THE IMPACT OF GEOLOGY ON WATER YIELD AND WATER QUALITY IN NORTH CAROLINA PIEDMONT HEADWATER WATERSHEDS

Understanding the impact of different types of geology on forest hydrology and water quality can offer valuable information to researchers and water resource managers in addressing water shortages during droughts. Four headwater watersheds (12- 40 ha in size) in the Carolina Slate Belt (CSB), Hill Demonstration Forest, Durham County, NC and two in the Triassic Basin (TB), Umstead Research Farm, Granville County, NC were compared in this study. The perennial stream channels in each watershed were monitored in 2007 for flow rate and water quality. The overstory vegetation in the riparian area and precipitation inputs were not significantly different across geologic areas. The CSB soils are generally deeper, less erodible, and contain less clay content than soils found in the TB. These differences in soil characteristics influenced annual, seasonal, and storm based hydrology and possibly water quality conditions. We found that annual water yield from the CSB streams was significantly different (Tukey test, p = 0.05) than annual water yield from the TB streams, 15% vs 20%, respectively. We also found a seasonal difference between geologic areas. During the dry months, the CSB streams had a higher flow rate than did the TB streams. However, during the wet months TB streams had a higher monthly outflow rate. The TB streams had significantly higher storm flows (8.5mm vs 5.5mm, Tukey test, p = 0.03) and runoff coefficients (14% vs 7%, Tukey test, p = 0.01) than CSB streams. Although the TB streams generally had higher annual exports of nitrate, ammonium, total phosphorus, total kjeldahl nitrogen, total organic carbon, and total suspended solids compared to the CSB streams, these data could be confounded by the farm fields in the TB watersheds. In conclusion, it appears that geology influenced hydrologic processes such as storm flow generation and soil water storage dynamics and to some extent water quality conditions in our study area. Thus, geology should be considered when planning and implementing forest best management practices (BMPs).