This volume analyzes past, current, and future conditions in ten river basins from six continents, each located in arid or semi-arid climates, equipped with engineered dams designed to increase year-round water availability for irrigated agriculture, cities, and environmental flow; and each important for world food production and basin economies. This introductory chapter details the history of the inquiry and the research project. In the Challenge section of the book, we ask how water supply and demand are changing as a result of climate change, reservoir sedimentation, depletion of groundwater, and declining environmental flows. We then present case studies of each of the selected rivers: How do they recognize challenges and how do they deal with them? In the Response section, we discuss three important options for improved water management: water-wise irrigated agriculture, carefully designed inter-basin water transfers, and strong stakeholder participation.

The multi-year research effort in this volume has demonstrated that (1) similar challenges are faced by all SERIDAS rivers, (2) problem recognition varies widely across basins, and (3) response strategies are not sufficiently grounded in the long-term goal of river sustainability. We recommend that river managers and stakeholders follow these guidelines: Determine the river’s dependable yield – the water supply, averaged over the period of the most severe drought experienced in the historical record. Use the dependable yield to define and implement a river management strategy that carefully balances human and ecological needs in the basin. To meet human needs, do more with less – practice water conservation. This is the best obtainable strategy for redefining and maintaining river sustainability.

This chapter explores the connection between groundwater and surface water. This is an important, but often overlooked, component of managing engineered rivers in arid lands. In order to make good management decisions, decision makers must understand the composition of water supplies and hydrological connections at work in their basin, including what portion of the overall water budget is supplied by groundwater, both in terms of availability and usage. Data gaps may impede this understanding in certain river basins. Directing policy and resources to meeting this information need should be a key priority.

The Jucar Basin faces the challenge of meeting an enormous demand for irrigation while water quality degrades from urban, industrial, and agricultural pollution. Relying on engineering solutions is not enough. Empirical evidence in Jucar indicates that water markets and institutional policies seem to deal with water scarcity more successfully than water pricing and irrigation subsidies. A first water governance priority is to convince farmers of substituting freshwater for the available urban recycled water. Second, seawater desalination plants must be upgraded so they will work at full capacity. More long-term governance goals are to curtail surface irrigation diversions and groundwater extractions, and reallocating water to urban, industrial, and environmental uses. These reforms will only work if they get the support and cooperation of farmers by compensating them for the reallocation of water from agriculture to other sectors.

The final chapter brings all the book’s threads together. It explores the similarities and differences between the Anthropocene defaunation and the ancient mass extinctions. It investigates how much time we have before we reach the extinction levels of the Big Five mass extinctions. It has been suggested that we should just let the defaunation run its course; but the consequences of that course of action appear dire. But all Is not lost. The fossil record shows that the Earth System is resilient, and if we quickly act to remove the climatic forcings we are creating, we have time to turn the situation around and avoid sliding into another full-blown mass extinction.

The chapter analyzes upstream-downstream issues in the Nile river basin. The large increase in population in the future and the resulting push for development will increase demand of the Nile flow in upstream riparian countries where headwaters are sourced. Variability in temperature and precipitation due to climate change will impact the heavily populated areas the most, which in the case of the Nile, is at the Delta in Egypt. This will have a deep economic impact as Egypt is the most prosperous country along the Nile. However, the increased development and engineering upstream will affect how much of the Nile is available at a specific time for Egypt. These tensions are already seen in the construction of the Grand Ethiopian Renaissance Dam. An agreement still has not been reached as countries are struggling with sharing ownership of the river. If sufficient flow is not released to Egypt, saltwater intrusion in Egypt can endanger current groundwater storage, pushing the country to invest in more desalination.

Agriculture, as the primary user of the world’s water resources, bears great promise in coping with water scarcity. Agriculture’s sub-sectors, such as plant production, livestock, and aquaculture, can significantly improve their practices that can lead to water savings, conservation of natural resources, climate change benefits, and co-benefits and can still increase levels of productivity and production to cater for a growing population. Accomplishing this requires a major change in the way we manage our water, land, and soil resources. We underline the key benefits of holistic approaches such as IWRM, nexus, and integrated landscape management, and provide examples from replicable practices.

Between 50 thousand and 10 thousand years ago many of the large vertebrate animals, the megafauna, went extinct on all continents except Africa. There the elephant, big cats, hippo and others are remnants of a megafauna that once occupied the entire globe. This is the youngest mass extinction in the fossil record and there have been several suggestions for what caused it: climatic instability as the planet emerged from the last ice age; the appearance of the first modern human hunters; and climatic instability driven by the impact of a comet. There is considerable scepticism about the impact scenario, so most workers fall into one of two camps – either climate change or humans were wholly responsible for the extinction. However, there is a growing body of opinion that supports the idea that neither climate nor the presence of humans on their own were enough to trigger the megafaunal extinctions: some combination of the two is needed.