Axel Bronstert

Land-use impacts on storm-runoff generation: scenarios of land-use change and simulation of hydrological response in a meso-scale catchment in SW-Germany

The impact of land-use change impacts on storm-runoff generation is presented based on a simulation study composed of three parts: (1) generation of spatially explicit land-use scenarios; (2) spatially distributed and process based hydrological modelling of runoff generation; (3) application of this procedure and demonstration of results for a predominantly agricultural meso-scale catchment in a loessy-soil landscape in SW Germany. Land-use scenarios are a prerequisite for assessing the influence of potential changes of land-use and/or land-cover on runoff generation. The land-use change modelling kit (LUCK) provides a method for the spatial transformation of overall trends of land-use into spatially distributed scenarios of land-use patterns, taking into account their topology in a true position mode. The assignment of land-use categories to each grid cell is realised in a spatially explicit manner, dependent on an evaluation of the site characteristics as well as its neighbourhood relationships. Based on these land-use scenarios, the influence of altered land-use characteristics on flooding is simulated using a modified version of the physically based hydrological model WaSiM-ETH. In order to extend the models capabilities with respect to adequate representation of land-use related runoff generation mechanisms, some additional mechanisms have been introduced: (1) A macropore module accounting for fast infiltration processes; (2) A siltation module decreasing hydraulic conductivity of the soil surface as a function of precipitation intensity and vegetation coverage. (3) Sub-grid variability considering the impervious and sealed portion of a grid cell. The location and lateral interaction of landscape elements within a catchment is captured by spatially explicit modelling on the basis of gridded information provided by the scenarios. The whole simulation procedure is applied to a meso-scale catchment in SW Germany. The results show that the influence of land-use conditions on storm-runoff generation depends greatly on the rainfall event characteristics and on the related spatial scale, i.e. the influence is only relevant for convective storm events with high precipitation intensities in contrast to long-lasting advective storm events with low precipitation intensities. However, convective events—and thus land-use conditions—are of very minor relevance for the formation of floods in large river basins because this type of rainfall event is usually restricted to small-scale occurrence.

Modelling of runoff generation and soil moisture dynamics for hillslopes and micro-catchments

Abstract The modelling of hillslope hydrology is of great importance not only for the reason that all non-plain, i.e. hilly or mountainous, landscapes can be considered as being composed of a mosaic of hillslopes. A hillslope model may also be used for both research purposes and for application-oriented, detailed, hillslope-scale hydrological studies in conjunction with related scientific disciplines such as geotechnics, geo-chemistry and environmental technology. Despite the current limited application of multi-process and multi-dimensional hydrological models (particularly at the hillslope scale), hardly any comprehensive model has been available for operational use. In this paper we introduce a model which considers most of the relevant hillslope hydrological processes. Some recent applications are described which demonstrate its ability to narrow the stated gap in hillslope hydrological modelling. The modelling system accounts for the hydrological processes of interception, evapotranspiration, infiltration, soil-moisture movement (where the flow processes can be modelled in three dimensions), surface runoff, subsurface stormflow and streamflow discharge. The relevant process interactions are also included. Special regard has been given to consideration of state-of-the-art knowledge concerning rapid soilwater flow processes during storm conditions (e.g. macropore infiltration, lateral subsurface stormflow, return flow) and to its transfer to and inclusion within an operational modelling scheme. The model is “physically based” in the sense that its parameters have a physical meaning and can be obtained or derived from field measurements. This somewhat weaker than usual definition of a physical basis implies that some of the sub-models (still) contain empirical components, that the effects of the high spatial and temporal variability found in nature cannot always be expressed within the various physical laws, i.e. that the laws are scale dependent, and that due to limitations of measurements and data processing, one can express only averaged and incomplete data conditions. Several applications demonstrate the reliable performance of the model for one-, two- and three-dimensional simulations. The described examples of application are part of a comprehensive erosion and agro-chemical transport study in a loessy agricultural catchment in southwestern Germany, and of a study on the sealing efficacy of capillary barriers in landfill covers.

Plot and field scale soil moisture dynamics and subsurface wetness control on runoff generation in a headwater in the Ore Mountains

Abstract. This study presents an application of an innovative sampling strategy to assess soil moisture dynamics in a headwater of the Weiseritz in the German eastern Ore Mountains. A grassland site and a forested site were instrumented with two Spatial TDR clusters (STDR) that consist of 39 and 32 coated TDR probes of 60 cm length. Distributed time series of vertically averaged soil moisture data from both sites/ensembles were analyzed by statistical and geostatistical methods. Spatial variability and the spatial mean at the forested site were larger than at the grassland site. Furthermore, clustering of TDR probes in combination with long-term monitoring allowed identification of average spatial covariance structures at the small field scale for different wetness states. The correlation length of soil water content as well as the sill to nugget ratio at the grassland site increased with increasing average wetness and but, in contrast, were constant at the forested site. As soil properties at both the forested and grassland sites are extremely variable, this suggests that the correlation structure at the forested site is dominated by the pattern of throughfall and interception. We also found a very strong correlation between antecedent soil moisture at the forested site and runoff coefficients of rainfall-runoff events observed at gauge Rehefeld. Antecedent soil moisture at the forest site explains 92% of the variability in the runoff coefficients. By combining these results with a recession analysis we derived a first conceptual model of the dominant runoff mechanisms operating in this catchment. Finally, we employed a physically based hydrological model to shed light on the controls of soil- and plant morphological parameters on soil average soil moisture at the forested site and the grassland site, respectively. A homogeneous soil setup allowed, after fine tuning of plant morphological parameters, most of the time unbiased predictions of the observed average soil conditions observed at both field sites. We conclude that the proposed sampling strategy of clustering TDR probes is suitable to assess unbiased average soil moisture dynamics in critical functional units, in this case the forested site, which is a much better predictor for event scale runoff formation than pre-event discharge. Long term monitoring of such critical landscape elements could maybe yield valuable information for flood warning in headwaters. We thus think that STDR provides a good intersect of the advantages of permanent sampling and spatially highly resolved soil moisture sampling using mobile rods.

Rainfall-runoff response, event-based runoff coefficients and hydrograph separation

Abstract Event-based runoff coefficients can provide information on watershed response. They are useful for catchment comparison to understand how different landscapes “filter” rainfall into event-based runoff and to explain the observed differences with catchment characteristics and related runoff mechanisms. However, the big drawback of this important parameter is the lack of a standard hydrograph separation method preceding its calculation. Event-based runoff coefficients determined with four well-established separation methods, as well as a newly developed separation method, are compared and are shown to differ considerably. This signifies that runoff coefficients reported in the literature often convey less information than required to allow for catchment classification. The new separation technique (constant-k method) is based on the theory of linear storage. Its advantages are that it is theoretically based in determining the end point of an event and that it can also be applied to events with multiple peaks. Furthermore, it is shown that event-based runoff coefficients in combination with simple statistical models improve our understanding of rainfall—runoff response of catchments with sparse data.

Capabilities and limitations of detailed hillslope hydrological modelling

Hillslope hydrological modelling is considered to be of great importance for the understanding and quantification of hydrological processes in hilly or mountainous landscapes. In recent years a few comprehensive hydrological models have been developed at the hillslope scale which have resulted in an advanced representation of hillslope hydrological processes (including their interactions), and in some operational applications, such as in runoff and erosion studies at the field scale or lateral flow simulation in environmental and geotechnical engineering. An overview of the objectives of hillslope hydrological modelling is given, followed by a brief introduction of an exemplary comprehensive hillslope model, which stimulates a series of hydrological processes such as interception, evapotranspiration, infiltration into the soil matrix and into macropores, lateral and vertical subsurface soil water flow both in the matrix and preferential flow paths, surface runoff and channel discharge. Several examples of this model are presented and discussed in order to determine the models capabilities and limitations. Finally, conclusions about the limitations of detailed hillslope modelling are drawn and an outlook on the future prospects of hydrological models on the hillslope scale is given.The model presented performed reasonable calculations of Hortonian surface runoff and subsequent erosion processes, given detailed information of initial soil water content and soil hydraulic conditions. The vertical and lateral soil moisture dynamics were also represented quite well. However, the given examples of model applications show that quite detailed climatic and soil data are required to obtain satisfactory results. The limitations of detailed hillslope hydrological modelling arise from different points: difficulties in the representations of certain processes (e.g. surface crusting, unsaturated–saturated soil moisture flow, macropore flow), problems of small-scale variability, a general scarcity of detailed soil data, incomplete process parametrization and problems with the interdependent linkage of several hillslopes and channel–hillslope interactions. Copyright

Regional integrated modelling of climate change impacts on natural resources and resource usage in semi-arid Northeast Brazil

Semi-arid regions are characterised by a high vulnerability of natural resources to climate change, pronounced climatic variability and often by water scarcity and related social stress. The analysis of the dynamics of natural conditions and the assessment of possible strategies to cope with drought-related problems require an integration of diverse knowledge including climatology, hydrology, and socio-economics. The integrated model introduced here dynamically describes the relationships between climate forcing, water availability, agriculture and selected societal processes. The model has been designed to simulate the complex human-environment system in semi-arid Northeast Brazil quantitatively and is applied to study the sensitivity of regional natural resources and socio-economy to climate change. The validity of the model is considered. Climate change is concluded to have an enormous potential impact on the region. River flow, water storage and irrigated production are specifically affected, assuming a continuous regional development and unfavourable but plausible changes in climate. Under plausible favourable changes in climate, these variables remain stressed. The impact of the integrated model and its applications on present policy making and possible future roles are briefly discussed.

Soil Moisture Estimation Under Low Vegetation Cover Using a Multi-Angular Polarimetric Decomposition

The estimation of volumetric soil moisture under low agricultural vegetation from fully polarimetric synthetic aperture radar (SAR) data at L-band using a multi-angular polarimetric decomposition is investigated. Radar polarimetry provides the framework to decompose the backscattered signal into different canonical scattering mechanisms referring to scattering contributions from the underlying soil and the vegetation cover. Multi-angular observation diversity further increases the information space for soil moisture inversion enabling higher inversion rates and a stable inversion performance. The developed approach was applied on the multi-angular L-band data set acquired by German Aerospace Centers ESAR sensor as part of the OPAQUE campaign in 2008. The obtained results are compared against ground measurements collected by the OPAQUE team over a variety of vegetated agricultural fields. The validation of the estimated against ground measured soil moisture results in an root mean square error level of 6-8 vol.% including all test fields with a variety of crop types.

Modelling river discharge for large drainage basins: from lumped to distributed approach

The paper presents an upscaled application of the HBV model to the German part of the Elbe drainage basin, and intercomparison of lumped and distributed versions of the model. The objectives of the work were (a) to check the model performance for large-scale basins, and (b) to compare the lumped and distributed versions of the model. Three versions of the HBV model, one lumped and two distributed, were applied first to a number of sub-basins of the Elbe with different hydrological regimes (area > 1000 km2), and then to the whole German part of the basin (area 80 657 km2). The model performed well in all cases. The distributed model versions are more data intensive but enabled better results to be achieved. The perspectives for using the model for large-scale water quality assessment, for climate change impact studies and for coupled land-atmosphere modelling are discussed.

1-, 2- and 3-dimensional modeling of water movement in the unsaturated soil matrix using a fuzzy approach

Modeling water movement in the unsaturated soil matrix is usually based on the numerical solution of the Richards equation. This approach requires much computational effort, therefore practical 2- or 3-dimensional applications are extremely rare. The purpose of this paper is to describe a computationally efficient and simple method. It is based on a transformation of the unsaturated Darcy law to a fuzzy rule system. The rule system is combined with the continuity equation yielding a fuzzy rule-based model for simulating the unsaturated flow. Basic definitions of fuzzy logic are given and the concept of the unsaturated flow model is outlined. The presented model performs well compared with a semi-analytical model and a 2-dimensional, numerical model. Furthermore the model has been incorporated into a physically-based and distributed, hydrological model. Model simulations for different types of hydrological situations show that the fuzzy rule-based approach is especially suitable for real-life applications.

Simple water balance modelling of surface reservoir systems in a large data-scarce semiarid region

Abstract Abstract Water resources in dryland areas are often provided by numerous surface reservoirs. As a basis for securing future water supply, the dynamics of reservoir systems need to be simulated for large river basins, accounting for environmental change and an increasing water demand. For the State of Ceará in semiarid Northeast Brazil, with several thousands of reservoirs, a simple deterministic water balance model is presented. Within a cascade-type approach, the reservoirs are grouped into six classes according to storage capacity, rules for flow routing between reservoirs of different size are defined, and water withdrawal and return flow due to human water use is accounted for. While large uncertainties in model applications exist, particularly in terms of reservoir operation rules, model validation against observed reservoir storage volumes shows that the approach is a reasonable simplification to assess surface water availability in large river basins. The results demonstrate the large impact of reservoir storage on downstream flow and stress the need for a coupled simulation of runoff generation, network redistribution and water use.