Lake Water Budget Models: Construction and Application to Support the Development of Lake Minimum Levels

This report describes the conceptualization of a water budget model and processes used for lake minimum levels development, with reference to a template model spreadsheet, Lake Water Budget Model Template.xlsx. For definitions of the terms “minimum level”, “P10”, “P50”, “P90,” and “Historic” used throughout this report, see Rules 40D-8.021 and 8.624, Florida Administrative Code. Water budgets (also called water balances) are widely used to represent hydrologic fluxes for lakes and other waterbodies (e.g., Healey et al., 2007; Mitchell & Jawitz, 2013). Using this approach, change in lake stage or volume can be calculated as the difference between its summed inflows and summed outflows over a specified time period. Examples of studies using water budgets for specific Florida lakes include Fellows & Brezonik (1980), Deevey (1988), Sacks et al. (1992), Belanger & Kirkner (1994), Grubbs (1995), Lee & Swancar (1997), Motz (1998), Sacks et al. (1998), Swancar et al. (2000), Motz et al. (2001), Watson (2001), Metz & Sacks (2002), McBride et al. (2010), Viridi & Sacks (2012), Swancar (2015), and McBride et al. (2017). Schiffer (1998) provides a generalized overview of the hydrology and water budgets of central Florida lakes, and Healy et al. (2007) explore the use of water budgets for water resource management, including an example from Florida. Water budgets have also been used by the Southwest Florida Water Management District for the development of minimum levels for numerous lakes. Among these studies, inflows universally include precipitation directly on the lake, while outflows include evaporation from the lake and leakage to the groundwater system. Depending on the lake and study, inflows can also include groundwater fluxes, overland flow, augmentation to the lake, and contributions from other surface waterbodies (such as streams), while outflows can include direct surface withdrawals and losses through structures or channels. The water budget model implemented by the District for the development of minimum lake levels is a calibrated spreadsheet model that tracks lake water inputs and outputs on a daily timestep to calculate an estimated lake water level. The default model includes precipitation, evaporation, surficial aquifer fluxes, Upper Floridan aquifer fluxes, overland flow, directly connected impervious area (DCIA) runoff, and surface outflow from a single fixed-elevation structure. A conceptual diagram of the water budget is shown in Figure 1. This conceptualization is generally adequate to describe the water balance for many District lakes. On an as-needed basis, channel inflows, additional surface outflows, augmentation, direct surface withdrawals, or other terms can be added. However, no model can perfectly represent the complex physical reality, and professional scientific judgement is essential when developing the models, interpreting their results, and applying them in decision-making processes. The following section describes the terms and equations used in the default water budget model, with reference to a template file, Lake Water Budget Model Template.xlsx.