Featured collection introduction: Water for megacities - challenges and solutions
The Earth has entered into Anthropocene, a new epoch dominated by people. The world’s urban population has grown more than four times during the past 60 years to 3.9 billion. Today, more people are living in the cities than in the countryside in most nations. Cities are growing bigger and faster than ever before (United Nations, 2014). Cities that have a population >10 million are commonly considered as megacities as defined by UN-HABITAT (Li et al., 2015b). Globally, there are about 28 megacities with approximately 13% of the world’s urban population (United Nations, 2014). Most of these megacities are found in Asia. By 2030 the world is projected to have 41 megacities with cities in Africa and Asia growing the fastest (United Nations, 2014). Megacities face many emerging challenges, from economic development and social stability to environmental changes in the 21st Century. Obviously, many of the water resource challenges in megacities are rooted in the rapid rise in competing water demands by people for multiple uses. Water problems arise when water demand cannot be met by water supply due to either natural (e.g., surface or groundwater exhaustion), socioeconomic (e.g., financial and governance), water quality, or environmental constraints. Meeting rapidly growing water demand in megacities often means sacrificing the environment such as water quality degradation, ecosystem damage, and/or unsustainable water use such as groundwater depletion and salt water intrusion. Competing water use by irrigated agriculture, thermoelectric power generation, and industrial and residential water use are common causes of water shortages for megacities, especially in arid or semiarid regions or during extreme drought years.