [Patterson, 1990], However, the maximum amount of soil water that can be used by vegetation, and thus becomes available as latent heat flux to the atmosphere, is also determined by the depth ofthe rooting system [Kleidon and Heimann, 1998], This impact of vegetation can be seen in the adjusted wmax field of Fig. 3b: whereas southern Africa and parts of Australia have rather low water storage capacities in the initial fields, the densitity of the vegetation there leads to rather high values after assimilation. The situation for South America turns out to be a little more complex: high w„KIX is predicted by the assimilation scheme for the Northeast, lower than initial wmax for the Cerrado southwest of that area, and again very high values at the edge of the Amazon rainforest (cf. Fig. 3c). For this latter region, the spatial pattern exhibited by the initial fields, with a large area ofhigh soil water holding capacity extending across the rather sharp rainforest/savanna boundary, is very different from the adjusted ones. This pattern found in the initial fields is probably unrealistic, because for the same climate, evergreen rainforest trees require more soil water storage than drought-deciduous savanna vegetation (see next subsection).

The simulated plant-available soil water content in the tropics is shown in Fig. 4 for the northern-hemisphere (March, top) and southern-hemisphere dry seasons (September, bottom). An important result for the northern tropics is that after assimilation, there is more soil moisture available in the Sahel region during March, where the zone of partially wet soils extends further north into the Sahara. This is also reflected by an increase of wmax in that region shown in Fig. 3c. For the southern tropics, which include most of the Amazon basin, it is important to note that an increase in wmax has lead to more available soil moisture during the dry season in most of southern Africa, the Brazilian northeast, parts of the Amazon rainforest, and northern Australia.

Figure 3. Maximum plant-available soil moisture, in mm, used as initial fields in the assimilation scheme (a), and as derived after maximising consistency with satellite derived fAPAR (b). Panel (c) shows the difference between he two fields as the effect of satellite assimilation

Figure 3. Maximum plant-available soil moisture, in mm, used as initial fields in the assimilation scheme (a), and as derived after maximising consistency with satellite derived fAPAR (b). Panel (c) shows the difference between he two fields as the effect of satellite assimilation

Figure 4. Simulated plant available soil moisture for the tropics with the BETHY land surface and vegetation model, for March (dry season in the north) and September (dry season in the south), (a) and (c) have been generated using the initial fields of maximum plant-availabe soil moisture, (b) and (d) with optimised fields of this parameter as derived through assimilation

Figure 4. Simulated plant available soil moisture for the tropics with the BETHY land surface and vegetation model, for March (dry season in the north) and September (dry season in the south), (a) and (c) have been generated using the initial fields of maximum plant-availabe soil moisture, (b) and (d) with optimised fields of this parameter as derived through assimilation

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