Using Soil Moisture Profile Event and Recession Dynamics from Multiple Landscape Positions to Constrain Groundwater Recharge Occurrence in a Forested, Seasonally-Water Limited Watershed in the Southeastern United States

Abstract:

Recharge is an important process that sustains stream baseflow, supporting water resources for humans and ecosystems. Recharge occurrence was quantified from soil moisture dynamics from four soil moisture profiles with different landscape positions at the Panola Mountain Research Watershed (PMRW), a 41-hectare forested, seasonally-water limited catchment in the Southeastern United States. Water content was monitored at three depths (15, 40, and 70 cm) for four landscape positions: a steep hillslope, a moderate hillslope, a toe slope, and a flat lowland. Profile data periods range from 0.8 to 11.8 years. Storage was calculated assuming a 1 meter depth. Groundwater recharge was estimated for events exhibiting drainage at 70 cm depth as the difference in soil moisture storage between the event maximum and either the start of the next event or when drainage about ceases. Precipitation and evapotranspiration (ET) that occurred after the maximum were accounted for. ET was assumed to be equal to calculated potential ET during these wet conditions. This approach provides a minimum estimate of recharge because the methodology does not consider drainage during soil moisture increases or from multiple soil moisture responses during an event. Recharge in the four profiles average between 31 to 58 cm/year. Watershed baseflow (as determined from hydrograph separation) averages between 22 to 31 cm/year for the corresponding profile data periods. Recharge in the steep hillslope profile was similar to baseflow, while recharge in the moderate hillslope profile was about twice as high as baseflow. Recharge exceeding baseflow indicates that a portion of recharge likely contributed to stormflow runoff. Seasonal variability in recharge was highest in the steep hillslope profile (74% in the dormant season, November–February, and 19% in the growing season, April–September) and lowest in the moderate hillslope (55% dormant season, 36% growing season). Infrequent occurrence of recharge during the growing season is attributed to soil moisture levels remaining below its field capacity due to high ET between events and particularly during droughts. These insights indicate the sensitivity of recharge and subsequent baseflow to changes in the magnitude and frequency of seasonal precipitation in seasonally water limited climates.