Abstract
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.
Highlights
The quantification of watershed nutrient loads in response to conservation practices, land use changes, and future climate projections is essential for developing sustainable watershed management plans [1,2]
The sensitivity of average annual total phosphorus (TP) loading to Lake Okeechobee was illustrated by plotting legacy and fertilizer phosphorus related model parameters (Table 1) in μ* − σ and μ − σ planes (Figure 3a,b)
Parameters that are located above the μ*= σ line on the lets) was most sensitive to the improved pasture legacy P parameter, indicating its critical role in P dynamics for the Taylor Creek Nubbin Slough (TCNS), followed by citrus fertilizer phosphorus
Summary
The quantification of watershed nutrient loads in response to conservation practices, land use changes, and future climate projections is essential for developing sustainable watershed management plans [1,2]. The use of field and watershed-scale hydrologic and water quality simulation models for the development and assessment of environmental plans (e.g., Basin Management Action Plans—BMAP, Total Maximum Daily Loads—TMDL, Conservation Effects Assessment Project—CEAP) has become common practice [14,15,16,17,18]. While such models are extremely useful in estimating the potential impacts of BMPs, they do not capture legacy nutrient aspects effectively due to limited knowledge about related biogeochemical processes affecting nutrient loss and residence times [19,20,21,22]. This limits our ability to accurately and explicitly answer questions such as the contribution of legacy nutrient pools to current and future nutrient losses, the unbiased effectiveness of conservation practices in the near-term future, and reasonable timeframes for desired water quality improvements [23,24,25]
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