SATURATED HYDRAULIC CONDUCTIVITY OF VEGETATED DUNES IN THE NEBRASKA SAND HILLS

Stability of fragile ecosystem is a topic of considerable interests in many semi-arid regions like the Nebraska Sand Hills (NSH). Here, vegetations are sparse and sediments are mostly sand and silt. Holocene eolian sand deposits overlie Quaternary and/or Pliocene alluvial sand and silt with low organic component. Vegetated dunes in NSH on average are 40 m high, 825 m long, and 1220m wide. Groundwater recharge of the dune-formed surficial aquifer is utilized by plants that stabilize the dune forms. Therefore, hydrologic cycling is the major factor in stability of the ecosystem. Knowledge of field hydraulic conductivity (Ks) is prerequisite of modeling the groundwater recharge and other processes in vadose zone. However, only isolated Ks measurements have been known in the NSH. Using direct in-situ measurements by constant head permeameter, we investigate spatial 3D patterns of Ks up to 2 m deep in a typical vegetated area in the framework of large-scale experiment. These include vertical and areal Ks distributions in the presence of vegetation. Data support hypothesis on profiles difference between the lowlands and the highlands.

Near the surface (20-cm depth), this difference is insignificant, and KS at all topographic elements are in the lower range (5-7 m/day) when macropores are not considered. We attribute this to homogenizing effects of biological activity (rodents, grazing animals, etc.). Ks magnitude is smaller because of entrapment of finer particles, more consistent vegetation conditions, better-developed soils, and compaction by grazing animals.

At intermediate depths within root zone, the increase in Ks at the lowlands (8-10 m/day range) is lagging behind the increase at highlands (10-15 m/day). Less bioturbated sand at the highlands (ridges is higher in Ks).

At the lower boundary of the root zone (200 cm depth), observed Ks values at highlands and lowlands are between 13 and 15 m/day and relatively uniform if unaffected by the water table.