Soil phosphorus (P) built up due to past management practices, legacy P, in the Lake Okeechobee Watershed (LOW) in south-central Florida, U.S.A., is often discussed as the root cause of lake eutrophication. Improvement of the lake’s water quality requires the identification of critical P sources and quantifying their contributions. We performed a global sensitivity analysis of the Watershed Assessment Model (WAM), a common evaluation tool in LOW environmental planning, using the Morris method. A pre-calibrated WAM setup (Baseline) of the LOW sub-watershed, Taylor Creek Nubbin Slough (TCNS), was used as a test case. Eight scenarios were formulated to estimate the contributions of various P sources. The Morris analysis indicated that total phosphorus (TP) loads were highly sensitive to legacy P in improved pastures, the major land use covering 46.2% of TCNS. The scenario modeling revealed that legacy P, inorganic fertilizers, and other sources contribute 63%, 10%, and 32%, respectively, to the Baseline TP load of 111.3 metric tons/y to the lake. Improved pastures, dairies, citrus, and field crops are the top TP load contributors. Our results have important implications for water quality improvement plans in the LOW and highlighted the need for accurate spatial mapping of legacy P and incorporation of such information in modeling efforts for watersheds demonstrating legacy P problems.

中文翻译：

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美国佛罗里达州受影响农业流域对遗留磷的流域响应和最佳管理实践

由于过去的管理实践而积累的土壤磷 (P)，美国佛罗里达州中南部奥基乔比湖流域 (LOW) 中的遗留磷经常被讨论为湖泊富营养化的根本原因。湖泊水质的改善需要确定关键的 P 源并量化其贡献。我们使用莫里斯方法对流域评估模型 (WAM) 进行了全局敏感性分析，流域评估模型是 LOW 环境规划中的常用评估工具。LOW 子流域 Taylor Creek Nubbin Slough (TCNS) 的预校准 WAM 设置（基线）用作测试案例。制定了八种情景来估计各种 P 源的贡献。Morris 分析表明，总磷 (TP) 负荷对改良牧场中的遗留磷高度敏感，主要土地用途覆盖 46。2% 的 TCNS。情景模型显示，传统 P、无机肥料和其他来源分别对湖泊 111.3 公吨/年的基线 TP 负荷贡献了 63%、10% 和 32%。改良的牧场、奶牛场、柑橘和大田作物是最大的 TP 负荷贡献者。我们的结果对 LOW 的水质改善计划具有重要意义，并强调需要对遗留 P 进行准确的空间映射，并将此类信息纳入流域建模工作中，以证明遗留 P 问题。