Graduation Year

2022

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Civil and Environmental Engineering

Major Professor

Mauricio E. Arias, Ph.D.

Committee Member

Mark Rains, Ph.D.

Committee Member

Mahmood Nachabe, Ph.D.

Committee Member

Qiong Zhang, Ph.D.

Committee Member

Hadi Charkhgard, Ph.D.

Committee Member

Rajendra Paudel, Ph.D.

Keywords

Designer Flows, Hydrologic Modeling, Phosphorus Dynamics, Shallow Lake, Trend Analyses, Watershed Hydrology, Nutrients

Abstract

Rivers around the globe have been altered as people have constructed water resources infrastructure to store water for different human needs, causing negative impacts on ecosystem processes and functions. Environmental flows have been introduced to restore deteriorated riverine ecosystems; however, environmental flows have not always been practical or adequate to restore environment conditions in rivers globally. Lake Okeechobee is a large subtropical lake that plays a pivotal role in South Florida water system providing various water demands for the surrounding communities as well as providing crucial services to the ecosystem of South Florida. However, hydrologic alterations and nutrient (phosphorus and nitrogen) excessive loadings to the lake from its watershed, have caused environmental problems in the lake and receiving water bodies, in particular harmful algal blooms. This doctoral dissertation aimed at designing flow regimes for complex regulated water systems to improve their water quality. The main hypothesis of this doctoral dissertation was that water flow and nutrient loads through multiple water structures of a complex regulated water system can be designed to simultaneously fulfill society water needs and specific ecological outcomes (i.e., nutrient management).

This dissertation first evaluated trend of flows and different water quality parameters (i.e., total phosphorus, total nitrogen, chlorophyll-a) in waters around Lake Okeechobee for the period 1974-2019. The study period was divided to account for the four operation schedules of Lake Okeechobee during the study period (i.e., 1978 Rules (1974-1990), RUN25 (1991-1999), WSE (2000-2007), and 2008 LORS (2008-2019)) to evaluate the specific impacts of the different operation schedules on flows and water quality parameters.

I found an increasing trend in water discharges to the Caloosahatchee River during the dry season for the entire period (1974-2019), and a decreasing trend in discharges to the St. Lucie Canal during the 2008 LORS operation schedule (2008-2019). I also found an increasing trend in phosphorus concentrations in the lake especially in the early 1978 Rules (1974-1990) schedule. Higher flows, combined with increasing TP concentrations in Lake Okeechobee and the Caloosahatchee, led to significant increases in monthly TP loads to the Caloosahatchee River by up to 122%-158% during the 2008 LORS (2008-2019). Trends in TN concentrations are very different and even opposite to those of TP, with either no changes or negative trends throughout the year around the lake. Though, higher flows in the Caloosahatchee resulted in a net TN load increase to the Gulf. Overall, results from the trend analyses demonstrated that reservoir operations could have long-term effects on nutrient status and exports; thus, modifying operations should be considered as a potential nutrient management tool.

To further understand the role of lake operations on nutrient dynamics, I developed the Lake Operation Optimization of Nutrient Exports (LOONE), a water balance-nutrient-optimization code programmed in Python. LOONE was used to simulate water balance and phosphorus dynamics of Lake Okeechobee for the period 1991-2018 incorporating three operation schedules: RUN25 (1991-1999), WSE (2000-2007), and 2008 LORS (2008-2018). LOONE’s water balance simulations were verified for the three operation schedules and LOONE performed very good, with R2 and NS exceeding 0.90. In addition, LOONE’s P module was calibrated for the period (2008-2018) where R2 = 0.4 and validated for the period (1991-2007) where R2 = 0.24. Although R2 was not high for P loads for the calibration/validation period, the model did capture P concentration general increasing and decreasing trends. LOONE was used to evaluate the effects of the proposed LOSOM operations on phosphorus concentrations in the lake and phosphorus exports into the Caloosahatchee River and St. Lucie Canal. LOSOM would decrease P loads into the St. Lucie Canal by 40% associated with reduction in high-volume discharges. P loads into the Caloosahatchee River would increase by 33% and P loads into the South that would be doubled because of increased flow volumes under the LOSOM run. In addition, LOONE was used to design optimal Lake Okeechobee discharges to minimize P exports to the Caloosahatchee and St. Lucie during the summer season as harmful algal blooms often spread during the summer season. LOONE provided optimal monthly discharges to the Caloosahatchee and St. Lucie that would reduce P exports during the summer season from 48.4 to 40.1 tons yr-1 into the Caloosahatchee, and from 50.1 to 35.9 tons yr-1 into the St. Lucie while maintaining Lake Okeechobee stages between 8 ft (2.44 m) and 18 ft (5.49 m). In addition, LOSA water demand deficit would decrease from 35,534 ac-ft (43.83 million m3) to 33,794 ac-ft (41.68 million m3) per year.

Excessive nutrient loads from nonpoint sources in Lake Okeechobee Watershed have been the major source of pollution to Lake Okeechobee. Various best management practices (BMPs) have been widely implemented in the watershed to mitigate nutrient loadings to the lake. This dissertation evaluated the effectiveness of BMP implementation in Lake Okeechobee Watershed on P and N reductions. The Watershed Assessment Model (WAM) was used to simulate flows and nutrients through the six northern Lake Okeechobee sub-watersheds (Upper Kissimmee, Lower Kissimmee, Indian Prairie, Taylor Creek/Nubbin Slough, Lake Istokpoga, and Fisheating Creek). WAM was calibrated for the period (1995-2006) for hydrology and nutrients for each of the six sub-watersheds, with R2 ranging from 0.43-0.93 for flow and 0.36-0.86 for nutrients. Validation for the period 2007-2018 resulted in R2 ranged from 0.43 to 0.95 for flow and 0.24-0.91 for nutrients. I used WAM to simulate three different BMP scenarios: baseline scenario (no BMPs implemented), current condition scenario, and maximum potential scenario (BMPs were implemented wherever possible). WAM simulations indicated that the current implemented BMPs reduced P and N loadings of Lake Okeechobee Watershed from 480 tons/year and 4446 tons/year to 465 tons/year and 3857 tons/year respectively. Further implementation of BMPs in the watershed could reduce P and N to 298 tons/year and 3066 tons/year. Breaking down of P and N reductions illustrated that specific sub-watersheds have the potential to reduce P (i.e., Taylor Creek/Nubbin Slough) and N (i.e., Upper Kissimmee), while specific sub-watersheds do not have much potential to reduce P or N. Our simulations suggested that other strategies are critical to further reduce P and N, especially in sub-watersheds that have low reduction potential with BMPs.

In summary, this doctoral dissertation evaluated the impacts of the different Lake Okeechobee operation schedules on flow and water quality trends. I found that phosphorus concentrations in Lake Okeechobee, Caloosahatchee River, and St. Lucie Canal were influenced by flow release patterns associated with the different operation schemes. This dissertation introduced LOONE to investigate effects of lake operations on water releases and phosphorus exports which were adopted as ecological outcomes to design alternative water discharge regimes. This dissertation designed optimal releases in a complex multi-outlet system to improve water quality which was tested against Lake Okeechobee and provided an alternative operation guide for the lake that considered phosphorus exports into the Caloosahatchee River and St. Lucie Canal. In addition, this dissertation evaluated the effects of BMP implementations in multiple watersheds on phosphorus and nitrogen load reductions. This dissertation addressed the potential of all appropriate BMPs to reduce phosphorus and nitrogen in distinguished land use watersheds. Finally, this dissertation provided alternatives that could improve water quality in Lake Okeechobee and estuaries while sustaining society water needs which could not be ignored.

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