INVESTIGATING THE INFLUENCE OF VERTISOLS ON RECHARGE IN NEBRASKA'S RAINWATER BASIN WETLANDS

The south-central region of Nebraska contains internationally important wetlands. The region, known as the Rainwater Basin, extends over 17 counties with about 34,103 wetland acres. It has been estimated that there were about 100,000 wetland acres in the early 1900s. This dramatic decrease in wetland acreage was caused by wetlands being removed or altered to create more production area due to the high value of agriculture to the region. These wetlands are vitally important for many species to survive on the semi-arid plains. However, they are most recognized for aiding migratory fowl as resting and breeding grounds as they make their journey between northern regions during summer and southern regions during winter.

The wetlands were formed over thousands of years by wind erosion in silt-loam loess soil. Soil development over time created vertisols on the basin floor. Today, these depressions are filled by water from precipitation or irrigation runoff. While inputs are known or easily identified, the outputs of water from these depressions are not well understood or quantified. It is assumed that the clay bottoms create an impermeable layer to water recharge and that ET is the only mechanism removing water. This assumption may be false however as recharge may be occurring during dry periods when desiccation cracks form in the basin floor soils.

Eight wetlands were picked to evaluate the hydrologic nature of the depressions in the basin. Drive-point wells were inserted into the soil at a depth of one meter to obtain soil hydraulic properties. A stilling well was inserted to determine the wetlands hydroperiod. Soil moisture sensors were inserted to obtain data on water movement in the top two meters of the soil profile. Analyses of the drive-point well data indicate that the degree of desiccation of the clay, which is influenced by soil moisture, greatly changes the hydraulic conductivity. When the soil is dry, it provides preferential pathways which can make conductivity values equivalent to a sand/gravel sediment. When saturated, conductivity values are extremely low and the basin floor acts like an aquitard impeding water movement. Preliminary results suggest that these wetlands may provide recharge as a pulse of water through the highly desiccated soil when sites are dry, but when wet, and thus “sealed” act as evaporation basins.