Mapping the future of water supply

TEMPE, ARIZONA // Imagine you are a city planner in 2025. The population of your desert city has grown at a steady three per cent annually for the past 15 years, dramatically increasing urban sprawl. Meanwhile, climate change has boosted temperatures, reducing the amount of water flowing into two river systems that supply the city. How will you keep the taps running? That is the kind of scenario confronting scientists and city planners inside the hi-tech Decision Theater drum at Arizona State University (ASU). They plug variables into the WaterSim, an interactive, multi-panel computer programme that gauges water supply in response to climate change, drought, population growth and changes in government policy. Imagine meteorologists are predicting a three-year drought. Can city officials make up the shortfall in supply by repairing chronic leaks in the city pipes? Michael Tschudi, who helped develop the WaterSim, adjusts the variables with the click of a mouse, watching red dials swing back and forth to indicate shifting supplies. The programme is highly flexible, able to take into account minute swings in temperature, changes in groundwater runoff and sudden population growth. Say the levels of the Colorado River drop precipitously: will Phoenix be able to draw enough water from underground reservoirs or should city officials impose regulations, such as a ban on private swimming pools? "We can plug in various options and see which gets us to where we need to be," Dr Tschudi said. The WaterSim is one of a host of projects at ASU's Decision Center for a Desert City, an institute trying to improve the sustainability of desert communities. Though much of their work is focused on the western US and Arizona in particular, the centre is attracting the attention of desert communities around the world, including in the Gulf. In October, it was awarded the Prince Sultan Bin Abdulaziz International Prize for Water, a biannual cash award from the environmental research centre at King Saud University in Riyadh. "The main thrust of our programme is water," said Patricia Gober, the centre's director. "We ask the question: How do we use our scarce recourses in the face of climate change and population growth?" As anyone living in the UAE knows, desert living presents special challenges and requires careful planning for the future. The centre, a part of ASU's school of sustainability, has helped city planners in Phoenix design new neighbourhoods, and draw up building permits that can accommodate the rapidly growing population. Now home to 4.2 million, it is estimated Phoenix could have 9m residents by 2050. It has traditionally been a city of single-family homes, many with an irrigated green lawn and a swimming pool out back. That means about two-thirds of the water used in Phoenix goes toward outdoor use - a rate that cannot continue if the city's population and temperatures continue to rise. The centre also studies the so-called "urban heat island" - a night-time phenomenon where cities retain heat absorbed from the Sun, making it less comfortable for residents and prompting them to use higher amounts of energy and water. Urban areas retain heat longer because of the prevalence of building materials such as asphalt and cement, which cool slowly. With 70 per cent of the developed world living in cities, finding ways to keep urban areas cooler will be crucial to reducing energy use, the scientists said. The effect of the heat island can be dramatic: rapid urbanisation in Phoenix over the past half-century has raised night-time temperatures by more than five degrees Celsius. Subhrajit Guhathakurta, an associate professor at ASU's school of planning, has calculated that water use in Phoenix has increased two per cent for every one per cent increase in night-time temperature, a staggering amount for a city where water is scarce. "One of the main ways to mitigate the heat island effect is with irrigated lands, but that takes more water," Ms Gober said. "The question is always: Where do we get the most bang for our buck?" Comparing weather data and detailed maps of new home constructions over the past 15 years, scientists at the centre were also able to map out which type of developments raised temperatures the most. Urban planners are now looking at zoning only for higher-density home developments for Phoenix, perhaps built around public swimming pools. They are already irrigating public parks and golf courses with untreated water, and offering tax incentives for families who replace their lawns with native desert plants. ASU scientists are also helping city planners in Phoenix understand how building materials affect the urban heat island. Harvey Bryan, of the ASU school of design within the school of sustainability, and his research assistants use infrared cameras to examine how different building materials retain heat. Dark asphalt and certain forms of cement glow white on the infrared screen, even hours after the Sun has set. In addition to Phoenix, Prof Bryan and his team have examined the cities of Las Vegas, Chicago and London, comparing satellite images with pictures shot from helicopters and infrared visuals taken at the pedestrian level. "At each level we get new data about how the various materials cool off," he said. They find that each city, because of its distinct environment, has a unique urban heat island and a unique solution to its problem. It is often simple solutions - not costly new technology - that can make the difference. In Chicago, for example, Prof Bryan has recommended the city install rooftop gardens, which appear to have a tremendous cooling affect in the northern, relatively humid climate. "The mayor likes the plan so much he is talking about making roof gardens a requirement on new buildings," Prof Bryan said. But that programme would not work as well in desert climates, where the lack of moisture means roof gardens would have to be watered. In desert cities such as Phoenix, he often recommends city planners pave their streets less deeply, and use lighter shaded pavement, which would reflect the Sun's rays. "If we continue to put in more pavement, we have to consider the impact on temperature," he said. "We believe we can bring down city temperatures by reducing the amount of dark asphalt." Ms Gober said the economic downturn in the United States, which has brought a real estate boom in Phoenix to a grinding halt, may have a silver lining from an environmental standpoint. "One positive thing that can come out of the recent housing crisis is that we take a breath and look where we are going and whether it is sustainable," she said. "We should take this moment to think about what kind of city we want to build." gpeters@thenational.ae