Hydraulic and physical properties of stony soils in a small watershed

The presence of rock fragments in soil layers can have a profound impact on measured hydraulic properties. Variation of surface soil hydraulic properties influences the amount, distribution, and routing of overland flow. The objective of this study was to assess the effect of rock fragments and soil texture on infiltration, hydraulic conductivity, and related physical properties in soils of a small watershed in northwestern Arkansas. Single-ring and tension infiltrometer measurements at three pressure heads (h = -0.03, -0.06, and -0.12 m) were completed on the surface soil layer at 42 sites along three transects crossing the watershed. Upland (Nixa, loamy-skeletal, siliceous, active, mesic Glossic Fragiudults) and side slope (Clarksville, loamy-skeletal, siliceous, semiactive, mesic Typic Paleudults) soils had significantly less rock fragments, lower infiltration rates (i), and lower hydraulic conductivities (K) at and near saturation compared with the valley bottom soil (Razort, fine-loamy, mixed, active, mesic Mollic Hapludalfs). Average infiltration rate at h = -0.03 m for all soils was only 9% of the ponded value suggesting that pores >1 mm in diameter dominated water flow under saturated conditions. At saturation, hydraulic properties tended to increase with rock fragment content while, at h = -0.12, the opposite was true. It is hypothesized that the source of rock fragments (weathering in place vs. colluvial and alluvial origin) and contact with the surrounding fine-earth fraction influence water flow by affecting hydraulic continuity near fragment surfaces. These relatively subtle morphological factors may have a disproportionate impact on water flow under near-saturation conditions in these soils.