Evaluation of Paired Watershed Runoff Relationships since Recovery from a Major Hurricane on a Coastal Forest—A Basis for Examining Effects of Pinus palustris Restoration on Water Yield

The objective of this study was to test pre-treatment hydrologic calibration relationships between paired headwater watersheds (WS77 (treatment) and WS80 (control)) and explain the difference in flow, compared to earlier published data, using daily rainfall, runoff, and a water table measured during 2011–2019 in the Santee Experimental Forest in coastal South Carolina, USA. Mean monthly runoff difference between WS80 and WS77 of −6.80 mm for 2011–2019, excluding October 2015 with an extreme flow event, did not differ significantly from −8.57 mm (p = 0.27) for the 1969–1978 period or from −3.89 mm for 2004–2011, the post-Hurricane Hugo (1989) recovery period. Both the mean annual runoff coefficient and monthly runoff were non-significantly higher for WS77 than for WS80. The insignificant higher runoff by chance was attributed to WS77’s three times smaller surface storage and higher hypsometrical integral than those of WS80, but not to rainfall. The 2011–2019 geometric mean regression-based monthly runoff calibration relationship, excluding the October 2015 runoff, did not differ from the relationship for the post-Hugo recovery period, indicating complete recovery of the forest stand by 2011. The 2011–2019 pre-treatment regression relationship, which was not affected by periodic prescribed burning on WS77, was significant and predictable, providing a basis for quantifying longleaf pine restoration effects on runoff later in the future. However, the relationship will have to be used cautiously when extrapolating for extremely large flow events that exceed its flow bounds.