Investigating hydrologic trends and attributing climate and human influence on major urban watersheds in the United States

This study investigates the long-term trends in precipitation, runoff and runoff coefficient in major urban watersheds in the United States. This study is important because we do not know for sure if there are trends in precipitation and runoff in the urban watersheds and also do not understand the influence of climate and humans on urban hydrology. Around 80% of the Unites States population lives in urban areas; urban or semi-urban watersheds contribute up to 8% of the urban water supply in the United States. Furthermore, a large proportion of surface water flows through urban watersheds. The seasonal Mann-Kendall trend test was performed on monthly precipitation, runoff and runoff coefficient data from 1950 to 2009, applied on 62 urban watersheds covering 21 major urban centers in the United States. The results indicate that only 5 out of 21 urban centers in USA showed an uptrend in precipitation, while the rest of the urban centers did not show any significant trend. Twelve urban centers showed an uptrend in runoff and runoff coefficient. However, 6 urban centers did not show any trend in runoff coefficient, and 3 urban centers showed a significant downtrend. The highest rate of change in precipitation, runoff and runoff coefficient was observed in the Houston urban watershed. To understand climate and human influence on urban watersheds, we attributed plausible causes for the trends in precipitation and runoff coefficient. Our analysis indicated that while a human only influence is noted in most of the urban watersheds, a combined climate (increase in precipitation) and human influence (increase in runoff coefficient) is observed in the most urban watersheds in the central United States. Climate only influence was found only in one urban watershed. However, discriminating and quantifying the relative contributions of climate and human influences on each urban center requires further investigation.