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publications > paper > groundwater control of mangrove surface elevation: shrink and swell varies with soil depth > results
Both seasonal and monthly lunar influences were important for the hydrological conditions at the site (Fig. 2). The highest monthly mean stages at Shark River were in September and October (-0.23 and -0.19 m, respectively), typical for this drainage. The high river stage was a result of the maximum discharge of accumulated water from the wet season (June-September, Fig. 2). Groundwater piezometric head pressure was also high during September and October (0.06 and 0.12 m, respectively) due to hydrological recharge from the wet season. Daily river stage was a reflection of monthly lunar tidal flooding, wet season river discharge, and annual sea level variability (thermal expansion, Provost 1973).
There was moderate correlation between the two hydrological metrics used in the multiple regression with an r = 0.72 for Shark River stage to groundwater piezometric head pressure. Tolerance values were above 0.547 and variance inflation factors were less than 1.829, suggesting that despite some correlation between predictor variables, collinearity was not a serious issue for these data (Neter et al. 1996; Quinn and Keough 2002).
The feldspar marker horizons did not become completely covered until 172 d after installation (September 10, 2002). The marker horizons were covered with mineral, organic, and root matter. The annual accretion rate was 6.64 ± 0.56 mm yr-1 (±1 SE). Sediment deposition values were intermittent in nature with high rates in October 2002 and March 2003 (Fig. 3). Slight erosion was evident during the November to December 2002 period (-1.8 mm) and the December 2002 to January 2003 (-0.8 mm) sampling.
Changes in absolute soil surface elevation for
both the deep-RSETs and original-SETs followed
a similar pattern (Fig. 3). Both devices recorded the
highest mean soil elevations at the end of the wet
season (8.89 mm on October 10, 2002, for the
original-SET and 15.14 mm on November 9, 2002,
for the deep-RSET) and the lowest mean elevations
during the dry season (January 9, 2003; -2.24 and -0.06 mm, respectively). The shallow-RSETs had
a distinctly different pattern of soil surface elevation,
with the highest elevation at the end of the wet
season (6.83 mm on November 9, 2002) and the lowest early in the wet season (-0.66 mm on June 3,
2002, Fig. 3).
RELATIONSHIPS BETWEEN SOIL ELEVATION AND HYDROLOGY
The daily rate of soil elevation change of the shallow-RSET was partially explained (Adjusted R2 = 0.16) by a negative relationship with the DRC of the river stage at the site (Table 2). That is, as river stage increased, the soil elevation that was influenced by the shallow soil zone decreased (Fig. 4). The rate of soil elevation change of the original-SET was positively related with the DRC of the groundwater head pressure (Adjusted R2 = 0.61; Fig. 4, Table 2). This model was run with a reduced data set (n = 28) due to a one time sampling error of original-SET number 2. The DRC of soil elevation for the deep-RSET had a strong positive relationship to the DRC of the groundwater head pressure (Adjusted R2 = 0.90; Fig. 4, Table 2). When groundwater head pressure increased the soil elevation increased for both the original-SET and the deep-RSET.
CONTRIBUTION OF EACH ZONE TO EXPANSION AND CONTRACTION OF THE ENTIRE PROFILE
We calculated the variation in thickness of each of the four constituent soil zones (Eq. 2) and the entire soil profile. We determined how much each of these soil zones contributed to absolute change of the entire profile by using a stepwise multiple regression model in which absolute change in the thickness of the entire profile was the dependent variable and the absolute changes in thickness for each soil zone were independent variables.
The contribution of each soil zone was not equivalent to the relative proportion of soil profile it comprised (Fig. 5). The bottom zone (4-6 m) accounted for 63% of the variation in the absolute change in thickness of the complete profile whereas the middle zone (0.35-4 m) accounted for only 22% (Fig. 5, Table 3). The bottom zone comprises only 31% of the entire profile whereas the middle zone comprises 63%. Accretion and the shallow zone were not significant contributors to the overall absolute change in thickness of the entire profile (Table 3).
U.S. Department of the Interior, U.S. Geological Survey
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