There has so far been no shared understanding of validity in agent-based simulation. We here conceptualise validation as systematically substantiating the premises on which conclusions from simulation analysis for a particular modelling context are built. Given such a systematic perspective, validity of agent-based models cannot be ensured if validation is merely understood as an isolated step in the modelling process. Rather, valid conclusions from simulation analysis require context-adequate method choices at all steps of the simulation analysis including model construction, model and parameter inference, uncertainty analysis and simulation. We present a twelve-step protocol to highlight the (often hidden) premises for methodological choices and their link to the modelling context. It is designed to aid modelers in understanding their context and in choosing and documenting context-adequate and mutually consistent methods throughout the modelling process. Its purpose is to assist reviewers and the community as a whole in assessing and discussing context-adequacy.@en

Water is a theme that fits very well on the critical interface between nature and society. To fully understand the impact of changes in water supply and demand, one needs to integrate knowledge from fields as diverse as meteorology, hydrology, soil and vegetation science, medicine, economy, law, and anthropology. Our ability to deal in a scientifically sound way with all factors and feedbacks that affect the hydrological cycle is still limited. Given the extent to which our societies are dependent on sufficient, good quality water, it is quite clear that it is necessary to analyse this multi-faceted theme in a comprehensive manner. This holds especially true in the Anthropocene with its fast changes in climate, land use, trade structures, and population distribution. Fortunately, an awareness of the necessity of integrated water science is growing rapidly as can be seen from recent global and regional initiatives, projects, and dialogues. Without attempting to be complete, we mention the Joint Water Project of the four Global Change Programs, UNESCO's HELP, the Challenge Program on Food and Water of the CGIAR, the Global Water Partnership, and the Dialogue on Water and Climate. Early on, the German Federal Ministry of Education and Research (BMBF) helped to start this trend through the GLOWA program. GLOWA, a German acronym for Global Change and the Hydrological Cycle, seeks to develop simulation tools and instruments, that allow the implementation of strategies for sustainable and future-oriented water management at the regional level (river basins of approximately 100,000km2). The objective is to predict the impact of global change, in its broadest sense, on all aspects of water availability and use. Five projects have been approved within the GLOWA program that study a total of six watersheds, aligned roughly along a North-South gradient: Elbe and Danube in Germany, Dra and Jordan around the Mediterranean, and Ouémé and Volta in West Africa. In this chapter, we focus on experiences within the GLOWA Volta Project. The watershed of the Volta is one of the poorest areas of Africa. Despite the presence of some precious mineral resources, average annual per capita income is estimated in the region at US $600 per year. The basin covers 400,000 km2 with 42% in Ghana, 43% in Burkina Faso and the remainder in Côte d'Ivoire, Mali, Togo, and Benin Figure 3.1.1). Rainfed and irrigated agriculture is the backbone of the largely rural societies and the principle source of income. Population growth rates exceed 2.5%, placing increasing pressure on land and water resources. Improved agricultural production in the West African Savannah depends on the development of (near) surface water resources and their effective use. Such water development programs will have an impact on the availability of downstream water resources, in particular on those of the Volta Reservoir. The Volta Reservoir has the largest surface of any manmade lake in the world and provides over 95% of the electricity in Ghana. Especially the more urbanized South depends to a large extent on this energy source for its economic development. Precipitation in the region is characterised by large variability, as expressed in periodic droughts. Unpredictable rainfall is a major factor in the economic feasibility of hydraulic development schemes, as witnessed by the major 1984 drought and the power shortages that plagued Ghana in 1998. Early results from the GLOWA Volta project show a strong dependence of rainfall on the state of the land surface. Changing land use and land cover is the main global change phenomenon within the basin (van de Giesen et al. 2001). To understand all aspects of the hydrological cycle in the Volta Basin one needs to take all these physical (atmosphere, land, water) and social aspects (population, economic development, institutions) into account. The major scientific challenge of the GLOWA Volta Project is, therefore, the integrated analysis of the physical and socio-economic factors that affect the hydrological cycle in the Volta Basin. The main content of this article concerns the experience and results of integrative research in GLOWA Volta. In the remainder of this article, we first give a general overview of the project structure, followed by a more in-depth discussion of two specific new methodologies and conclusions.

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