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

Abstract

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 (q v ), bulk density, soil texture, saturated hydraulic conductivity, and organic matter in addition to compaction, vegetative cover, and cattle density. Our results show significant q v differences between terracette benches and risers in the upper 10 cm, with benches exhibiting higher mean q v 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.Abbreviations: EMD, site with east aspect and moderate-to low-density cattle stocking; OC, organic carbon; PSA, particle size analysis; WHD, site with west aspect and highdensity cattle stocking.

Modeling of terracette‐hillslope soil moisture as a function of aspect, slope and vegetation in a semi‐arid environment

“…The value of bench and riser morphological classifications in relation to site slope is important in defining the role of terracettes in these semi‐arid ecosystems. This can be exemplified by coupling the mechanistic understanding of terracettes and θ v by Corrao et al (), and the simulated θ v values for various aspects, LAIs and slopes from this research with the morphological classification of terracette features by TLS for the estimation of how total terracette slope θ v varies as a function of these biophysical conditions (Figure ). Because terracette benches were shown to have greater θ v than risers regardless of the differing cattle use conditions, as measured and simulated in this study, the prediction of bench width and riser length allows the calculation of percent‐area for both features when coupled with slope of a terracette hillside.…”

“…The example in Table shows an average of more than 4% greater soil moisture for the observed conditions of the WHD site in this study exhibiting an overall slope of 26% and lower LAI, as opposed to the EMD site with an overall slope of 49% and a greater LAI. The differences in LAI between these two sites was primarily driven by soil compaction and cattle stocking density as discussed in previous research (Corrao et al, , ).…”

“…Similarly, the varying stocking densities produced a wide range of vegetative cover conditions ranging from 95% cover on the ungrazed control to 4% on the most intensively used WHD site. A more detailed description of site and plot characteristics are provided in (Corrao et al, , )…”

“…Similarly, the varying stocking densities produced a wide range of vegetative cover conditions ranging from 95% cover on the ungrazed control to 4% on the most intensively used WHD site. A more detailed description of site and plot characteristics are provided in (Corrao et al, 2015(Corrao et al, , 2016 Soil sampling A total of 725 soil samples were collected between 2013 and 2015 using an ICT International Model 0200 soil bulk density sampler (5.7 cm i.d. by 10 cm long).…”

“…Similarly, overland flow simulations have demonstrated that microtopography, similar to terracettes, can reduce the net runoff and erosion in semi-arid systems during rainfall events by moderating flow velocities through reduced slope-length, and depression storage (Liu and Singh, 2004;Mayor et al, 2008). The research of Corrao et al (2016) demonstrating greater soil moisture on terracette benches as opposed to risers and significantly higher soil moisture on terracette sites as opposed to non-terracette sites under active gazing, is the only study we are aware of that quantifies plot-scale soil moisture patterns related to terracette microtopography in semi-arid rangelands.…”