Study: Urbanization Has Profound Impact on Surface Hydrology

What are the consequences of human tinkering with land cover and hydrology on surrounding native ecosystems and biodiversity? This question forms the backdrop for a case study proffered by an Arizona State University research team and published in the journal BioScience, which found that one of the most profound impacts of urbanization is the "reconfiguration of surface hydrology."

According to a Sept. 3 press release, lead author John Roach, now with Simbiotic Software in Missoula, Mont., ASU professors Nancy Grimm, and J. Ramon Arrowsmith and other former graduate students mapped water resources and connectivity and tracked land-use change in the Indian Bend Watershed . The researchers found that construction of artificial lakes and canal systems along with extensive groundwater pumping have had "unintended impacts on nutrient cycling."

"As Phoenix grew from a small settlement to the large urban center it is today, it built an extensive canal network to bring water from the Salt, Verde, and Colorado rivers to agricultural fields and city taps," says Roach. "While these canals enabled farmers to grow crops in the desert, they also cut across stream channels, disrupting the flow of water and sediments from tributary networks to the main channel. In pristine streams, sandbars and other patches created where these sediments collect are often ideal places for nutrient cycling. By starving streams of their historic supply of this material, canals accidentally alter the way nutrients are cycled in stream ecosystems."

"Prior to these alterations, channel systems like those of Indian Bend Wash were ephemeral, storm precipitation-driven systems with only a limited connection to the groundwater (via loss from the channel bed)," notes Ramon Arrowsmith, professor with School of Earth and Space Exploration in ASU's College of Liberal Arts and Sciences. "Now, the surface and subsurface hydrologic network is short circuited with water entering the channel from well and canal sources, and water leaving by important evaporation, seepage, and canal redirection."

The authors emphasize how modern urban water systems shatter any limitations imposed by the topographic contours of a region. The Central Arizona Project cuts a blue swatch across the Sonoran Desert and subdivides watersheds, to deliver a reported 1.7 × 109 cubic meter per year (or 1.5 million acre-feet) of surface water to the area. In addition, the pumping of groundwater has dropped the water table 90 meters and connected surface and subsurface flows, "not only increasing the spatial and temporal availability of water, but having the unintended effect of increasing the flux of NO3 through urban waterways by returning nitrogen leached from historic fertilizer applications to surface flows."

One concern is the potential impact on riparian species, the "integrity of native ecosystems and the continued delivery of goods and services from these ecosystems."

"We were surprised by how frequently the concentration of nitrate in surface waters was determined by the turning of a tap," Roach notes. "Because the groundwater below the greater Phoenix ecosystem contains a lot of nitrate, when groundwater wells are turned on, the concentration of nitrate in the canals and streams receiving this water goes up. This nitrogen, in turn, can act as fertilizer, stimulating unwanted growth and producing changes in what the stream looks like that are independent of the decision to deliver more water to city lawns."

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