Chris Leibundgut

A noncalibrated rainfall‐runoff model for large, arid catchments

A distributed, field-based rainfall-runoff model was developed for the 1400-km2 arid catchment of Nahal Zin, Israel. No calibration with measured flow data was performed. The model used rainfall radar input applied over a catchment that was spatially disaggregated into different terrain types according to hydrologically relevant surface characteristics. Hortonian overland flow generation on each type was parameterized independently using values of initial loss and temporal decay of infiltration determined from existing field experiments. Delimited by topography, this catchment wide pattern of rainfall excess was distributed over 850 tributary catchments (model elements). Runoff delivery from the model elements to the adjoining channel segments was timed by applying a mean response function determined in an environmentally similar experimental catchment. Inside the channel network the MVPMC3 method of the Muskingum-Cunge technique was used for streamflow routing, accounting for channel dimensions and roughness. For each channel segment a constant infiltration rate was applied to account for transmission losses and discontinued when the wetting front reached the bottom of the available alluvial storage. Within two model tests, one separate for the routing component (October 1979) and one for the complete model (October 1991), observed hydrographs and reconstructed peak discharges were successfully simulated. The spatially distributed model output showed that during the October 1991 test, tributaries produced preceding peaks that wetted the channel alluvium before the main flood had arrived and transmission losses lost their significance downstream. Total maximum model uncertainty was estimated including the uncertainty ranges of each model parameter. In general, this study shows that field-based data on generation and losses of runoff may be incorporated into a distributed hydrologic model to overcome calibration with the poor data records of arid high-magnitude events.

Runoff Generation Processes on Hillslopes and Their Susceptibility to Global Change

Global change will influence hillslope hydrological processes for a variety of reasons. On the one hand, climate change might alter the hydrological input, i.e. precipitation and snow melt, which might cause an increase or decrease in the intensity of specific hillslope processes. For instance, overland flow might be amplified by increased rain intensities (Horton 1933) or by reduced infiltration due to surface crusts (Yair 1990) or increased hydrophobicity (Doerr et al. 2002), triggered by longer and more pronounced drought periods. However, overland flow could also be significantly influenced by antecedent moisture conditions of the substrate that were either altered due to wetter climate and reduced evapotranspiration at a site or due to different snow and snow melt regimes, changing the hydrological input for a specific precipitation event. On the other hand, global change in the form of land use changes will play a key role in defining the dominant runoff generation processes on hillslopes (cf. summary given in DVWK 1999).

Modelling nitrogen dynamics for a mesoscale catchment using a minimum information requirement (MIR) concept / Modélisation des dynamiques de l'azote pour un bassin versant de taille moyenne, utilisant le concept de besoin minimum d'information

Abstract Based on the water balance model LARSIM (Large Area Simulation Model), a model for the simulation of nitrogen transport was developed in a mesoscale catchment in southwest Germany. To meet the needs and constraints in river basin management, the nitrogen model was developed following the concept of minimum information requirement (MIR). The modelling concept uses only few calibration parameters and only easily accessible input data. Water balance, runoff generation and nitrogen transport were simulated on a 1-km2 grid of sub-areas in which different land-use classes and soil characteristics were accounted. Temporal variability of the storage of mobile nitrogen were described using a monthly based mass balance. Nitrogen mobilization and transport was simulated using monthly values of different runoff components and data for soil properties, topography, hydrogeology and river network. The simulation was calibrated and validated using streamflow from two gauging stations and observed nitrogen concentrations at the catchment outlet, showing reasonable results for both streamflow and nitrogen dynamics. The results of the model application are discussed in the context of uncertainty problems and their implications for water management.