Infiltration into frozen soil as affected by ripping
Frozen soil contributes to water runoff and soil erosion in the inland Pacific Northwest, especially on fields that are fall seeded. Field experiments were conducted on a Walla Walla silt loam near Pendleton, Oregon, to evaluate the use of chisel tillage to increase water infiltration into frozen soil. Tillage at site 1 was with a single 50-mm wide straight chisel. Tillage at site 2 was with l00-mm wide twisted shovels. Water infiltration measurements were made in the fall, in the winter when the soil was frozen, and in the spring. Water was applied at a constant rate using a rainfall simulator. A finite element model (UNSAT2) was used to verify field measurements of water infiltration for the case when the depth of frozen soil was less than the depth of tillage. The finite element mesh was constructed using a vertical slit to imitate the chisel trough. Tillage spacing to minimize runoff was estimated from historic precipitation records, soil macroporosity features, and unfrozen soil permeability. Undisturbed soil cores, 200 ¥ 200 ¥ 250-mm deep, were impregnated with paraffin and sliced into serial sections to provide details on macroporosity. Cumulative water infiltration was greatest on chisel treatments, compared to no tillage when the depth of frozen soil was less than the depth of tillage. Increased infiltration on the chisel treatments was attributed to the presence of macropore channels. Serial sections of soil cores collected after the soil thawed confirmed the presence of macroporosity. Simulated water infiltration agreed with field measurements and showed that water infiltration was limited by the permeability of underlying unfrozen subsoil. Results also suggest that shallow tillage improves water infiltration only when the depth of frozen soil does not exceed the depth of tillage.