Water quality trends under rapid agricultural expansion and enhanced in-stream interception in a hilly watershed of Eastern China

Conflicts between agricultural intensification and the increasing demand for clean water resources are growing worldwide. This study sought to understand how the negative consequences of agricultural expansion in fragile hilly watersheds can be mitigated by ecologically based engineering practices. We analyzed long-term and seasonal water quality trends in two sub-watersheds of the Tianmu Lake watershed in Eastern China. The Zhongtian and nearby Zhucao sub-watersheds are very similar in terms of climate, topography and other features that can influence water quality. Both are experiencing rapid expansion of tea plantations, but the Zhongtian River contains an engineered system of overflow dams and cascade wetlands that is absent from the Zhucao River. The multi-year averaged reduction (2009-2018) of total nitrogen (TN) and total phosphorus (TP) from upstream to downstream reaches was 10%-15% greater in the engineered Zhongtian River compared to the non-engineered and free flowing Zhucao River, which has no interventions to reduce nutrient concentrations. Average annual reductions in TN, TP, and total suspended solids (SS) downstream of the engineered system reached 0.5%-4.0% of their multi-year averaged concentrations over this time interval. These reductions occurred despite a 2.3-fold expansion of tea plantation area in the engineered watershed, which contrasts with deteriorating water quality in the non-engineered watershed that had a 0.4-fold expansion of tea plantation area. Our results underscore the value of such engineered systems to improve water quality and help reconcile competing advantages of agricultural development and environmental protection in hilly watersheds, where there is limited in-stream processing of nutrients and the effects of human activities are substantial.