Stream water quality data are essential for understanding watershed processes and managing water pollution, but the effort and expense of stream monitoring limit how many watersheds can be studied. For 59 small watersheds in the Chesapeake Bay drainage, we compared water quality measurements from inexpensive spot sampling to data from costly automated monitoring that used 1–3 yr of continuous flow measurement and weekly, temporally composited water sampling. Mean nitrogen (N) levels ranged from 0.01 to 16 mg N/L among streams. There were important temporal variations in N concentrations at each site, but the differences among sites were much greater. Spot samples were very effective at accurately and precisely placing average stream N levels within the N gradient among streams draining N‐enriched watersheds. Among watersheds, nitrate (NO₃) and total N concentrations from spot samples were very strongly correlated with means from weekly composite sampling (R² > 97%). We confirmed this result for independent data for 85 larger watersheds in the Chesapeake Bay Non‐tidal Network. NO₃ concentration from a single March spot sample was highly correlated (R² > 92%) with flow‐weighted average total N concentration synthesized from five years of monitoring. Spot sampling effectively quantifies average N status across N‐enriched watersheds because most N moves as dissolved NO₃ in subsurface flow and that flux is much less variable than the episodic surface transport of particulate materials. For questions answered by quantifying average N levels, spot sampling can assess more watersheds at much lower cost than automated sampling, so it should be more widely used to support cost‐effective N research and management. For materials that are mainly bound to particulates, such as phosphorus, spot sampling is much less effective.

中文翻译：

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廉价的现场采样提供了流域氮状况的出乎意料的有效指标

溪水水质数据对于了解流域过程和管理水污染至关重要，但是溪流监测的工作量和费用限制了可以研究多少个流域。对于切萨皮克湾流域的59个小流域，我们将廉价的现场采样的水质测量结果与昂贵的自动监测数据进行了比较，该数据使用了1-3年的连续流量测量以及每周一次的临时混合水采样。流中的平均氮（N）水平范围为0.01至16 mg N / L。每个部位的氮浓度存在重要的时间变化，但部位之间的差异更大。点状样品在将富含氮的集水区排水的溪流中，将平均溪流N水平准确准确地置于N梯度内非常有效。在流域中，硝酸盐（NO₃）和现货样品中的总N浓度与每周复合采样的平均值非常相关（R ²  > 97％）。对于切萨皮克湾非潮汐网络中85个较大流域的独立数据，我们证实了这一结果。一个单一的3月斑点样品中的NO ₃浓度与 五年监测合成的流量加权平均总N浓度高度相关（R ² > 92％）。点采样有效地量化了富氮流域的平均氮状况，因为大多数氮以溶解的NO _3的形式移动在地下流动中，该通量的变化远小于颗粒物质的表面散布。对于通过量化平均氮水平回答的问题，现场采样可以以比自动采样低得多的成本评估更多的流域，因此应更广泛地用于支持经济高效的氮研究和管理。对于主要与微粒结合的材料（例如磷），点采样的效果要差得多。