Hydrology of a Forested Wetland Complex in an Urbanizing Area of the Texas Gulf Coast and Clean Water Act Implications

Swales in wetland complexes can provide evidence of hydrologic connectivity for wetlands on the Texas Gulf Coast, supporting the idea that many coastal wetlands in Texas are vitally connected to navigable waters covered by the Clean Water Act. In this study, runoff that accounted for more than 18% of rainfall was observed from a representative ?isolated? wetland complex?wetland depressions and upland areas interconnected by shallow erosional features?southeast of Houston, Texas between March 2005 and April 2010. Annual runoff ranged from 0% in 2005 to 27% in 2007. This result was surprising, given the presumably isolated nature of the wetlands.

The wetland complex was predominantly forested, with emergent vegetation dominating some of the depressions. Measured hydrologic fluxes included: (1) rainfall, using a tipping-bucket rain gauge supplemented with official weather station data; (2) surface runoff, using a v-notch weir to measure discharge from a wetland swale; (3) transpiration of Quercus nigra (18.0 cm diameter) and Quercus pagodafolia (15.9 cm diameter) using the heat-dissipation sap flux method; (4) groundwater level changes, using piezometers, and (5) soil moisture changes, using soil moisture probes. Watershedscale evapotranspiration was estimated using the Hargreaves model. Surface runoff, although intermittent, occurred during 25 of 57 months. Monthly runoff ranged from 0% of rainfall to 57% of rainfall. Soil moisture loss trended with increased transpiration rates, where the Q. nigra specimen transpired 11.6 to 35.8 L d-1 and Q. pagodafolia specimen transpired 2.43 to 13.8 L d^-1. Moisture was depleted rapidly in the upper soil layer, emphasizing the importance of considering local weather patterns when identifying wetlands and making jurisdictional decisions.

The results of this study call into question regulatory presumptions about coastal plains wetlands (at least 400,000 ha in Texas alone), of which roughly 50% are considered geographically isolated. One way to improve implementation of federal rules for wetlands similar to those in this study, which are reasonably close to both navigable and non-navigable streams, is for regulatory agencies to determine whether the wetlands are adjacent to a navigable water before making other decisions that would lead to a presumption that significant nexus does not exist.