Effects of microtopography and capillarity on specific yield in wetlands

The hydraulic parameter specific yield commonly is used in hydrologic simulation of surficial systems to relate a water flux to a change in water level. Wetland specific yield is often assumed to be a constant value equal to the difference between saturation and residual moisture content for water levels below land surface and equal to a value near unity for water levels above land surface. Microtopographic variations and soil capillarity, however, can result in values of specific yield that vary continuously with water level, particularly in environments such as wetlands with a shallow water table and/or periodic inundation. The effects of microtopography and capillarity on specific yield of wetlands in the Florida Everglades were evaluated at three sites for which microtopographic data were available. Substantial discrepancies were noted between the traditional concept of wetland specific yield and that determined considering microtopographic and capillarity factors. The results indicate the need to routinely incorporate these factors in estimation of specific yield as part of wetland hydrologic simulation.