Around 80% of the population lives in urban areas in the United States, and urban or semi-urban watersheds contribute to up to 8% of the water supply in US cities. Since a large proportion of surface water flows through urban watersheds, it's important to study these drainage basins in rapidly growing urban areas.

The researchers performed Mann-Kendall trend analysis on monthly precipitation (P), runoff (Q) and Q/P data from 1950–2009 obtained from 62 urban watersheds covering 21 major urban centres in the US. The results indicate that only five out of 21 urban centres in the US showed an uptrend in precipitation, while the remaining urban centres did not show any significant trend.

12 urban centres showed an uptrend in Q/P. However, six urban centres did not show any significant trend in Q/P and three urban centres showed a significant downtrend. The highest rate of change in precipitation, runoff, and Q/P was observed in the Houston urban watershed.

Results from this study also indicate that while a human-only influence is observed in most of the urban watersheds, a combined climate and human influence is observed mostly in the central United States.

This study is part of the US Department of the Interior's WaterSMART (Sustain and Manage America's Resources for Tomorrow) Program, whose mandate is to help water managers plan for climate change and other threats to water supplies, and take action to secure water resources for the communities, economies, and ecosystems they support. Further studies should focus on quantifying relative climate and human influences on each urban centre and their impact on water availability.

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