No abstract

. The relation between the occurrence of plant species in environments varying in moisture status and groundwater regime was tested using numerical methods. The groundwater regime during the vegetation period was expressed by means of four parameters, the average (AVG), mean highest (HIGH), mean lowest (LOW) groundwater level and the maximum fluctuation (AMP). 67 records of five vegetation types were selected from hydrologically stable sites in brook valleys in the northern part of The Netherlands. Response curves were calculated for 30 representative species. Calculated optima for AVG, HIGH and LOW are strongly correlated to each other. The vegetation reacts independently from overall wetness to the amount of fluctuation of the groundwater level (AMP). Response curves of single species as well as combinations of both present and absent species were used to find the best set of indicators for each parameter. The use of combinations of species clearly improves the indicating value of vegetation records. The vegetation appears to be the most sensitive to the parameter HIGH, which can thus be considered to be a key factor in controlling vegetation composition. The four parameters can be predicted satisfactorily only in the middle part of the investigated gradient. This is not only due to arithmetic artifacts, inherent to the applied method, but also to the fact that at average groundwater levels below ‐ 60 cm or above 0 cm other factors become predominant.

No abstract

Infiltration experiments have been performed at three sites along a well-known catena under virgin tropical rain forest using a portable sprinkling infiltrometer.Experimentally determined infiltration curves are presented. Infiltration curves are also simulated on the basis of the Mein-Larson equation. The parameters for this model have been obtained from the infiltration curves (saturated conductivity) and simple soil moisture determinations (fillable porosity). The agreement between experimentally determined and modelled infiltration is reasonable, provided (a) saturated conductivity as derived from the experimental data is corrected, (b) a storage parameter, also derived from the experimental data, is added to the Mein-Larson model, and (c) the decline in soil porosity with depth is either small or occurs abruptly at shallow depth.Comparison of observed infiltration rates with rainfall intensity shows that Horton Overland Flow has to occur naturally at least on the middle and lower section of the catena.Despite the fact that most parameters can be estimated in principle from basic soil data, it remains advisable to obtain sprinkling infiltrometer field measurements, because of soil variability due to dynamic surface conditions, macroporosity, air entrapment, and irregularity of the wetting front. KEY WORDS Tropical rain forest Forest hydrology Modelling Infiltration Mein-Larson equation

No abstract