Calibration of paired watersheds: Utility of moving sums in presence of externalities

Historically, paired watershed studies have been used to quantify the hydrological effects of land use and management practices by concurrently monitoring two similar watersheds during calibration (pre-treatment) and post-treatment periods. This study characterizes seasonal water table and flow response to rainfall during the calibration period and tests a change detection technique of moving sums of recursive residuals (MOSUM) to select calibration periods for each control-treatment watershed pair when the regression coefficients for daily water table elevation (WTE) were most stable to minimize regression model uncertainty. The control and treatment watersheds were one watershed of 3−4 year-old intensely managed loblolly pine (Pinus taeda L.) with natural understory, one watershed of 3−4 year-old loblolly pine intercropped with switchgrass (Panicum virgatum), one watershed of 14−15 year-old thinned loblolly pine with natural understory (control), and one watershed of switchgrass only. The study period spanned from 2009 to 2012. Silvicultural operational practices during this period acted as external factors, potentially shifting hydrologic calibration relationships between control and treatment watersheds. MOSUM results indicated significant changes in regression parameters due to silvicultural operations and were used to identify stable relationships for WTE. None of the calibration relationships developed using this method were significantly different from the classical calibration relationship based on published historical data. We attribute that to the similarity of historical and 2010−2012 leaf area index (LAI) on control and treatment watersheds as moderated by the emergent vegetation. While the MOSUM approach does not eliminate the need for true calibration data or replace the classic paired watershed approach, our results show that it may be an effective alternative approach when true data is unavailable, as it minimizes the impacts of external disturbances other than the treatment of interest.