Comparison with soil moisture measurements from Amazonia

So far, the analysis has shown that requiring concistency with observed vegetation cover has had an impact on the simulated soil water content in large parts of the tropics, with a tendency towards greater maximum soil water storage. In fact, it has been discovered that often trees in tropical rainforests can survive prolonged dry periods by developing very deep roots [Nepstad et al., 1994], with important consequences for the soil water balance. It has also been argued that, at least in water limited environments, plants should optimise the rooting strategy for water use, which leads to much deeper roots and higher soil water capacity than traditionally assumed in climate or vegetation models [Kleidon and Heimann, 1998].

To investigate whether the model simulations shown before are consistent with observations, the BETHY model is run on the basis of daily precipitation data for 1992 reported by Nepstad and co-workers - with temperature data for 1992 unchanged - for their Paragominas site (3°S, 48°W). Initial rooting depth was set to 2 m, with computed from soil texture information as in a previous application of the model [Knorr, 1997].

Simulated and satellite-derived fAPAR for this site are show in Fig. 5a. The model run with a-priori parameters shows a pronounced decrease m fAPAR during the dry season as a result of leaf shedding forced by declining soil water, which disagrees both with the satellite data, and with published reports for this site. If, however, simulated fAPAR after assimilation is compared to satellite-derived values, the agreement improves considerably, with a lower variability for the model calculated than for the satellite derived values. In fact, satellite-derived values of fAPAR tend to be higher during the dry season, and lower during the wet season. This is most likely a result of residual contamination of the satellite signal by high water vapour concentration during the wet season. This particular result demonstrates how data assimilation can also help to filter out noise in input data by accounting for both model and data uncertainties.

More important for the surface energy balance than fAPAR is the actual evapotranspiration. As Fig. 5b shows, high evapotranspiration rates persist through the dry season after assimilation, although somewhat reduced due to declining soil water reserves, while they approach zero with the a-priori setup of the model. This large difference in the energy and water balance between the two model versions can be attributed entirely to persisting soil water reserves during the dry season, as Fig. 5c shows: while in the a-priori model version with 2 m root depth, the soil dries out almost completely, observations [Nepstad et al., 1994] reveal much higher reserves in that part of the soil. The difference, however, is not attributable to less evapotranspiration, but to higher soil water storage; this is revealed by Fig. 5d: the slope of the simulated and observed soil water curves at the beginning of the dry season (May to June) are all similar, but in the assimilated case, the model infers very large water storage in accordance with measurements down to a depth of 8 m, a depth where there were still active roots. (It should be noted that this particular site was found to be free of soil water intrusion).

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(b) évapotranspiration [mm/month]

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> M K

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Paragominas ; (3*S 4rw63m)

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(b) évapotranspiration [mm/month]

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(c) plant-available soil moisture [mm] (0. ,2m depth}

250 200 150 100 so 0

A measured

Paragominas

(d) plant-available soil moisture [mm] (0...8m depth}

(d) plant-available soil moisture [mm] (0...8m depth}

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Figure 5. Satellite derived and simulated fAPAR (a), simulated évapotranspiration (b), and NPP (e), and simulated plant-available soil moisture vs. measurements by Nepstad et al. (1994) for the top 2 m (c) and 8 m (d), all for the Paragominas site in the eastern Amazonian rainforest (3°S, 48°W). The BETHY model was run in three modes with observed daily rainfall data: without assimilation (a priori), with assimilation of the satellite derived fAPAR values (assimilated, all except c), and with satellite-derived fAPAR used as forcing {prescribed, only e)

To better understand the mechanism through which changes in wmax can lead to an improved consistency with observed fAPAR, and to contrast this method with simply prescribing the LAI according to satellite observations, Fig. 5e shows the simulated net primary productivity (NPP) also for the case of prescribed fAPAR: when the LAI is set the observed constant value of 5, NPP declines rapidly during the dry season and reaches large negative values caused by persisting plant respiration at closed stomata. By contrast, in the normal a priori run, LAI declines to stabilise NPP around zero, while in the assimilated case, NPP persists rather unchanged. This latter type of carbon balance has been found for a rainforest site with a similar climate [Grace et al., 1995], who have carried out eddy correlation measurements of C02 fluxes in Rondonia, Brazil. This indicates that, because of the carbon costs of maintaining extensive foliage, the presence of green vegetation can be used to deduce a source of transpired soil water. In a climate model that contains an interactive vegetation component, this information could then be used to check its consistency with rather easily observable satellite information.

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