In response to concerns regarding the health of streams and receiving waters, the United States Environmental Protection Agency established a total maximum daily load for nitrogen in the Chesapeake Bay watershed for which practices must be in place by 2025 resulting in an expected 25% reduction in load from 2009 levels. The response of total nitrogen (TN) loads delivered to the Bay to nine source reduction and land use change scenarios was estimated using a Spatially Referenced Regression on Watershed Attributes model. The largest predicted reduction in TN load delivered to the Bay was associated with a scenario in which the mass of TN as fertilizer applied to agricultural lands was decreased. A 25% decrease in the mass of TN applied as fertilizer resulted in a predicted reduction in TN loading to the Bay of 11.3%, which was 2.5–5 times greater than the reductions predicted by other scenarios. Eliminating fertilizer application to all agricultural land in the watershed resulted in a predicted reduction in TN load to the Bay of 45%. It was estimated that an approximate 25% reduction in TN loading to the Bay could be achieved by eliminating fertilizer applied to the 7% of subwatersheds contributing the greatest fertilizer‐sourced TN loads to the Bay. These results indicate that management strategies aimed at decreasing loading from a small number of subwatersheds may be effective for reducing TN loads to the Bay, and similar analyses are possible in other watersheds.

中文翻译：

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切萨皮克湾氮负荷对源头减少和土地利用变化情景的响应：SPARROW分析

为了回应对溪流和接收水域健康的担忧，美国环境保护署在切萨皮克湾流域确定了氮的最大每日总装载量，为此必须在2025年前采取措施，从而使装载量预计减少25％从2009年的水平开始。使用流域属性上的空间参考回归模型，估算了输送到海湾的总氮（TN）负荷对九种源减少和土地利用变化情景的响应。预计向海湾输送的总氮负荷量最大的减少与一种情况有关，在这种情况下，作为肥料施用到农田上的总氮量减少了。施氮量减少25％，导致预计到海湾的总氮量减少11.3％，为2。比其他方案预测的减少量大5-5倍。取消在流域的所有农业用地上施用肥料，预计到海湾的总氮负荷将减少45％。据估计，通过消除施肥到7％的小流域，将最大的肥料来源的TN负荷施加到该海湾，可以使该海湾的TN负荷减少约25％。这些结果表明，旨在减少少量子流域负荷的管理策略可能对减少流向海湾的TN负荷有效，而在其他流域进行类似的分析也是可能的。据估计，通过消除施肥到7％的小流域，将最大的肥料来源的TN负荷带到海湾，可以使海湾的TN负荷减少约25％。这些结果表明，旨在减少少量子流域负荷的管理策略可能对减少流向海湾的TN负荷有效，而在其他流域进行类似的分析也是可能的。据估计，通过消除施肥到7％的小流域，将最大的肥料来源的TN负荷施加到该海湾，可以使该海湾的TN负荷减少约25％。这些结果表明，旨在减少少量子流域负荷的管理策略可能对减少流向海湾的TN负荷有效，而在其他流域进行类似的分析也是可能的。