Estimation and Analysis of Baseflow in Drainage Channels in Two Tile-Drained Watersheds in Illinois

Abstract

This study was conducted to develop understanding of baseflow characteristics of tile-drained watersheds in two Illinois watersheds, namely the Big Ditch (Site A) and the Upper Embarras River (Site B) watersheds. In each of the watersheds, a channel section was selected, and rectangular cutthroat flumes were installed at upstream and downstream ends in the channel sections. The stream segment at Site A had two tile outlets draining into it. For Site B, there was no tile draining directly to the stream section. Flow rates were measured at each of the flumes and tile outlets at 15 min intervals. Observation wells were installed on both sides of the channel section at each site to monitor the water table elevations and to estimate baseflow to the channels. From all the stream flow measurements at both sites, hydrographs were developed for storm events of different intensities. In total, 31 hydrographs were studied. A mass balance analysis was conducted to estimate the baseflow volume per unit length of the channels. The Dupuit equation was also applied to determine baseflow at three transect locations in the channel sections for both sites during the study period. The mass balance analysis revealed that when tile outlets are present (Site A), the average fraction of baseflow was about 93% and the average percentage of tile flow was about 7% of the total flow contribution within the channel section. For the channel section at Site B (with no tile outlets), only baseflow contributed to the total flow contribution within the channel section. The average baseflow per unit length of the stream segment at Site A was 3.59E-04 m2 s-1, and that for Site B was 2.07E-04 m2 s-1 during storm events. It was observed that baseflow rates were dependent on precipitation and antecedent moisture condition; flow rates at Site A were higher than those at Site B. Differences in soil types also affected the baseflow rate at each of the sites.