Integrated Surface Water and Groundwater Modeling to Study Water Level Fluctuations of Isolated Lakes in Florida

This paper discusses the results of an integrated hydrologic modelling to study potential adjustment to the established minimum flow and levels for lakes in Northwest Hillsborough County, Florida. The northwest area of Hillsborough County is characterized by numerous lakes and wetlands many of which are interconnected and managed via control structures or groundwater augmentations. Some lakes in the area are isolated and have much smaller contributing watershed and rarely reach their established minimum water levels. It is surmised that as a result of drainage alteration and land development some of these lakes are more impacted than others. In 2004–2005, challenges were made to the application of Southwest Florida Water Management District's Minimum Flow and Level rule (MFL) for Category 3 lakes (Raleigh, Rogers, and Starvation) and have, to some degree, raised the same issues related to land use/drainage alterations and lake water level impacts. The primary purpose of this study was to examine whether the drainage alteration and land development within the area have significantly affected the natural flow into the subject lakes, and determine whether such alterations warrant adjustment of the proposed lake minimum levels. To this end, an integrated surface and ground water model of the basin using the MIKE SHE software was developed to look into the inter-relationships among surface and ground water, water supply pumping in the adjacent well fields and drainage alterations in the past several decades. The study was conducted in three phases. Phase I consisted of data collection and digital terrain model (DTM) development for the proposed study area. During phase two, a complete watershed evaluation was performed, including hydrologic and hydraulic features inventory, surface water/subsurface water assessment, as well as a preliminary integrated modeling plan and approach. In phase III, the subject of this paper, an integrated hydrologic model of the site was developed to assist in the development of watershed management plan, with emphasis on the assessment of the potential impacts on lake water levels resulting from human activities such as drainage alteration and land development. To study the impacts, the calibrated model was used to simulate both the existing condition and historic landuses to compare water levels and develop strategies to lessen the impacts on the lake water levels.