Alfred Becker

Development and test of a spatially distributed hydrological/water quality model for mesoscale watersheds

Abstract The new watershed model SWIM was developed in order to provide a comprehensive GIS-based tool for hydrological and water quality modelling in mesoscale watersheds (from 100 to 10 000 km 2 ), which can be parametrized using regionally available information. SWIM is based on two previously developed tools—SWAT and MATSALU. The model integrates hydrology, vegetation, erosion and nitrogen dynamics at the watershed scale. SWIM has three-level disaggregation scheme and is coupled to the Geographic Information System GRASS. A robust approach is suggested for the nitrogen modelling in mesoscale watersheds. Model test and validation were performed sequentially for hydrology, crop growth, nitrogen and erosion in a number of mesoscale watersheds in the German part of the Elbe drainage basin. Firstly, the hydrological module was tested in five watersheds of different size with different topography, soil and spatial resolution of input data. After that the test was performed for the crop module in the state of Brandenburg, for the nitrogen module in the rural basin of Stepenitz, and for the erosion module in the mountainous Mulde basin. A comprehensive scheme of spatial disaggregation into subbasins and hydrotopes combined with reasonable restriction on the subbasin size allows the assessment of water resources and water quality to be performed with modest data requirements at the regional scale. The direct connection to land use and climate data provides a possibility to use the model for analysis of climate change and land use change impacts on hydrology and water quality.

Biospheric aspects of the hydrological cycle; preface

Abstract The Core Project Biospheric Aspects of the Hydrological Cycle (BAHC) of the International Geosphere Biosphere Programme (IGBP) addresses the biospheric aspects of the hydrological cycle through experiments and modelling of energy, water, carbon dioxide and sediment fluxes in the soil– vegetation–atmosphere system at a variety of spatial and temporal scales. Active regulation of water, energy and carbon dioxide fluxes by the vegetation make it an important factor in regulating the Earth’s hydrological cycle and in the formation of the climate. Consequently, human induced conversion of vegetation cover is an important driver for climate change. A number of recent studies, discussed in this paper, emphasise the importance of the terrestrial biosphere for the climate system. Initially, these studies demonstrate the influence of the land surface on tropical weather and climate, revealing the mechanisms, acting at various scales, that connect increasing temperatures and decreasing rainfall to large-scale deforestation and other forms of land degradation. More recently, the significance of the land surface processes for water cycle and for weather and climate in temperate and boreal zones was demonstrated. In addition the terrestrial biosphere plays a significant role in the carbon dioxide fluxes and in global carbon balance. Recent work suggests that many ecosystems both in the tropics and in temperate zones may act as a substantial sink for carbon dioxide, though the temporal variability of this sink strength is yet unclear. Further, carbon dioxide uptake and evaporation by vegetation are intrinsically coupled, leading to links and feedbacks between land surface and climate that are hardly explored yet. Earth’s vegetation cover and its changes owing to human impact have a profound influence on a lateral redistribution of water and transported constituents, such as nutrients and sediments, and acts therefore as an important moderator of Earth’s biogeochemical cycles. In the BAHC science programme, the importance of studying the influence of climate and human activities on mobilisation and river-borne transport of constituents is explicitly articulated. The terrestrial water and associated material cycles are studied as highly dynamic in space and time, and reflect a complex interplay among climatic forcing, topography, land cover and vegetation dynamics. Despite a large progress in our understanding of how the terrestrial biosphere interacts with Earth’s and climate system and with the terrestrial part of its hydrological cycle, a number of basic issues still remain unresolved. Limited to the scope of BAHC, the paper briefly assesses the present status and identifies the most important outstanding issues, which require further research. Two, arguably most important outstanding issues are identified: a limited understanding of natural variability, especially with respect to seasonal to inter-annual cycles, and of a complex ecosystem behaviour resulting from multiple feedbacks and multiple coupled biogeochemical cycles within the overall climate system. This leads to two major challenges for the future science agenda related to global change research. First, there is a need for a strong multidisciplinary integration of research efforts in both modelling and experiments, the latter extending to inter-annual timescales. Second, the ever increasing complexity in characterisation and modelling of the climate system, which is mainly owing to incorporation of the biosphere’s and human feedbacks, may call for a new approach in global change impact studies. Methodologies need to be developed to identify risks to, and vulnerability of environmental systems, taking into account all important interactions between atmospheric, ecological and hydrological processes at relevant scales. With respect to the influence of climate and human activities on mobilisation and river-borne transport of constituents, the main issues for the future are related to declining availability and quality of ground data for quantity and quality of water discharge. Such assessments as presented in this paper, in combination with community wide science evaluation, have lead to an update of the science agenda for BAHC, a summary of which is provided in the appendix.

Regionalisation of the base flow index from dynamically simulated flow components — a case study in the Elbe River Basin

Abstract This study investigates possibilities for the regionalisation of flow components within large river basins. One main purpose is the derivation of an empirical relationship for the estimation of the average base flow index (i.e. the ratio of base flow to total flow, BFI), which can be included into simplified models in form of decision support systems aimed at rapid integrated water resources assessment. Based on results from the hydrological models ARC/EGMO and HBV in 25 mesoscale sub basins of the macroscale Elbe River Basin, the modelled BFI is regionalised for the whole German part of the Elbe Basin, divided into 114 subbasins. For regionalisation of the average BFI multiple regression (MREG) and two geostatistical approaches (ordinary kriging (OK) and external drift kriging (EDK)) are applied using different physical catchment attributes and observed climate data as independent variables. The average base flow index is strongly related to topographical, pedological, hydrogeological and precipitation characteristics and less influenced by land cover/land use properties of the catchments. As indicated by cross validations, both EDK and MREG are suitable for regionalising the BFI with coefficients of determination of 0.80 and 0.77, respectively. Furthermore, the plausibility of the estimated BFI values is shown by comparisons with parameters calculated from statistical and fractal analysis of daily flow time series, which represent a strong relation to the regionalised BFI values.

Integrated modelling of hydrological processes and nutrient dynamics at the river basin scale

The paper presents a generic modelling approach for modelling nutrient transfer from soil to surface water in mesoscale river basins. The approach is tested in application of the spatially-distributed coupled hydrological / water quality model SWIM for modelling hydrological processes and nitrogen dynamics in the upper Stepenitz basin (subbasin of the Elbe, 575 km2). Both hydrological and hydrochemical validations of the model were successful for this basin. The differences in nitrogen cycling for different soils revealed by the simulation provide a basis for further scenario evaluation and eventually for recommendations for nonpoint source pollution control at the river basin scale.

Mesoscale ecohydrological modelling to analyse regional effects of climate change

Hydrological processes and crop growth were simulated for the state of Brandenburg (Germany) using the hydrological/vegetation/water quality model SWIM, which can be applied for mesoscale river basins or regions. Hydrological validation was carried out for three mesoscale river basins in the area. The crop growth module was validated regionally for winter wheat, winter barley and maize. After that the analysis of climate change impacts on hydrology and crop growth was performed, using a transient 1.5 K scenario of climate change for Brandenburg and restricting the crop spectrum to the three above mentioned crops. According to the scenario, precipitation is expected to increase. The impact study was done comparing simulation results for two scenario periods 2022–2030 and 2042–2050 with those for a reference period 1981–1992. The atmospheric CO2 concentrations for the reference period and two scenario periods were set to 346, 406 and 436 ppm, respectively. Two different methods – an empirical one and a semi-mechanistic one – were used for adjustment of net photosynthesis to altered CO2. With warming, the model simulates an increase of evapotranspiration (+9.5%, +15.4%) and runoff (+7.0%, +17.2%). The crop yield was only slightly altered under the “climate change only” scenario (no CO2 fertilization effect) for barley and maize, and it was reduced for wheat (−6.2%, −10.3%). The impact of higher atmospheric CO2 compensated for climate-related wheat yield losses, and resulted in an increased yield both for barley and maize compared to the reference scenario. The simulated combined effect of climate change and elevated CO2 on crop yield was about 7% higher for the C3 crops when the CO2 and temperature interaction was ignored. The assumption that stomatal control of transpiration is taking place at the regional scale led to further increase in crop yield, which was larger for maize than for wheat and barley. The regional water balance was practically not affected by the partial stimulation of net photosynthesis due to higher CO2, while the introduction of stomatal control of regional transpiration reduced evapotranspiration and enlarged notably runoff and ground water recharge.

Runoff Processes in Mountain Headwater Catchments: Recent Understanding and Research Challenges

Runoff generation in mountain catchments is one of the most complex hydrological processes. It is highly variable in space and time, depending on the combination of three main controlling factors: (1) climate, (2) soil and geology, and (3) vegetation. The different combinations of these three factors determine the water balance of landscape units, including soil moisture dynamics, evapotranspiration and runoff generation. When assessing runoff generation, not only the runoff amounts need to be considered, but also the relative streamflow contributions of surface and subsurface runoff, which may differ considerably between areas (Buttle 1998). An overview of runoff mechanisms and components in different environments is given in Uhlenbrook and Leibundgut (1997) and Bonell (1998). The main focus of this paper is on subsurface stormflow, the least understood flow component.

Regional analysis of global change impacts: Concepts, tools and first results

The paper introduces an approach for the analysis of global change impacts on river basins or regions. This approach is quite general and can be transferred to any region or river basin of interest on earth. The first application of the approach was in the Elbe river basin, with primary focus on the hydrologic model part and on the integration of crop growth and nitrogen dynamics. Finally, concepts for the integration of socio-economic aspects in the analysis are introduced.

Global Change and Mountain Regions — an IGBP Initiative for Collaborative Research

Mountain regions are of special importance for global change research. Due to the strong altitudinal gradients many mountain regions provide unique opportunities to detect and analyse global change processes and phenomena. Therefore, integrated interdisciplinary collaborative research activities are suggested to be implemented globally in a well coordinated way to understand, model and predict environmental change processes in mountain regions and, where needed, make proposals towards sustainable land, water and resources management. The required research is suggested to be structured around four activities and a number of specific tasks to be briefly described in the following. Moreover, suggestions will be made for the implementation and international coordination of the research.

Responses of Hydrological Processes to Environmental Change at Small Catchment Scales

Chapter D.2 deals with fundamental hydrological processes and their modelling at “small catchment scales”. We specifically define such catchments as having areas from ~ 10-1 km2 to 103 km2, known as the hydrological micro- to meso-scale. Since the exchange processes between the land surface and the atmosphere (energy, water etc.) at small scales are already treated in Chapt. A.2, the primary focus in this chapter is on so-called “wet hydrology”, i.e. soil moisture dynamics, runoff generation and resulting lateral flows of water and associated transports of Sediments, chemicals and nutrients. The processes at and below the land surface in soils and aquifers represent an important part of the terrestrial phase of the hydrological cyde and associated biogeochemical cycles.

Conclusions: Scaling Relative Responses of Terrestrial Aquatic Systems to Global Changes

From an even cursory reading of the material presented in Part D, it should be quite apparent that terrestrial aquatic Systems encompass a broad set of biogeophysical landscape features and complex processes. Terrestrial aquatic Systems include water, waterborne material, sediment and biota in Vegetation, the soil unsaturated zone, groundwaters, wetlands, rivers, lakes and artificial water bodies such as reservoirs and canals. The fundamental drivers of water circulation and related material fluxes (for nutrients, carbon, particulate matter, pollutants) are multiple and combine physical, chemical and biological processes including open water evaporation, precipitation, infiltration, water runoff generation, water routing, erosion, leaching, weathering, silting, evapotranspiration, biological uptake and bacterial degradation. Together with their associated coastal zones, terrestrial aquatic Systems constitute what we define as Continental aquatic Systems (CAS) (Fig. D.94).