Water, Development, and Migration
QUANTIFYING THE COST OF DAY ZERO–LIKE EVENTS Building on the lessons from prior studies, a new approach was developed to analyze the economic impact of day zero-like water scarcity events in cities. To do so, a global database was assembled that links urban areas with their specific water sources. This unique data set from the Nature Conservancy and McDonald (2016) links global cities with the locations from which they receive their municipal water supplies. It gives the location and name of water sources for more than 500 medium and large cities in the world. The water points identified by the Nature Conservancy and McDonald (2016), and the cities they link to, are used to identify when cities are likely to be experiencing a water supply shock. This is done by overlaying a global database of historical weather from Matsuura and Willmott (2018) on top of the water points data. By looking into the past and examining what the typical rainfall is around these water points, it is possible to identify years in which rainfall is significantly below average. When several of these years are stacked together, it becomes likely that the water points are experiencing a prolonged drought, and the city is therefore facing a water shortage. Thus, the same “rainfall shocks” that are used in prior chapters can identify urban “water shocks,” where cities are likely experiencing reduced water supplies. Details of how these water shortages are identified are explained in box 4.1. Finally, econometric analysis is used to estimate the impact of these shocks on city-level growth.
BOX 4.1: The Resilience of Urban Water Systems Urban water systems around the world are designed to cope with droughts of varying frequency, lengths, and severity. It is neither practical nor affordable to design a system capable of providing water through any possible length of drought, so water supply systems are typically planned to meet a design standard, expressed as a return period (for example, maintain supplies without any restriction on use through a drought with a return period of 1 in 50 years) (Watts et al. 2012). There is no formula or standard engineering prescription to setting the “design” drought. The standard comes down to a typical risk-based trade-off between what the water utility or community in question is prepared to accept in terms of the frequency, severity, and duration of water use restrictions associated with drought and what they are prepared (or able) to pay to avoid these restrictions (Erlanger and Neal 2005). Although the length of the “design” drought that urban water systems can handle varies depending on context, worldwide experiences show that long droughts lasting three or more years are typically more taxing for water supply systems. As shown in box continues next page
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