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Title: ROCK FRAGMENT CONTENT AND SPATIAL VARIATION OF SOIL HYDRAULIC PROPERTIES

Author
item Sauer, Thomas
item Logsdon, Sally

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/13/2002
Publication Date: 11/1/2002
Citation: SAUER, T.J., LOGSDON, S.D. ROCK FRAGMENT CONTENT AND SPATIAL VARIATION OF SOIL HYDRAULIC PROPERTIES. SOIL SCIENCE SOCIETY OF AMERICA JOURNAL. 2002. V. 66. P. 1947-1956.

Interpretive Summary: When rain falls, the water can enter the soil or flow across the surface (runoff) to streams and lakes. If manure, fertilizer, or pesticides have been applied to the soil recently, the water will carry some of the nutrients or chemicals with it. It is important to understand what affects water movement into soil to help prevent pollution of surface and ground water. In this experiment, water movement into soils with different amounts of rocks was measured. The soil on the hill tops and slopes contained fewer rocks than the soil at the bottom of the valley. The soil with more rocks in the valley bottom was able to absorb water faster than the other soils. Results of this study will help farmers and ranchers who apply fertilizers, manures, or pesticides to their fields reduce the risk of water pollution by avoiding application on areas with greater risk of runoff.

Technical Abstract: The presence of rock fragments in soil layers can have a profound impact on measured hydraulic properties. Spatial variation of surface soil hydraulic properties influences the amount, distribution, and routing of overland flow. The objective of this study was to assess the spatial variation of infiltration, hydraulic conductivity, and related soil physical properties in soils with varying amounts of rock fragments. 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 3 transects across a watershed in northwestern Arkansas. At saturation, hydraulic properties tended to increase with increasing rock fragment content while, at h = -0.12, the opposite was true. Upland (Nixa) and side slope (Clarksville) soils had significantly less rock fragments, lower infiltration rates (i), and lower hydraulic conductivities (K) at and near saturation compared to the valley bottom soil (Razort). Average infiltration rate at h = -0.03 m for all soils was only 9% of the ponded value suggesting that pores > 1 mm-diameter dominated water flow under saturated conditions. It is hypothesized that the source of rock fragments (weathering in place vs. colluvial/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.