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A Hydrological Budget (2002-2008) for a Large Subtropical Wetland Ecosystem Indicates Marine Groundwater Discharge Accompanies Diminished Freshwater Flow
Rainfall A seasonal and bimodal pattern was observed in all 7 years of rainfall data (Fig. 2a) with high rainfall occurring in the wet season (June-October). While the rainfall for 2002-2008 averaged over the SRS was similar to a longer term average annual rainfall (1,366 mm, based on 1980-2009 at Royal Palm Ranger Station, ENP), inter-annual variability was observed (Table 3; Fig. 2 (top)) with 2002 receiving the lowest (1,142 mm) and 2005 the highest (1,448 mm). While daily rainfall varied considerably between the stations, indicating spatial patchiness in precipitation on a daily time scale, monthly rainfall amounts did not vary greatly between stations. Monthly rainfall did however vary from year to year (Fig. 5). Uncertainty in rainfall data is due to several factors: instrument precision (0.254 mm on a daily reading translates to 92 mm/year), spatial variability (insignificant on a monthly scale), and inter-annual variation.
Inflow and Discharge from SRS Over 2002-2008, surface water inflows commenced well into the wet season and lagged behind rainfall by 1-2 months (Fig. 2 (middle), Fig. 4, Fig. 5). There was considerable inter-annual variation in the inflows, with the high of 691 mm in 2005 coinciding with the highest precipitation in 2002 through 2008, part of which was contributed by the hurricanes Katrina and Wilma) while almost no inflow occurred in 2007 (Table 3). Inflows were 25-50% of discharges from SRS in each year (Table 3; Fig. 4) with the exception of 2003.Outflows varied from 1,399 mm in 2005, the year with the highest rainfall, to a low of 502 mm in 2003 (Table 3).
Evapotranspiration The Shuttleworth PM model yielded ET values that varied seasonally: 5-7 mm/day in summer and 1-4 mm/day in winter (Fig. 3, Fig. 4, and Fig. 5). Mangrove communities were associated with higher ET than sawgrass on account of greater LAI and vegetation height (Fig. 3). Other models showed a similar seasonal variation but with different magnitudes (Table 1); for instance the FAO PM model had higher ET values than the Shuttleworth PM model, possibly reflecting the agricultural bias of the FAO PM model that is designed to work for a single species under well-watered conditions; thus the FAO PM model yielded similar potential ET estimates for South Florida as the USGS (http://sofia.usgs.gov/eden/evapotrans.php). The Priestley-Taylor equation yielded values twice as large; this model has been reported to overestimate ET in humid areas in summer (Yoder et al. 2005; Suleiman and Hoogenboom 2007). ET estimates obtained from latent heat flux data showed the same seasonal pattern as the other models but was lower in magnitude as has been noticed elsewhere (Bidlake et al. 1996) whereby latent heat based measurements are almost always lower than values yielded by meteorological vapor transport models.
On account of several hurricanes and tropical storms that passed through the area over 2004-2007, the weather towers were severely damaged, leading to large gaps in data over this period. Hence, while we calculated ET over the entire 2004-2008 data availability period, for the budget we consider 2008 as a "model" year with complete daily ET estimates calculated from SRS6 eddy flux tower data. The monthly ET values obtained by Abtew's simple radiation model for 2004 and 2008 did not differ significantly (p>0.99). ET calculated from SRS6 meteorological data can be considered representative of SRS, because net radiation, the single most important driver of ET in the Everglades (Abtew 1996) was found to be very similar at the four eddy flux towers spread over an area of almost 5,000 km2 in the month of August 2008 when we had data from all four weather towers. Error in ET measurements stems from instrumental precision (taken as 5% from Price et al. 2006) as well as from differences in vegetation composition and soil moisture at a given instant of time (Fig. 3).
Water Levels Water levels in SRS varied by less than 1.5 m on a seasonal basis (Fig. 2c). The highest water levels were observed in September-November of each year. Water levels were at their lowest in May of each year.
Water Budget and Net Groundwater Discharge The annual change in water storage in SRS varied from deficit to surplus, from a net gain of 303 mm at the end of 2008 to a net loss of 216 mm in 2006 (Table 3). The magnitude of annual change in water storage was orders of magnitude lower than the other water budget components (Fig. 4). Over the period studied, 2002-2008, rainfall was the largest input, averaging 1,295 mm/year while ET was the largest output averaging 1,367 mm/year (Table 3). Outflows exceeded inflows and all other inputs combined, which led to a net groundwater input into the SRS (termed Groundwater Discharge or GWD) on an annual basis. GWD varied seasonally, with the largest inputs occurring in May-July, and a net recharge or input from the slough surface water in Jan-April (Fig. 4). There was a large annual variation with the highest value of net GWD into the SRS (673 mm) in 2008 and a net recharge of groundwater (88 mm) occurring in 2003 (Table 3; Fig. 5). The estimate of GWD varied with the ET model used (Table 1). The values of GWD obtained in the water budget result from the selection of the modified Shuttleworth PM model to estimate ET.
Surface Water and Groundwater Salinity Surface water salinity varied seasonally between 0 and 23.5 psu. Groundwater salinity also varied seasonally, with the maximum annual values increasing from less than 5 psu in 2003 to about 15 psu in 2008 (Fig. 6). With monthly data, analysis by lag correlation indicated that surface water salinity in SRS had a significant positive correlation with GWD (Fig. 7) when leading by 1 month (p<0.03). The same pattern was demonstrated in the daily data, where the surface water salinity expressed a significant positive correlation with GWD when leading by 24-34 days (p<0.05).
Surface Water and Groundwater Level Both groundwater and surface water levels at site SH2 co-vary over monthly and yearly time scales (Fig. 8 (top)). Daily groundwater level is consistently higher than surface water in early wet season (end of May to August). Monthly averages indicated that at SH2, groundwater discharge to the surface water was dominant in June-August, with surface water recharging the groundwater being dominant during the other months (Fig. 8 (bottom)).
U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 04 September, 2013 @ 02:04 PM (KP)