Evaluating water-quality trends in agricultural watersheds prioritized for management-practice implementation
Many agricultural watersheds rely on the voluntary use of management practices (MPs) to reduce nonpoint source nutrient and sediment loads in surface waters; however, the water-quality effects of MPs are often uncertain. The effectiveness of many MPs has been demonstrated by plot- and field-scale studies but there is little research about collective effects in larger watersheds. Thus, additional insights about MP performance are needed from monitoring-based studies to inform effective load-reduction strategies. We interpret water-quality responses collected from as early as 1985 through 2020 in three agricultural Chesapeake Bay watersheds that were prioritized for MP implementation, namely, the Smith Creek (Virginia), Upper Chester River (Maryland) and Conewago Creek (Pennsylvania) watersheds. These watersheds represent geological settings and agricultural activities common to the Chesapeake Bay watershed and may therefore provide results representative of the broader region. We synthesized patterns in MP implementation, climatic conditions, land use, and nutrient inputs to better understand how natural and anthropogenic factors affected monitored nutrient and sediment loads. Relations between MP implementation and expected water-quality improvements were not consistently identifiable. Although the number of MPs increased in all watersheds since the early 2010s, most monitored loads did not decrease. Sediment loads increased in the Smith Creek and Upper Chester River watersheds and decreased in Conewago Creek. Nitrogen and phosphorus loads increased or remained stable in the Smith Creek and Upper Chester River watersheds despite the widespread use of nutrient-reducing MPs. In the Conewago Creek watershed, nutrient-load decreases may have been related to point-source upgrades and (or) sediment load decreases. In the Smith Creek watershed, a 36-year time-series model suggests a causal link between changes in manure inputs and the flow-normalized export of total nitrogen. We hypothesize that increases in nutrient inputs, to keep pace with intensifying agricultural activities, may overshadow some expected MP effects. Although MPs may have stemmed additional water-quality degradation, water-quality improvements may rely on lowering surplus manure and fertilizer applications so that nutrient inputs align with local crop demand. Collectively, these results highlight the complexity of watershed responses to MPs and the need to continue assessments of MP performance with long-term monitoring-based studies.