FINE SCALE VARIATIONS OF SURFACE WATER CHEMISTRY IN AN EPHEMERAL TO PERENNIAL DRAINAGE NETWORK IN THE HUBBARD BROOK EXPERIMENTAL FOREST, NH, USA

Catchment scale stream chemistry has been monitored at Hubbard Brook Experimental Forest, NH, USA, for nearly 50 years. Remarkable chemical responses from changing environmental conditions, such as increases in pH and decreases in sulfate and calcium, have been recorded. Although monitoring at the catchment outlet has been informative, it is important to also study spatial variations in stream chemistry across the drainage network within the catchments. By understanding the processes producing small scale spatial stream chemistry variations, we may be able to mechanistically interpret long term records. In this study, we used fine scale sampling to describe spatial and temporal surface water chemistry variations in a first order 0.41 km² headwater catchment at Hubbard Brook Experimental Forest. The spatial patterns and temporal persistence seen in stream chemistry were used to evaluate the location and extent of sources controlling surface water chemistry. We discovered a range of stream water chemistry that is as variable as that seen at the scale of the entire Hubbard Brook Valley (35 km²). We examined surface and subsurface characteristics in an attempt to understand controls on stream chemistry (e.g. upslope accumulated area, soil horizon development, type of parent material). Three potential sources and mechanisms driving surface water chemistry variation were identified: groundwater from distinct soil types, riparian zone and near stream exchanges, and isolated seeps as distinct groundwater inputs. As water table depth and configuration influence soil development and chemistry of groundwater sources to the stream, changes in spatial patterns in stream chemistry may serve as an indicator of the impacts of shifting hydrology, in response to a changing climate, on key hydropedologic processes.