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Evaluation of Methods to Calculate a Wetlands Water Balance
W. Lee Daniels
G. Michael Fitch
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The development of a workable approach to estimating mitigation site water budgets is a high priority for VDOT and the wetlands research and design community in general as they attempt to create successful mitigation sites. Additionally, correct soil physical, chemical and biological properties must be restored that are appropriate to the intended wetlands biota in order for the sites to function similar to a natural sites that they are replacing. The major objectives of this research program were to evaluate the currently recommended procedures for estimating wetland water balances and to characterize the soil and hydrologic regime present at natural and constructed sites and their interaction with wetlands biota. This report records our efforts to develop an estimated overall water budget at VDOT's Ft. Lee mitigation site along with a summary of our previous water budget studies at Manassas. Detail on supporting studies is also provided along with an overall summary of multi-year research results and implications. In this report, the terms water balance and water budget are used almost interchangeably. In our view, however, water budgets are developed by humans to interpret actual wetland water balances. It was concluded that the use of the Pierce (1993) approach for developing mitigation wetland water budgets is prone to a number of errors in surface water charging estimates and ET estimates via the Thornthwaite method. The Pierce approach is most appropriate for estimating water budgets in surface water driven emergent/shrub-scrub systems with little ground water flux that rely upon berms or other water control structures to detain and pond water over impermeable soils or strata. Additionally it was found that the development of soil redox features, particularly the quantity and distinctness of oxidized rhizospheres can be reliably used to interpret hydric soil development sequences in mitigation wetlands. However, the reestablishment of an appropriate mitigation site wetness regime to one that appears to meet jurisdictional wetness criteria will not always guarantee the success of desirable hydrophytic vegetation over time.