|Bonta, James - Jim|
Submitted to: Gumpensteiner Lysimeter Congress
Publication Type: Proceedings
Publication Acceptance Date: 4/7/1997
Publication Date: N/A
Interpretive Summary: Much of our basic understanding of the process of infiltration of water into soil during rain storms is based on early theory and computer models, assuming that water moves uniformly through each soil layer. Several years of more recent field and laboratory studies have documented rapid infiltration of rainfall within a few large soil pores while in much of the esurrounding soil there is no infiltration. This process has been well documented to occur in the soil profile, and we can now show that it also occurs in the fractured stone bedrock layers beneath the soil. We measured water flowing down through the soil and bedrock 2.5 m below the soil surface in isolated 0.6- by 2.0-m sections. In March 1994, one section had water draining through it at 3300 mL per hour while adjacent sections produced flow at only 1 and 69 mL per hour. During the first 4 months of 1995, one section produced 326 L of water while an adjoining section produced only 2 L. These numbers show that the water that flows down through the soil and fractured bedrock to replenish groundwater supplies does not always move uniformly through these zones. Flow in "preferential" pathways may allow surface-applied chemicals to by-pass the filtering capacity of the soil; or just the opposite, it may allow clean rain water to by-pass chemicals held in the soil. These findings contribute to our knowledge and understanding of how water and chemicals move once they are below the soil surface, and as a result, this research will assist in developing land use management practices which are more environmentally friendly.
Technical Abstract: Lysimeters at the North Appalachian Experimental Watershed (NAEW) near Coshocton, Ohio USA were used to evaluate preferential flow through the soil and parent material and its influence on sustained crop production. The Coshocton lysimeters are 2.44 m deep, containing 1- to 1.5-m undisturbed, residual soil profiles overlying >1 m of sandstone or shale bedrock. Eight separate pans comprising the bottom of each lysimeter, wer instrumented to assess spatial and temporal variation in percolation. Downward flow through the parent material occurs mainly at atmospheric pressure through spatially variable cracks. In March 1994, when the soil profile was nearly saturated, one pan produced percolation at 3300 mL hr-1 while the adjoining pans were producing at 1 and 69 mL hr-1. During the first 4 months of 1995, one pan produced 326 L of percolation while the adjoining pan produced only 2 L. Some pans respond immediately to intense rainfall with high percolation rates, indicating direct connection to continuous, preferential flow paths in the soil and through the parent material. Response of other pans is consistently delayed, often continuing to produce percolation for days or weeks after the rain. These observations help explain the movement of agricultural chemicals through our soils and enhance the evaluation of long-term management practices on our soil and groundwater resources.