Abstract:There are several indications that changes in land cover have influenced the hydrological regime of various river basins. In addition, the effects of climate change on the hydrological cycle and on the runoff behaviour of river catchments have been discussed extensively in recent years. However, it is at present rather uncertain how, how much and at which spatial scale these environmental changes are likely to affect the generation of storm runoff, and consequently the flood discharges of rivers. Firstly, this paper gives an overview of the possible effects of climatic and land-use change on storm runoff generation. Secondly, it discusses models dealing with the hydrological response to climate and land-use variations, including both the downscaling of climate information from global circulation models and the way flood forecasting models represent land-use conditions. Finally, two modelling studies of meso-scale catchments in Germany are presented: the first shows the possible impacts of climate change on storm runoff production, and the second the impacts of land-use changes.

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Land-use changes effects on floods are investigated by a multi-scale modelling study, where runoff generation in catchments of different sizes, different land uses and morphological characteristics are simulated in a nested manner. The macro-scale covers the Rhine basin (excluding the alpine part), the upper meso-scale covers various tributaries of the Rhine and three lower meso-scale study areas (100-500 km 2 ) represent different characteristic land-use patterns. The main innovation is the combination of models at different scales and at different levels of process representation in order to account for the complexity of land-use change impacts for a large river basin.The results showed that the influence of land-use on storm runoff generation is stronger for convective storm events with high precipitation intensities than for long advective storms with low intensities. The simulated flood increase at the lower meso-scale for a scenario of rather strong urbanization is in the order of 0 and 4% for advective rainfall events, and 10-30% for convective rain storms with a return period of 2-10 years.Convective storm events, however, are of hardly any relevance for the formation of floods in the large river basins of Central Europe, because the extent of convective rainstorms is restricted to local occurrence. Due to the dominance of advective precipitation for macro-scale flooding, limited water retention capacity of antecedent wet soils and superposition of flood waves from different tributaries, the land-use change effects at the macro-scale are even smaller, for example at Cologne (catchment area 100 000 km 2 ), land-use change effects may result in not more than 1-5 cm water level of the Rhine. Water retention measures in polders along the Upper and Lower Rhine yield flood peak attenuation along the Rhine all the way down to the Dutch border between 1 and 15 cm.

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<p>A major aim of physically based distributed hydrological models is an adequate representation of hydrological processes, including runoff generation processes. A significant proportion of runoff is generated through the subsurface, i.e. by groundwater flow or unsaturated subsurface stormflow. However, in the case of high rainfall intensity and/or low soil-surface infiltrability, surface runoff may strongly contribute to total runoff, too, either through saturation excess (“Dunne-type surface runoff”) or infiltration excess (“Hortonian surface runoff”). Both types of surface runoff can be rather important if antecedent wetness is high and parts of the catchment area are saturated (leading to saturation excess), or if the maximum infiltration rate into the soil surface is less than the actual rainfall intensity (resulting in infiltration excess). Even though the latter process can be very important during high-intensity rainstorms, both for flood generation and for matter transport linked with surface runoff, an appropriate consideration of this process in catchment models is still challenging. Actually, budgeting between the actual rainfall intensity and the soil surface infiltration capacity is required. This may appear simple in principle, but there are a number of challenges in the details: First, the ‘real’ rainfall intensity may vary tremendously in time increments much smaller than the time step of the model. The soil surface infiltrability can also be significantly reduced, e.g. by crusting, compaction or rain energy-induced sealing of the soil surface or through hydrophobic effects.</p> <p>Otherwise, soil infiltrability can be strongly enhanced as a consequence of preferential flow paths / macropores caused by e.g. bioturbations or other voids.</p> <p>Finally, there is high variability of such soil surface features appearing at a rather small spatial scale, below the typical spatial modelling unit.</p> <p>This contribution presents observational data and model approaches to deal with these challenges. We show results from combined infiltration and infiltration-excess experiments and observations at three different spatial scales. Then, we present a model approach based on a double-porosity soil, thus enabling the combined modelling of high infiltration rates and dampened soil moisture distribution after termination of infiltration, as observable in the field. Furthermore, we present an approach to model the effects of soil surface conditions on actual infiltration capacity and its variation.</p> <p>We show simulation results where these approaches improved the overall plausibility and explanatory power of the model concerning surface runoff generation and soil moisture dynamics. For instance, model results of infiltration experiments at the plot and hillslope/field scales show that it is possible to simulate high infiltration rates jointly with a relatively slow movement of moisture within the soil matrix, field phenomena often observed in the case of heavy rainfall. Other simulation efforts deal with the non-linear and space-time variable effects of soil surface conditions. This is a rather important feature for flood generation in the case of high rainfall intensity and low soil infiltrability.</p>

Physically based distributed hydrological models aim at an adequate representation of hydrological processes, including runoff generation. A significant proportion of runoff is generated through the subsurface, that is, by groundwater flow or unsaturated subsurface stormflow. However, in the case of high rainfall intensity and/or low soil-surface infiltrability, surface runoff may strongly contribute to total runoff, too, either through saturation excess ("Dunne-type surface runoff") or infiltration excess ("Hortonian surface runoff"). Both types of surface runoff can be rather important if antecedent wetness is high and parts of the catchment area are saturated (leading to saturation excess), or if the maximum infiltration rate into the soil surface is less than the actual rainfall intensity (resulting in infiltration excess). Even though the latter process can be very important during high-intensity rainstorms, both for flood generation and for matter transport linked with surface runoff, an appropriate consideration of this process in catchment models is still challenging.Actually, budgeting between the actual rainfall intensity and the soil surface infiltration capacity is required and there are a number of challenges in the details: First, the "real" rainfall intensity may vary tremendously in time increments much smaller than the time step of the model. The soil surface infiltrability can also be significantly reduced, for example, by crusting, compaction or sealing of the soil surface or through hydrophobic effects. Otherwise, soil infiltrability can be strongly enhanced as a consequence of preferential flow paths/macropores caused by, for example, bioturbations or other voids. Finally, there is a high variability of such soil surface features at a small spatial scale, below the typical spatial modelling unit. We present observational data and approaches to deal with these challenges. We show results from combined infiltration/infiltration-excess experiments and observations at three spatial scales. Then, we present a model approach based on a double-porosity soil enabling the combined modelling of high infiltration rates and dampened soil moisture distribution after termination of infiltration, as observable in the field. Furthermore, we present an approach to model the effects of soil surface conditions on actual infiltration capacity. These approaches improved the plausibility and explanatory power of the model concerning surface runoff generation and soil moisture

Reasons for the StudyBoth the landscape and the river systems in large parts of Central Europe have undergone major changes in the past, and there is no doubt that these environmental changes have altered the nature of floods in this region. But due to the complexity of the processes involved, the magnitude of their impact on storm-runoff generation and subsequent flood discharge in the river system is still uncertain. This uncertainty offers a vivid platform for various contradictory opinions on this topic, quite often disregarding scale-dependencies and boundary conditions in a way that ensures public attention rather than relevance.The work presented in the following focuses on three main questions, strictly referring to the spatial scale and the boundary conditions for which statements are made:Which runoff generation mechanisms are likely to be affected by land-use and land-cover changes at the mesoscale? (2) To what degree can flooding be mitigated by water retention measures in the landscape at the mesoscale? (3)How does the influence of land-use and land-cover changes on storm-runoff generation depend on catchment characteristics and spatial scale as well as event characteristics and temporal scale?The investigation does not address the influences of river training conditions and retention along the river courses on flood-wave propagation. 267J. Marsalek et al. (eds.). Advances in Urban Stormwater and Agricultural Runoff Source Controls,[267][268][269][270][271][272][273][274][275][276][277][278]