Hydrology of an impounded lotic wetland - Subsurface hydrology

Although the importance of hydrology to wetland ecosystem productivity is r ecognized, there are few detailed analyses of shallow subsurface water flow (<1 m depth) through wetland ecosystems. Shallow subsurface water flow pat hs and average linear velocity were quantified with nested lysimeters in a riverine wetland located in Hale County, AL. Water-table elevation was spat ially 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 decrea sed across much of the wetland. From December 1996 through July 1997, avera ge monthly precipitation increased to 110 mm with a concomitant increase in water-table elevation throughout the wetland. During periods of low averag e monthly precipitation, wetland subsurface water was recharged from the hy porheic 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 a verage linear velocity was greatest in the upper 20 cm of the wetland compa red to lower sediment depths. Vertical average linear velocity was similar among most sample intervals and was not correlated to antecedent precipitat ion. In laboratory mesocosm experiments, evapotranspiration by the emergent macrophyte Juncus effusus increased the rate of subsurface water flow in m esocosms with sand sediments, while there was no macrophyte effect in mesoc osms with predominantly clay sediments.