Z3 - Central Laboratory Unit and the influence of pedo-hydrological changes on the water and nutrient cycle in catchment areas, considering ENSO-impacts in the meso-scale.

In period III the former subproject B2 is integrated in subproject Z3. The following abstract and summary refer mostly to B2.

Abstract
Within the experimental and modelling research in the Nopu-catchment (Central Sulawesi) the water balance model WASIM-ETH was successfully applied for the first time in a tropical catchment. Land use scenarios are simulated and experimental result show a high significance of subsurface runoff generation, changes of infiltration characteristics with land use type and duration and a high increase in nutrient output with forest conversion. The main objectives of this sub-project are (1) to improve the parametrisation and forecasting quality of the hydrological model WASIM-ETH for tropical catchments on the micro- and mesoscale, (2) to apply a regionalisation concept for upscaling and (3) to simulate the impacts of ENSO and land use change on the water balance and water resource availability.

Summary
Long term experimental and modelling research on the effects of forest conversion on runoff generation, water balance and nutrient output in the Inner Tropics is rare. With the generation of hydrological and hydrochemical data since the end of 2001, B2 hypotheses are tested successfully by applying the water balance model WASIM-ETH for the first time in a tropical catchment. The model application showed a good agreement between the simulated and measured water balance using a “multiple response validation”. Land use scenarios are simulated and experimental results show high importance of subsurface runoff generation, changes of infiltration characteristics with land use type and age and high increase in nutrient output with forest conversion. Main results from phase 2 are: (1) forest conversion leads to an increase in river discharge by 220 mm (2002 to 2004, subcatchment slash&burn), (2) scenario simulation (natural forest into agroforestry with cocoa and slash&burn) shows an increase for all runoff components with cacao plantations (+ 17% total river discharge) and with slash&burn ( + 82% for overland flow), (3) during floods fast runoff (mainly quick interflow) contributes up to 60-80% to total runoff, (4) for all main cations soluble nutrient output in 2004 was 4-5–fold higher in the slash&burn subcatchment compared to natural forest catchment, (5) plot erosion measurements carried put by the indonesian counterpart indicate relatively low soil erosion (1.6 t/ha*y cocoa, 2,4 t/ha*y maize) compared with the natural forest plot (0.2 t/ha*y).

For the third phase upscaling methods with relief-soil moisture classes and pedohydrological parametrisation (HRU´s) will be used for the model application of WASIM-ETH in the mesoscale Gumbasa catchment. In relation to focus 2 B2 will concentrate on the analysis, evaluation and simulation of the different effects of two agroforestry land use types and the enhanced conversion to cacoa agroforestry on the water balance. To study regional effects of ENSO impacts, the successfully adapted water balance model WASIM-ETH can be used for the simulation with regard to water yield change, change of evapotranspiration rates and groundwater recharge in the Gumbasa watershed.

The main objectives of this sub-project are (1) to improve the parametrisation and forecasting quality of the hydrological model WASIM-ETH for tropical catchments in the micro- and mesoscale, (2) to use a regionalisation concept for upscaling, (3) to simulate the impacts of ENSO and land use change on the water balance and water resource availability and (4) to analyse and simulate long term trends in water cycling and nutrient output affected by ongoing smallholder forest conversion.