Abstract:The areal infiltration behaviour of a grass field is studied using a data set of 78 sprinkler infiltration experiments. The analysis of the experimental data shows a distinct event dependency: once runoff begins, the final infiltration rate increases with increasing rainfall intensity. This behaviour is attributed to the effects of small-scale variability. Increasing rainfall intensity increases the ponded area and therefore the portion of the plot which infiltrates at maximum rate. To describe the areal infiltration behaviour of the grass field the study uses two different model structures and investigates different approaches for consideration of subgrid variability. It is found that the effective parameter approach is not suited for this purpose. A good representation of the observed behaviour is obtained by using a distribution function approach or a parameterization approach. However, it is not clear how the parameters can be derived for these two approaches without a large measurement campaign. The data analysis and the simulations show the great importance of considering the effects of spatial variability for the infiltration process. This may be significant even at a small scale for a comparatively homogeneous area. The consideration of heterogeneity seems to be more important than the choice of the model type. Furthermore, similar results may be obtained with different modelling approaches. Even the relatively detailed data set does not seem to permit a clear model choice. In view of these results it is questionable to use very complex and detailed simulation models given the approximate nature of the problem. Although the principle processes may be well understood there is a lack of models that represent these processes and, more importantly, there is a lack of techniques to measure and parameterize them.

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

<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>