The green dilemma for arid cities
Greenspace contributes to both human health and wildlife conservation, but maintaining urban parks and street trees requires water. So should arid cities plant more trees in a hotter and drier world? The science of “socioecohydrology” aims to find out.
Most cities import their water from outside their city limits, sometimes over distances of hundreds of miles. Los Angeles is a famous example, beginning with the construction of the Los Angeles aqueduct in 1913, which directed runoff away from the Sierra Nevada Mountains more than 200 miles (322 km) away.
In total, Los Angeles County currently imports about 60% of its water from remote sources. Why? The region is naturally dry, receiving only 15 inches (380 mm) of rainfall a year. Ecologically, Los Angeles is a shrubland with few, if any, trees at low elevations before the city was built. From its green lawns to its palm tree-lined streets, the landscapes of LA are virtually all irrigated.
In fact, more half of the water consumption of LA is used outdoors for irrigation, with even higher proportions in the inland deserts. All over the dry, western region of the U.S., cities transfer massive amounts of water from mountain snowmelt to outdoor irrigation to sustain urban greenspace.
The case for more trees
Urban trees in dry cities are something of a paradox: the hotter and drier it gets, the more cities benefit from the shading and evaporative cooling effects of tree canopies. There’s abundant evidence that urban trees and parks can offset urban heat island effects. But trees also need water, and that water is increasingly in short supply. In 2015, the western U.S. experienced a major drought, with severely diminished snowpack in the Sierras to supply the LA aqueduct. Longer, hotter summers are becoming more common in arid regions, but so are droughts. So should we add more trees to these dry cities, or not?
Ecohydrology: the intersection of ecology and hydrology
The science of hydrology, the study of the water cycle, can help us understand how much water is available to meet urban needs. In Los Angeles, hydrologic research showed the effects of groundwater extraction on water table depth. On the biological side, ecology tells us about the water needs of urban plants and landscapes. Ecology and hydrology together are now called "ecohydrology." Our ecohydrologic studies showed that urban trees in LA account for only about 30% of landscape water consumption – the rest is attributable to lawns.
But urban water use isn’t just governed by the environment. Human water needs are also a product of water regulations and pricing, systems of government, values, and cultural and historical norms. All of these factors vary in space and time, and as a result, urban water delivery systems are a complex network of people, infrastructure, water agencies and rules.
Socioecohydrology – the study of the human-dominated water cycle – combines the social sciences, ecology, and hydrology to find ways of optimizing tradeoffs between human and ecological water needs. In Los Angeles, our team made up of water engineer Erik Porse, cultural geographer Stephanie Pincetl, hydrologist Terri Hogue, and ecologists from my lab found that LA County could reduce water imports by 30% without groundwater overdraft while still irrigating greenspace if reasonable conservation measures were put into place.
This study used a model of network complexity built by Erik Porse (Artes) to account for the many human and natural influences on water supply and demand in LA. But there are many other approaches for studying and applying socioecohydrology to help solve water problems around the world. We'll be looking at more of them here soon to find out how water-limited cities globally are solving the urban greenspace dilemma.