QUANTIFYING THE HYDROLOGIC INTERACTIONS ASSOCIATED WITH THE PLANT WATER STRESS FUNCTION AND EVAPOTRANSPIRATION PARTITIONING IN A WETLAND ECOSYSTEM

The soil moisture regime of a wetland is a dominant control on plant establishment and persistence and biogeochemical processes such as denitrification and carbon sequestration. Quantifying the soil moisture regime requires an understanding of important hydrologic fluxes such as evapotranspiration, infiltration, and groundwater upwelling. Specifically, the feedback mechanisms between soil moisture, vegetation, transpiration, and evaporation are a critical aspect of wetland ecohydrology, yet are not well understood or accounted for in hydrologic models. One component of this interaction is the plant water stress function, which usually reduces root water uptake at both the dry (water-limited) and wet (oxygen-limited, anaerobiosis) ends of the soil moisture spectrum. A second component is the partitioning of evaporation and transpiration, which is dependent on leaf area and the amount of exposed bare ground (both characterized by leaf-area index). We quantify the effects of modifying the plant water stress function and evapotranspiration partitioning on the soil moisture regime using a 1-D variably-saturated hydrologic model of an archetypical floodplain wetland in southwestern Wisconsin. We utilize three different plant water stress functions: 1) a widely-used model that linearly reduces root water uptake between two constant pressure heads in the anaerobiosis zone, 2) a recently proposed model that defines the anaerobiosis zone based on soil type, plant characteristics, and soil depth, and 3) a new model that does not reduce root water uptake near saturation representing the effects of aerenchyma (a spongy tissue that facilitates the transfer of gases between plant roots and shoots). Values for leaf-area index represent conditions before and after floodplain restoration at two field sites where topsoil was removed to restore pre-settlement hydrologic conditions. Results show large differences in the soil moisture regime by only changing the plant water stress function and leaf-area index. In order to accurately predict wetland hydrology and the ecosystem processes that are controlled by the soil moisture regime, more research needs to be done on effectively characterizing the plant water stress function for various plant species and/or communities.