Water Resources Research Act Program

Details for Project ID 2013MS183B

Non-linear downward flux of water in response to increasing wetland water depth and its influence on groundwater recharge, soil chemistry, and wetland tree growth

Institute: Mississippi
Year Established: 2013 Start Date: 2013-03-01 End Date: 2014-12-31
Total Federal Funds: $37,260 Total Non-Federal Funds: $74,592

Principal Investigators: Gregg Davidson

Abstract: Many oxbow lake-wetland systems in the Mississippi River floodplain are perched above the regional water table, resulting in a downward hydraulic gradient. Fine grained sediments that accumulate in these environments limit downward flow, but fallen tree trunks and limbs introduce heterogeneity and isolated pockets of higher hydraulic conductivity. Normally, flux is proportional to the gradient, but previous work by the PI suggests that the relationship between water depth and downward flow in these systems can be non-linear. Studies in Sky Lake, in the Delta region of Mississippi, have documented minimal vertical movement of water until a threshold water depth is reached. Above the threshold, abrupt changes in soil chemistry have been observed as water begins moving downward, which may in turn influence the growth of wetland trees. Recharge to the regional groundwater may be likewise influenced, where significant downward flow only occurs above a threshold water depth. The groundwater response beneath these wetlands is currently unknown. The proposed study will investigate the influence of wetland water level and the importance of a threshold depth on (1) recharge to the local groundwater, (2) redox potential and oxygen concentrations in the wetland soils, and (3) sap flow rate and changes in trunk circumference in cypress trees as a measure of biological activity and growth related to the changes in soil chemistry. Water level is currently under continuous monitoring in Sky Lake using data logging pressure transducers (uninterrupted 10-year record). For the new study, responses in the local groundwater level will be monitored in two wells near the wetland perimeter; the vertical hydraulic gradient in the root zone in the wetland will be monitored in shallow piezometer nests; redox, oxygen, and conductivity probes will be deployed in a grid in the soil to record the timing and magnitude of chemical changes as water levels rise and fall; sap flow probes will be inserted in nearby cypress trees to monitor changes in the sap flow rate to correlate with water level and soil chemistry changes; and dendrometer bands will be placed around trees as a direct measure of growth.