Soil Moisture Differences between Terracette Benches and Risers on Semiarid Rangeland Hillslopes

Abstract

Core Ideas Hoof impact on terracetted hillslopes affects bulk density and compaction between bench and riser. Increased soil moisture is not plant available and doesn't increase forage production. Greater water content on benches is due to shifts to smaller pore sizes with compaction. Reduced vegetative cover due to root impedance results in greater water content. Microtopographic features known as terracettes are found throughout semiarid rangelands. Their soil properties and hydrologic function, however, are virtually unknown. This research aimed to quantify whether or not terracetted hillslopes retain more soil water. The objectives of this research were to: (i) assess soil moisture measured at two terracetted field sites comparing bench and riser; (ii) identify to what extent microtopography, soil properties, and land use affect soil moisture; and (iii) quantify differences in soil moisture at the field scale between terracetted and control sites (grazed and ungrazed) in eastern Washington. We measured volumetric water content (θv), bulk density, soil texture, saturated hydraulic conductivity, and organic matter in addition to compaction, vegetative cover, and cattle density. Our results show significant θv differences between terracette benches and risers in the upper 10 cm, with benches exhibiting higher mean θv than risers across all sites during both wet (+6.14%) and dry (+6.63%) seasons. Soil texture and organic matter did not vary between bench and riser features, and microtopography itself was not driving observed soil moisture differences. Soil moisture differences were attributed to land use (i.e., cattle) impact on soil bulk density and vegetative cover. Greater water content on terracette benches is partially attributed to shifts to smaller pore sizes with compaction and a reduction in transpiration resulting from lower vegetative cover due to root impedance. However, this increased water storage is not plant accessible and does not contribute to increased forage production. This work is intended to provide a mechanistic understanding of terracette hydrology for semiarid rangeland management.