Recent Acceleration of the Terrestrial Hydrologic Cycle in the US Midwest

Most hydroclimatic trend studies considered only a subset of water budget variables; hence, the trend consistency and a holistic assessment of hydrologic changes across the entire water cycle cannot be evaluated. Here we use a unique 31year (1983-2013) observed data set in Illinois (a representative region of the U.S. Midwest), including temperature (T), precipitation (P), evaporation (E), streamflow (R), soil moisture, and groundwater level (GWL), to estimate the trends and their sensitivity to different data periods and lengths. Both the Mann-Kendall trend test and the least squares linear method identify trends in close agreement. Despite no clear trends during 1983-2013, increasing trends are found in P (8.73-9.05mm/year), E (6.87-7.47mm/year), and R (1.57-3.54mm/year) during 1992-2013, concurrently with a pronounced warming trend of 0.029-0.037 degrees C/year. However, terrestrial water storageis decreased by -2.0mm/year (mainly due to declining GWL), suggesting that the increased R is caused by increased surface runoff rather than baseflow. Monthly analyses identify warming trends for all months except winter. In summer, P (E) exhibits an increasing (decreasing) trend, leading to increasing R, soil moisture, GWL, and terrestrial water storage. Most trends estimated for different subperiods are found to be sensitive to data lengths and periods. Overall, this study provides an internally consistent observed evidence on the intensification of the hydrologic cycle in response to recent climate warming in U.S. Midwest, in agreement with and well supported by several recent studies consistently reporting the increased P, R and E over the Midwest and Mississippi River basin.