Multi-Scale Variations in Streamwater Chemistry and Hydropedological Implications for Hotspot Development

Headwater streams comprise the vast majority of stream network length in watersheds and affect regional water quality. However, the spatial variation of water quality in headwater regions often remains unknown. In this study, we show that spatial processes differentially affect biogeochemical condition and pattern across a multiple scales in a headwater stream network. Synoptic spatial surveys of streamwater chemistry at both fine grain and broad extent coupled with network-based geostatistical analysis, allowed us to quantify spatial patterns over a range of scales from 1^st order ephemeral and intermittent streams to 4^th and 5^th order perennial streams. In addition, synoptic surveys at a fine spatial grain (50 m) in a small watershed (0.41 km²) showed the same range of chemical variation as its larger basin (36 km²) sampled at coarser grain (100 m). Spatial structure was apparent at either a single scale or at multiple nested scales in the larger basin, suggesting separate processes operating simultaneously within the stream network and surrounding terrestrial landscape. We explored the fine-scale patchiness in stream chemical patterns, with particular emphasis on dissolved organic carbon (DOC), as an example to illustrate hotspot development due to hydropedological variation in a watershed. Hydropedological variations refer to soil morphological differences that directly relate water table regime, flowpaths, and saturation frequency to soil development. Soils have been digitally mapped using topographic and bedrock-related metrics determined from a LiDAR-derived digital elevation model (DEM) and field surveys of bedrock outcropping. Spatial patterns of DOC concentrations were associated with the distribution of soil types and their thresholds to stormflow generation. The hydrologic connection of shallow soils in channel head regions above the stream network where lateral podzols developed appeared to be a source for DOC hotspots in the stream network. Overall, our study highlights the importance of understanding the structural organization within a watershed, particularly hydropedological variation, in assessing controls on the spatial patterns of streamwater chemistry and developing tools for predicting the stream chemical variation in headwater stream networks.