Impacts of Global Change on the Hydrological Cycle in West and Northwest Africa

Human activity affects the Earth’s climate mainly via two processes: the emission of greenhouse gases and aerosols and the alteration of land cover. Climate research conducted in the past several years indicates that most of the observed increase in global average temperatures over the past few decades is very likely ¹ due to the observed increase in greenhouse gas concentrations from human activities. It is likely that without the cooling effects of atmospheric aerosols, greenhouse gases alone would have caused a greater global temperature rise than has actually been observed. Research also indicates that human influences on the climate are expected to increase in the future, mainly because greenhouse gas emissions will continue to rise. Consequently, global average surface warming is projected for the 21^st century. These projections depend largely on the scenarios used to represent greenhouse gas emissions. In general, however, the projected warming is greatest over the land and most high northern latitudes, with relatively less change over the Southern Ocean and parts of the North Atlantic Ocean. Warming is typically projected to be greater in arid regions than in humid regions. Projected precipitation increases are very likely at high latitudes, while decreases are likely over most subtropical land regions. This projected change in precipitation is a continuation of recent trends.

As previously mentioned in chapter I-1, Global Change is generally considered to consist of three components: demographic change, global environmental change, and the impacts of globalization. Strong population growth and migration are particular aspects of the ongoing Global Change in Africa. The present population growth in Africa, for example, is 2.32% p. a., compared to a worldwide increase of 1.24% p. a. between 2000 and 2005.

As discussed in chapter I-3, the climatic, ecological, and hydrologic environments in the Ouémé and Drâa catchments are very different. A scarcity of hydro-meteorological and vegetation data is common to both regions and is typical for many African catchments, both for spatial density of measurement stations and for longterm homogeneous time series. These data are necessary for process studies, trend analyses, as well as for model initialization and validation. The measurement concepts developed within IMPETUS incorporated these challenges and provided a database for knowledge-based adaptations to a changing environment.

Water is the key to sustainable development. It is necessary for domestic use, for husbandry, industry, and irrigation, and it is necessary for sustaining ecosystem functions. Water availability is, on one hand, dependent on meteorological conditions, and on the other, on terrestrial processes. According to Falkenmark and Rockström (2004), the ratio of blue to green water is determined at the boundary between the pedosphere and the atmosphere. In this concept, blue water is the visible part of the water, which is available at the surface or as soil water or groundwater. In contrast, green water is the non-visible part, which returns as water vapor into the atmosphere. Green water may be productive (transpiration) or unproductive (evaporation). Integrated Water Resource Management (IWRM) mainly concentrates on blue water, although conserving green water is an important aspect.

The anthroposphere may be defined as the part of the environment that is made or modified by humans. Put differently, the anthroposphere is the sphere of the earth system or its subsystems where human activities constitute a significant source of change through the use and subsequent transformation of natural resources, as well as through the deposition of waste and emissions. Since the end of the 18^th century, population growth and the technology advances have made humans the dominant drivers of change to the earth system as a whole and most of its subsystems (Crutzen 2002)¹. In low- and middle-income countries, human activities are still heavily based on the use of natural resources, renewable or non-renewable. The cultural habits, technological knowledge, and preferences determine the state and change of regional anthropospheres.

The overall goal of IMPETUS aimed at developing an interdisciplinary, integrative approach to mitigating region-specific risks of Global Change as they relate to the hydrological cycle for two differing catchments. The target areas were the Ouémé basin in Benin and the Drâa catchment in Morocco. Ultimately, decision makers in Benin and Morocco were given tools that have helped them analyze decision-making problems and understand the phenomena underlying these problems. This then allowed these parties to assess the impact of their decisions and to implement sustainable management options for water resources that are so vital to life. Results of these different options can be compared and evaluated for different scenarios using Spatial Decision Support Systems (SDSSs), Information Systems (ISs), and Monitoring Tools (MTs). These tools are described in Part II of this book. Target years were chosen as 2025 for Benin and 2020 for Morocco based on pre-existing, long-term government strategy papers.

Regional climate models that take land use and land cover changes into account indicate a general decrease in rainfall and prominent surface heating in sub-Saharan Africa until 2050 (see sect. II-3.2). The high population growth predicted in this period is expected to cause rapid land use changes and to strongly influence water availability and demand. In this context, the IMPETUS research project was not intended to offer a prescription of rigid options for sustainable management of the hydrological cycle. Rather, it was intended to support the decisionmaking process within project countries. For this purpose, tools have been developed that allow for the comparison and balancing of different options with the aid of Spatial Decision Support Systems (SDSSs), Information Systems (ISs), and Monitoring Tools (MTs). The intention is to help decision makers by providing them with efficient and user-friendly tools for analyzing and managing decisionmaking problems and underlying phenomena. Target areas for this research were the Ouémé basin in Benin and the Drâa catchment in Morocco. Target years were chosen as 2025 for Benin and 2020 for Morocco based on pre-existing, long-term government strategy papers.

Decision support is not only a question of mere technical assistance, but also of multiple facets. Besides scientific and organizational aspects, special attention must be paid to the societal aspects of decision support, because a decision can only be supported and taken when the social frame is considered. The different aspects of decision support and software development are presented here. Within the IMPETUS research project, the SMILE (Scientific Model Integration pipeLine Engine) framework has been developed, which integrates dynamic simulation models, information systems and monitoring tools for decision support. The developed framework is based on modern software technologies and tries to balance numerous contradicting aspects like the implementation of complex solutions with convenience of use. As examples, two Spatial Decision Support Systems, one information system and one monitoring tool are presented and discussed. The flexibility of the SMILE framework in handling different aspects of decision support is shown.

In order to investigate the effects of Global and Regional Change on water resources and related issues, it is mandatory to develop a targeted, sound, and foresighted environmental assessment at appropriate geographic scales. This assessment must integrate social, technological, environmental, economic, and demographic issues. Scenarios that include expected developments in agriculture, economy, demography, and environment have become a state-of-the-art tool in environmental assessment and management (e.g., Gaiser et al. 2003; Millennium Ecosystem Assessment of UNEP (UNEP 2005); Alcamo 2008). Scenarios are consistent and plausible images of alternative futures that are comprehensive enough to support decision-making. Scenarios are not predictions or forecasts, but alternative development routes of complex systems. They enhance the information basis for decision-making through identifying the following: (1) the most important driving forces at the national and regional level; (2) sub-regional developments or events that are of national relevance; (3) the most important inter-linkages between national and regional development; and (4) the most important knowledge gaps and unanswered questions, which point to further actions needed. A meaningful scenario analysis must estimate a certain range of plausible developments that will enable decision-makers in public policy or private entities to deduce suitable advice from the results.

Coping with the impacts of Global Change is a major challenge for the vulnerable semi-arid to arid Drâa catchment, as well as for Southern Morocco as a whole, because of its already dry conditions and limited water availability. People rely on water not only for drinking, but also to secure their livelihood by irrigation for subsistence or cash crop farming, live stock production, small enterprises, and tourism. Other environmental issues pertinent to the area include land degradation by overgrazing, soil and water salinization due to intensive irrigation agriculture and depletion of the groundwater reservoirs by overuse of groundwater for irrigation purposes. This chapter highlights examples of the ways in which Global Change may affect the future of the Drâa catchment.

IMPETUS has developed decision support tools for evaluating management options to address water-related problems for different scenarios of Global Change. The tools consist of Spatial Decision Support Systems (SDSSs), Information Systems (ISs), and Monitoring Tools (MTs). The so-called SMILE (Scientific Model Integration pipeLine Engine) framework has been developed, which integrates these components (see chap. II-2). The overall system is intended to be used by decision makers and stakeholders and is available to everyone interested scientifically or involved in water management (see the end of sect. II-2.3). This book was not intended as an instruction manual for the decision support tools. Rather, its purpose is to elucidate scientific results linked to these tools.