SEASONAL MODELING OF CATCHMENT WATER-BALANCE - A 2-LEVEL CASCADING MODIFICATION OF TOPMODEL TO INCREASE THE REALISM OF SPATIOTEMPORAL PROCESSES

A general problem of hydrological modelling is parameter identificatio n for the driving processes. To examine the longterm dynamics of the w ater balance of a small (4.2 km(2)) forested catchment in south-east G ermany (Lehstenbach), TOPMODEL has been adapted as a two-level cascadi ng approach. Only the lower cascade is allowed to respond dynamically. This modified TOPMODEL version accounts for the observations that sur face runoff only takes place in a small portion of the catchment and t hat water flow in large portions of the catchment occurs through groun dwater aquifers with a lateral recharge to the downslope regions, Wate r from an upper catchment region is transferred to a lower storage. Th e border between the two areas is represented as a topographic index ( ATB) threshold that can be varied in the model. The best fits are obta ined if only 60% of the catchment area is allowed to react dynamically . A substantial improvement of the runoff description has been achieve d by a moderate increase of model complexity. Results of a Monte Carlo simulation showed that the model structure has a flat global optimal solution. In order to quantify the boundary conditions, we combined di rect estimates of tree and understorey transpiration, maps of tree age and understorey cover to estimate empirically the total catchment eva potranspiration. The context of a dynamic hydrological model allows an evaluation of ecological data in the context of catchment scale dynam ics. Vapour pressure deficit can be assumed to be the major driver of vegetation-atmosphere water transfer. Soil moisture does not affect tr ee transpiration in this catchment. The results show that transpiratio n measurements can be scaled to the catchment scale in spite of variat ions between sites of up to 100%. (C) 1997 by John Wiley & Sons, Ltd.