Hydrology of an impounded lotic wetland—subsurface hydrology

Abstract

Although the importance of hydrology to wetland ecosystem productivity is recognized, there are few detailed analyses of shallow subsurface water flow (<1 m depth) through wetland ecosystems. Shallow subsurface water flow paths and average linear velocity were quantified with nested lysimeters in a riverine wetland located in Hale County, AL. Water-table elevation was spatially and temporally variable and was directly correlated to precipitation at the wetland. In the beginning portion of the study (May to November 1996), average monthly precipitation was 73 mm and water-table elevation, decreased across much of the wetland. From December 1996 through July 1997, average monthly precipitation increased to 110 mm with a concomitant increase in water-table elevation throughout the wetland. During periods of low average monthly precipitation, wetland subsurface water was recharged from the hyporheic zone of the main inflow stream (northern side of the wetland). With increased precipitation, subsurface water recharge shifted from the stream hyporheos to recharge from an upgradient area (entire northern portion of the wetland) dominated by alder, Alnus serrulata. Horizontal and vertical average linear velocity was greatest in the upper 20 cm of the wetland compared to lower sediment depths. Vertical average linear velocity was similar among most sample intervals and was not correlated to antecedent precipitation. In laboratory mesocosm experiments, evapotranspiration by the emergent macrophyte Juncus effusus increased the rate of subsurface water flow in mesocosms with sand sediments, while there was no macrophyte effect in mesocosms with predominantly clay sediments.