At the continental U.S. scale, a lot of research has focused on hydrologic change in the latter half of the 20th century. On the one hand, this period has experienced a significant increase in concentrations of atmospheric greenhouse gases, and its relations to intensified hydrologic cycle also appeals to scientists. On the other hand, a large number of stream gages are available to provide flow data in this period. Previous findings seemed consistent in that stream flow (moderate and low flow) significantly increased, and was dominated by climate (precipitation). However, most previous studies were based on Hydro-Climate Data Network (HCDN) dataset with a limited period of ~1950s?1988.

Recently, Prof. XU Xianli in Institute of Subtropical Agriculture, Chinese Academy of Sciences(ISA) and his colleagues in both US and China, revisited the Continental US Hydrologic Change in the latter half of the 20th Century, using international Model Parameter Estimation Experiment (MOPEX) dataset with a restricted period of ~1950s?2000 for 302 watersheds (gages) across continental US. This dataset is more representative of the latter half of the 20th century than HCDN. In contrast with previous studies in which 50% or more of the studied watersheds presented significant change in flow, Prof. XU and his colleagues found that only 20–30 % of watersheds present increasing trends in flow, and most (> 65 %) watersheds presents non-significant trends. Similar to previous studies, the watersheds with increasing trends in streamflow and baseflow are concentrated in Midwest and high plain (North-Central area) of USA. Climate contributes more to streamflow change (61±25 % vs. 39±25 %) but slightly less to baseflow change than human activity (49±26 % vs. 51±26 %) and much less to baseflow index change than human activity (?5±61%vs. 105±61%). A step change at ~1971 in streamflow and baseflow was found for 35–45% but not for a large proportion of watersheds.

This study provides new insights on the latter half of the 20th century’s hydrologic cycle for the continental US. Based on different datasets, they found some quite different results from previous studies, and it demonstrates that data sources matter in identifying hydrologic change and its causes.

The study entitled " Revisiting Continental U.S. Hydrologic Change in the Latter Half of the 20th Century” has been published on http://link.springer.com/article/10.1007%2Fs11269-013-0411-3.(Water Resources Management , Volume 27, Issue 12, pp 4337-4348).