Bench and Riser Soil Water Content on Semiarid Hillslopes with Terracettes

Abstract:

Microtopographic features known as terracettes are found throughout many semiarid rangelands. These path-like features roughly perpendicular to the slope are frequently traversed by grazing animals on steep hillslopes. The soil properties and hydrologic function, however, are virtually unknown. This research aimed to identify differences in soil properties between terracette bench and riser features, and their influence on soil water content for two terracetted sites and two non-terracetted control sites (grazed and ungrazed) in Eastern Washington State. Measurements of volumetric water content ($\theta_v$), bulk density, soil texture, saturated hydraulic conductivity, pH, and EC$_a$ were collected along with compaction, vegetative cover and cattle density throughout the 2013 and 2014 field seasons. Results show small but significant volumetric water content differences between terracette benches and risers in the upper 10 cm with benches exhibiting higher mean $\theta_v$ than risers throughout the year. Soil bulk density on benches (1600 kg m$^{-3}$) was significantly higher than that of risers (1300 kg m$^{-3}$) with no differences in soil texture. The saturated hydraulic conductivity on benches was roughly half of that for risers. No significant soil differences were noted below 20 cm depth. Terracetted sites showed greater field-averaged $\theta_v$ compared to non-terracetted sites suggesting a positive trend with animal stocking rates. Higher water content on terracette benches is attributed to shifts in pore size distribution with compaction, and a reduction in root-water uptake due to plant-root impedance. This increased soil water does not however increase forage production as it is not accessible to plants.