Data from: Long-term data reveal patterns and controls on streamwater chemistry in a forested stream: Walker Branch, Tennessee

We present 20 years of weekly streamwater chemistry, hydrology, and climate data for the Walker Branch watershed in eastern Tennessee, USA. Since 1989, the watershed has experienced a ~1.0˚C increase in mean annual temperature, a ~20% decline in precipitation, and a ~30% increase in forest evapotranspiration rates. As a result, runoff has declined by ~34%. We evaluate long-term trends in streamwater concentrations and fluxes for 9 solutes and use wet deposition data to calculate approximate watershed input-output budgets. Dissolved constituents were classified as geochemical solutes (Ca2+, Mg2+ and SO42-) or nutrients (NH4+, NO3-, SRP, TSN, TSP, and DOC). Geochemical solutes are predominantly controlled by discharge and long-term changes in catchment hydrology have led to significant trends in the concentrations and fluxes of these solutes. Further, trends in geochemical solute concentrations indicate shifting soil flowpath contributions to streamflow generation through time, with deep groundwater having a greater proportional contribution in recent years. Despite dramatic changes in watershed runoff, there were no trends in inorganic nutrient concentrations (NH4+, NO3-, and SRP). While most nutrients entering the watershed are retained, stream fluxes of nutrient solutes have declined significantly as a result of decreasing runoff. Nutrient concentrations in streamwater exhibit large seasonality controlled by in-stream biological uptake. Stream benthic communities are sensitive to hydrologic disturbance and changes in the frequency or intensity of storm events through time can affect nutrient fluxes. Stream NO3- concentrations are also sensitive to drought, with concentrations decreasing (increasing) if conditions during the 3 years prior to the time of sampling were drier (wetter) than the long-term mean. Future changes in the incidence of storm events, as well as the number and duration of droughts, have the potential to significantly alter watershed nutrient losses. Our analysis indicates that changing climates can differentially affect watershed element cycles through changes in biogeochemical process rates or through changes in catchment hydrology. Furthermore, climate change can include both long-term trending in mean climate variables, as well as changes in the frequency and intensity of storms and droughts, with each of these types of change having distinct effects on biological and geochemical processes governing different solutes.

本数据集提供了美国田纳西州东部沃克支流流域（Walker Branch watershed）长达20年的周尺度河水化学、水文与气候数据。自1989年以来，该流域年平均气温升高约1.0℃，降水量减少约20%，森林蒸散速率提升约30%，受此影响径流量下降约34%。本研究针对9种溶质的河水浓度与通量长期趋势展开分析，并利用湿沉降数据估算流域的输入-输出收支平衡。溶解组分被划分为地球化学溶质（包括Ca²+、Mg²+与SO₄²-）与营养盐（包括NH₄+、NO₃-、SRP、TSN、TSP与DOC）。地球化学溶质主要受径流量调控，流域水文的长期变化已导致此类溶质的浓度与通量出现显著趋势。此外，地球化学溶质浓度的变化趋势表明，随时间推移，土壤水文路径对径流形成的贡献正发生改变，近年来深层地下水的贡献占比有所提升。尽管流域径流量发生显著变化，无机营养盐（NH₄+、NO₃-与SRP）的浓度并未出现显著趋势。尽管进入流域的大部分营养盐均被截留，但营养盐溶质的河道通量仍因径流量减少而显著下降。河水中的营养盐浓度呈现显著季节性特征，这一特征受河道内生物吸收作用调控。河道底栖生物群落对水文扰动十分敏感，而风暴事件的频率与强度随时间的变化也会对营养盐通量产生影响。河道NO₃-浓度对干旱也较为敏感：若采样前三年的气候条件较长期均值更干旱（湿润），则其浓度会下降（上升）。未来风暴事件发生频率、干旱次数与持续时间的变化，均有可能显著改变流域的营养盐流失情况。本研究分析表明，气候变化可通过改变生物地球化学过程速率或流域水文条件，对流域元素循环产生差异化影响。此外，气候变化既包括气候变量均值的长期趋势性变化，也涵盖风暴与干旱事件的频率与强度变化，而每一类变化对调控不同溶质的生物与地球化学过程均具有独特影响。