Quantitative Metrics of Soil Structure and Relationships to Hydraulic Properties in a Vertic Argiudoll

Abstract

Soil structure is a fundamental property referring to the morphology of soil aggregates and the network of void spaces between them. Structure affects many pedogenic, hydrological, and other ecosystem service processes. While its importance is generally recognized, the tortuous nature of soil structure and its variable size and expression make this property difficult to quantify, especially at the pit scale. The absence of quantitative soil structure metrics also inhibits the ability to accurately model water flux. This research explores the application of multistripe laser triangulation (MLT) scanning to a soil profile in the field. MLT scan data were analyzed for their ability to quantitatively characterize soil structure. The study site was located near Lawrence, KS in a Grundy soil series with vertic properties, where soil moisture sensors were installed in a lysimeter next to an exposed profile. Several logistical problems concerning MLT field operations and data processing are addressed in this work including: ambient light, MLT scanner positioning in relation to the soil surface, and post-processing procedures for the resulting data. MLT scans capture the profile surface along with areas of missing data, termed surface scan gaps (SSGs), which represent preferential flow paths (PFPs) actually observed in the soil. Metrics describing SSGs were first studied to determine whether the digital data could be related to conditions observed in the field. These metrics were then examined in relation to soil hydraulic parameters, especially saturated hydraulic conductivity (Ks) and water retention curve (WRC) parameters. Soil moisture data collected at the lysimeter, in conjunction with atmospheric data from an adjacent tower, were used as inputs for Hydrus 1-D to predict, then separately to verify hydraulic parameters that were obtained using quantitative soil structure metrics. Several close relationships were identified with WRC parameters such as α and n, as well as relationships with Ks. These connections, enabled by quantification of soil structure as a continuous rather than categorical variable through field-based measurements, present an opportunity to inform soil water flux models and advance the understanding of mechanisms underlying field-scale cycling of soil water.