Calibrating Hydric Soil Field Indicators to Long‐Term Wetland Hydrology

Abstract

Jurisdictional wetlands are required to be saturated to the surface for 5% or more of the growing season in 5 out of 10 yr, but practical field methods for confirming this are lacking. This study determined whether hydric soil field indicators were related to wetland hydrology requirements. Water table levels were monitored daily for 2.5 yr in a toposequence of nine soil plots that included well to poorly drained members (Oxyaquic Paleudults and Typic Albaqualfs). Monitoring data were used to calibrate a hydrologic model that simulated water table levels from inputs of hourly rainfall data. Forty years of rainfall data were then used with the model to compute long‐term daily water‐table levels in each plot. These data were summarized as “saturation events”, which are the frequency that water tables were at or above preselected depths for at least 21 d. Twenty‐one days was the average period needed for Fe reduction to begin in these saturated soils. This condition must occur for hydric soil field indicators to form. Regression equations were developed to relate saturation events to percentages of redoximorphic features. The r 2 values for relationships between percentages of redoximorphic features and saturation events were >0.80 for depths of 15 cm, and >0.90 for depths between 30 and 90 cm. Results showed that the depleted matrix field indicator, in which redox depletions occupy >60% of the horizon, occurred in soils that were saturated for 21 d or longer at least 9 yr out of 10. This indicated the depleted matrix indicator occurred in soils that were saturated nearly twice as long, and more frequently, than the minimum requirements needed to meet wetland hydrology requirements.