The monitoring project was designed to develop a water budget for selected agricultural impoundments and to provide data for hydrologic analysis. The water budget data will be used to determine the water storage characteristics of the impoundments. The hydrologic analysis will evaluate the effect of seepage on water storage. A hydrologic simulation model will be constructed to model water movement for the reservoir and the area of the grove that interacts with the reservoir. This includes the area of the grove from which storm water is pumped into the reservoir and the area of the grove from which water is removed for irrigation. The monitoring data from the reservoir will be used to calibrate the model.

Water Budget

The agricultural impoundments will be monitored to provide data for calculating a water budget as given in the following equation:

(Rain + pumpage) – (evapotranspiration + seepage + discharge + returnflow) = D storage

The inflows and outflows will be monitored or estimated as well as the change in water storage in the impoundment (Table 7.2). The inflows are rainfall and pumped inflow from the grove following storm events. There may be several inflow pumps depending on the size and configuration of the impoundment. At the GFC grove the inflow from another section of the impoundment is also monitored. The rainfall will be monitored with automated tipping-bucket raingage to get hourly rainfall depths and a manual raingage to get weekly totals for comparison. Each inflow pump will be equipped with a rpm sensor to determine the number of revolutions of the pump and the volume pumped. All of the pumps are diesel pumps and the speed can vary depending on the requirements of the grove. The manufacture’s pump curve is used to estimate discharge. This is calibrated by measuring the discharge. The ditch water level is monitored to provide the head for the pump.

Impoundment outflows are more difficult to monitor than inflows. The outflows include evaporation and transpiration, offsite discharge, return flow to the grove and seepage. Offsite discharge is determined by monitoring the upstream and downstream water levels at the discharge structure and using the appropriate culvert formula for calculating flow. Similarly the return flow is estimated using the water level in the impoundment and the water level in the toe ditch outside the impoundment and calculating flow using the culvert flow equation with the appropriate drop-structure. Evaporation and transpiration are estimated using the estimated potential evapotranspiration (PET) and the appropriate crop conversion coefficients. The PET is estimated using weather data collected at each site and the crop coefficients are used to estimate the actual ET from the PET values. The extent and type of vegetation in each impoundment will be mapped to determine the combined ET for the site. The evaporation from open water will be estimated and combined with the ET estimates. The remaining outflow, seepage, will be estimated using several techniques. These are discussed in detail below

Wetland Water Level

The water level is monitored in the interior of the impoundment at selected locations. These data are necessary for quantifying the storage characteristics of the impoundment. Unlike deep, open-water reservoirs where the water-level in uniform, the water levels in impoundments varies with the natural land topography and density of vegetation. Impoundments are simple structures consisting of a cast-in-place levee that surrounds the native landscape. The native land consists of wetland and upland vegetation, which often provides considerable resistance to flow. Typically water stacks up inside the impoundment near inflow pumps rather than forming a level pool in the impoundment. This is due to dense vegetation that commonly grows near the inflow pump. Consequently it is necessary to monitor the water level at several locations in the impoundment that are not connected by continuous open water.

It is also necessary to monitor different locations in the impoundment due to variations in ET. The ET varies due to differences in the type and density of the vegetation. The vegetation types vary as a function of the original landscape, the depth of water, the imposed hydroperiod, and the fertility of the site. Greater vegetation density will produce higher ET rates that may result in lower water levels. With higher vegetation density the flow of water from deeper areas to the areas of greater ET will be restricted resulting in differences in water level and storage capacity. These dynamics are important to understand to evaluate the storage capacity of the impoundments.

Seepage Assessment

Seepage may be a significant component of the water budget for impoundments in southwest Florida (Barton et al. 1998, CDM 1996). Seepage occurs where impoundments have been constructed on porous geologic materials or levees are not compacted. Seepage may have a positive or negative effect on water storage. It may be positive where water pumped into the impoundment seeps out recharging the local water table aquifer and can be recovered by pumping out of the impoundment during a dry period. Missimer (1996) demonstrated this effect. Seepage is negative when the water is lost to ground water and can not be recovered. The degree to which the ground water recharge is useful depends on the depth of the ditches in the impoundment, the vertical conductivity of the bed material, the lateral conductivity of the foundation below the dikes and depth of the confining layer. The local groundwater storage effect of seepage may be a means for improving local water supply.

The possible loss of water through seepage needs to be monitored for each impoundment. There are four general cases for seepage loss. These include vertical loss, seepage through the levee, seepage under the levee, and deep lateral seepage. Vertical loss may be the most important. These impoundments were selected because they were constructed over shallow clay layers that were within 20 ft deep. It was expected that the clay layer would make vertical seepage losses small. Vertical losses will be calculated to evaluate this assumption. This may be evaluated by monitoring the piezometric head in the aquifer below the confining zone and determining the gradient. Using estimated permeability the seepage rates will be calculated.

Where possible deep groundwater wells will be drilled at three locations near each impoundment. At these sites we intend to drill wells that connect with the lower Tamiami aquifer at approximately 120-ft depth. The piezometric head in the lower Tamiami aquifer will be monitored in order to calculate the gradient with the water table aquifer and estimate deep seepage loss. These wells will be instrumented with continuous water-level monitoring devices. Where it is not practicable to drill wells existing monitoring wells may be instrumented to monitor the piezometric head the underlying aquifer.

There are three modes of lateral seepage that should to be evaluated at each impoundment. Seepage through the levee is an important mode of water loss when the water level in the impoundment is substantially greater than the water level in the outside dike. The difference in water level may be 8 to 10 ft when the impoundment is full. Where impoundment levees are formed without compaction from available soil materials, which is the common practice, the levee may have a high seepage rate. The rate of seepage can be evaluated by direct measurement using a seepage meter (Ballanger, 1986) or calculated using the gradient method. The seepage meter method requires the temporarily installation "seepage meters" in the impoundment and measurement the water loss. The seepage meters will be installed at 8 to 16 locations in each impoundment for a period of 2-3 days. Seepage will be measured at different water levels at each impoundment. The gradient method uses the head gradient and the hydraulic conductivity of the levee material to estimate seepage loss. Hydraulic conductivity will be determined using samples collected from several locations at each impoundment.

Lateral seepage loss through the foundation material is another important process of water loss. This process involves water movement from the impoundment through the native soil that makes up the foundation of the levee to the toe ditch at the base of the levee. Seepage loss may be measured using a seepage meter if there are sites where good contact can be made with the bed sediment in the impoundment. Where thin layers of organic material provide a seal against seepage the use of the seepage meter may provide erroneously high values. The gradient method can be used to estimate the seepage providing good estimates of lateral hydraulic conductivity are obtained for the foundation material. These values will be obtained using the borehole drawdown method at several locations on the levee.