publications > scientific investigations report > surface-water and ground-water interactions > recharge and discharge estimates determined by independent techniques > estimates from the South Florida Water Management Model

Each of the following sections details the use of one technique to estimate recharge and discharge in WCA-2A. Each technique has is strengths and weaknesses in terms of cost and sophistication of analyses and modeling. The comparison between techniques reveals that the problem of estimating recharge and discharge in the Everglades is a scale dependent one. Each technique has a differing level of sensitivity, and thus detection capability, for determining the shorter or longer timescale components of recharge and discharge. Results of each approach are reported in sequence and then all results are compared in a final section.

Estimates from the South Florida Water Management Model

Even though the South Florida Water Management Model (SFWMM) is one of the most important tools being used to understand the hydrology of the Everglades, it has not often been used to specifically investigate recharge and discharge in the WCAs (South Florida Water Management District, 1999). Recharge and discharge in the central Everglades are considered by the SFWMM, but these values are not reported in a format that is directly interpretable as such in the standard output of the SFWMM (Ken Tarboton, South Florida Water Management District, written commun., 2003). Water-balance results from the SFWMM mainly report fluxes in surface water and ground water that cross basin boundaries. A simple estimate of recharge and discharge is possible using the water balance results directly, but the result would be an underestimate because recharge fluxes in the interior part of the wetland are not necessarily considered.

In the present study recharge and discharge were estimated for WCA-2A using the results of the SFWMM in a new interpretive framework. Individual mass-balance equations for surface water and ground water were written based on the SFWMM. Terms and numerical values were then inserted for WCA-2A as supplied from SFWMM results for certain specific time periods. The mass balance equations were then solved for recharge and discharge. Since these recharge and discharge estimates should be sensitive to fluxes in the interior part of WCA-2A, they can be compared with other estimates of recharge and discharge described later in this report.

Figure 5. Schematic diagram showing hydrologic fluxes of the South Florida Water Management Model in an application to the water budget of a Water Conservation Area, central Everglades, south Florida. (Modified from South Florida Water Management District, 1999). STQSIN and STQOUT are surface water flows through water control structures entering or leaving the basin, respectively; RAINFALL is the rainfall on the basin; ETP and ETS are evaporation and/or transpiration leaving the basin from ponded surface water and saturated subsurface zones, respectively; SWSTOCH and GWSTOCH are the changes with time in water storage in surface water and ground water of the basin, respectively; GWIN and GWOUT are the water fluxes entering and leaving ground water in the basin by regional ground-water flow, respectively; LSPGIN and LSPGOUT are the water fluxes entering or leaving the surface-water basin by shallow ground-water flow beneath selected levees, respectively; PERCOLATION is a regional-scale computation of water movement across the wetland sediment surface based on (2X2 mi grid cell) data for a specified domain; SW_GW_RESIDUAL is a residual (an unmeasured quantity) associated with both the surface-water and ground-water balance equations. [larger image]

Mean annual water-balance fluxes for WCA-2A were obtained (based on SFWMM version 3.5) representing the periods 1979-1990 (calibration run), 1991-1994 (verification run), and 1965-1995 (base simulation). Using those results and the SFWMM documentation (South Florida Water Management District, 1999), time-averaged mass-balance equations for WCA-2A were written individually for surface water and ground water. The new equations included complete expressions for vertical fluxes of water across the sediment surface. The terms in these mass-balance equations are illustrated in figure 5. Equation 1 is a mass balance equation for surface water, equation 2 is a mass balance equation for ground water (used mainly as a check on the first equation), and equation 3 computes a net flux between surface water and ground water by summing total recharge and discharge.

0 = STQSIN + RAINFALL + LSPGIN - STQSOUT - ETP - LSPGOUT - STOCH - PERCOLATION - SW-GW_RESIDUAL,     (1)

0 = GWIN - GWOUT - ETS - GWSTOCH + PERCOLATION + SW-GW_RESIDUAL,     (2)

NET EXCHANGE = Recharge - Discharge = LSPGOUT + PERCOLATION + SW-GW_RESIDUAL - LSPGIN.     (3)

where STQSIN and STQSOUT are surface water flows through water control structures entering or leaving the basin, respectively;

RAINFALL is the rainfall on the basin;

ETP and ETS are evaporation and/or transpiration leaving the basin from ponded surface water and saturated subsurface zones, respectively;

SWSTOCH and GWSTOCH are the changes with time in water storage in surface water and ground water of the basin, respectively;

GWIN and GWOUT are the water fluxes entering and leaving ground-water basin by regional ground-water flow, respectively;

LSPGIN and LSPGOUT are the water fluxes entering or leaving the surface-water basin by shallow ground-water flow beneath selected levees, respectively;

PERCOLATION is a regional-scale computation of water movement across the wetland sediment surface based on (2 x 2 mi grid cell) data for a specified domain;

SW-GW_RESIDUAL is a residual (an unmeasured quantity) associated with both the surface-water and ground-water balance equations. Its magnitude can be calculated by difference using either equation 1 or 2. Since this residual term is comparable in magnitude to the other water balance fluxes, it is used to account for vertical fluxes not specifically accounted for by LSPGIN, LSPGOUT, or PERCOLATION. A positive value indicates recharge and a negative value indicates discharge.

All of the variables are positive numbers except for SWSTOCH, GWSTOCH, PERCOLATION, and SW-GW_RESIDUAL, which can be positive or negative depending on the direction of the flux. Changes in storage variables are positive if water storage increases, and negative if water storage decreases. Vertical exchange fluxes (PERCOLATION and SW-GW_RESIDUAL) are positive if recharge occurs and negative if discharge occurs.

Although equations 1-3 are not documented as part of the SFWMM, the variables are exactly as represented in the documentation, both in name and in magnitude (South Florida Water Management District, 1999, p. 12-56). However, recasting the mass balance equations in terms that are most relevant to recharge and discharge did require the definition of one new variable, SW-GW_RESIDUAL, in order to compute residual terms for the equations 1 and 2 by difference. Residual terms that are specific to surface water and ground water have not previously been computed or interpreted by the SFWMD, yet the magnitude of those residuals was found to be large, which has important implications for the estimation of recharge and discharge.

The surface-water mass balance for WCA-2A included all surface-water inflows and outflows from the SFWMM water balance, including a calculation of "percolation" (South Florida Water Management District, 1999, p. 37). According to Ken Tarboton of SFWMD (written commun., 2003), the PERCOLATION flux is calculated for areas of the Everglades where ponded surface water occurs (which generally includes the majority of WCA-2A) by determining the vertical flux of that water across the wetland sediment that must occur to maintain a hydrostatic distribution of pressure head in surface water and ground water. PERCOLATION tends to be relatively large in magnitude and negative in sign in WCA-2A, which defines it as a discharge flux that transfers water from ground water into WCA-2A surface water. The SW-GW_RESIDUAL computed in the present investigation is also relatively large but positive in sign. The SW-GW_RESIDUAL flux should not be confused with RESIDUAL, which is defined in standard SFWMD water-budget reports. The RESIDUAL term is computed by summing all surface-water and ground-water balance terms (which results from combining equations 1 and 2). RESIDUAL fluxes reported by SFWMD for WCA-2A tend to be more than an order of magnitude smaller than the SW-GW_RESIDUAL fluxes reported here, likely because SW-GW_RESIDUAL accounts for unmeasured fluxes of water across the interface between surface water and ground water. In the case of WCA-2A, the SW-GW_RESIDUAL flux accounts for an unmeasured component of recharge greater than the estimate for levee seepage expressed as the LSPGOUT term in equation 1.

In order to estimate recharge in WCA-2A, the positive vertical fluxes across the interface (LSPGOUT, SW-GW_RESIDUAL, and sometimes PERCOLATION) were summed. Likewise, discharge was computed by summing the negative vertical fluxes (LSPGIN and, usually, PERCOLATION). The sum of all these vertical fluxes is the NET EXCHANGE (equation 3), which was usually positive in WCA-2A. A small positive NET EXCHANGE suggests that a small amount of recharge tends to occur on a net basis in WCA-2A.

The calculations of recharge and discharge for WCA-2A based on SFWMM version 3.5 are given in table 2. The results of the 1991-1995 "verification" run and 1979-1990 "calibration" run each have the expected characteristic that the "exchange" fluxes, recharge and discharge, are similar in magnitude with only a small difference which represents a net flux due to recharge in the central area of WCA-2A (table 2). Beyond the general pattern of similarity between recharge and discharge fluxes, there was not particularly good agreement between the two different simulations. The shorter of the two model runs, the 1991-1995 "verification" run, produced estimates of recharge and discharge that are 3 to 4 times larger compared with the longer 1979-1990 "calibration" run. There is no obvious explanation for these differences. That recharge and discharge fluxes should be linked in the interior areas of WCA-2A is supported by detailed measurements in WCA-2A. For example, on the basis of a 5-year record of daily calculations of recharge and discharge, Harvey and others (2004) showed alternating periods of recharge, discharge, and neutral flux conditions in WCA-2A. Peak magnitudes and time periods dominated by recharge and discharge were comparable, with reversals on various time scales ranging from weeks to months. Those detailed results support the general pattern of equality illustrated by recharge and discharge results from the SFWMM. A more detailed comparison between all estimates of recharge and discharge is given later in the report.