Introduction - Challenges and Limitations in Global Water Research
Lucas Menzel, University of Heidelberg, Germany
Current discussions on environmental problems (including the pollution and the increasing scarcity of water resources) often have a narrow perspective on climate change or are consciously or unconsciously beamed towards climate change which frequently provokes a diffuse feeling of helplessness towards a global development which hardly can be kept under control. In fact, the impact of mankind on hydrological systems has come up to global dimensions, but an analysis of related problems requires a broader and balanced view on processes of global change (including, inter alia, climate change). If we necessarily want to analyse the many interdependencies (e.g., between rising living standards, water use and climate variability) in greater detail however, we identify two major obstacles: First, our knowledge of the feedback of a changed water system on the biosphere and on mankind is largely unknown or in its infancy. For example, hydraulic engineering allows the control of about 50% of the water to which humans have access. The impact of the many impounded rivers and lakes, the irrigated areas as well as the inter-basin transfers of water on the regional and global climate conditions can hardly be estimated. Second, we are lacking (long-term) observation data series and station networks which are required to construct a global picture of the climatic conditions and the water cycle.
The challenges and possible threats related to global change also affect hydrological research which currently undergoes a change in focus, driven by increasing global and complex water problems. Therefore, hydrology is increasingly confronted with and coupled to interdisciplinary research questions. This requires an integrated consideration of both physiographic and climatic effects which determine hydrological processes and water availability as well as socio-economic drivers which govern human water use. This includes the development of a new generation of global models to determine large scale water availability, to simulate water use behaviour and to apply scenarios of global change for the estimation of future world water conditions. Results from a number of investigations with such global water models (in combination with scenarios of global change) show that the growth and the economic development of human population will probably have the strongest impact on future water stress conditions. Climate change will lead to an aggravation of the water situation, in some regions however it could also contribute to an alleviation of water scarcity (note however that the adverse impact of climate change on other sectors, such as on human health, is not considered in these types of investigations).
It is obvious that the 21st century will see heavy water crises (water scarcity, drought) as a consequence of population growth, increasing water demand and climate change. However, the investigations with global water models demonstrate that the different scenarios lead to a wide range of possible water futures, thus introducing high uncertainties. This makes it difficult to come to decisions regarding possible water management options. In addition, the scenarios as well as the global models introduce a number of (partly unacceptable) shortcomings and uncertainties: For example, the data used to drive the models are constructed from various data sources, and since observed data are scarce and incomplete over many regions on earth, certain techniques (such as gap-filling methods, the selection of auxiliary information, data homogenizations and spatial interpolation techniques) need to be applied in order to generate a complete data set. Therefore, it is difficult to reproduce small-scale structures and processes with this type of data. A consequence of these findings is that a global view on water resources and possible water futures is required but that we also need a continuation of small-scale investigations on hydro-meteorological processes in different (hot spot-) regions in order to better understand global change processes and to improve our global investigations.