MACROPOROSITY AND INITIAL MOISTURE EFFECTS ON INFILTRATION RATES IN VERTISOLS AND VERTIC INTERGRADES

In Vertisols and vertic intergrades, spatial and temporal variability remains a challenge in water flow and chemical transport studies. Infiltration measurements were made with tension infiltrometers operating at supply potentials ≥ −0.24 m in 42 clay-textured horizons from seven Vertisols and three vertic Alfisols. Variability of near saturated infiltration was shown to be linked closely to soil macroporosity and moisture. Data revealed negatively sloped relationships when soil macroporosities were plotted against initial gravimetric soil water contents and when apparent steady-state infiltration rates were plotted against initial water contents. A positive linear relationship was observed between apparent steady-state infiltration rate at 0-m supply potential and soil macroporosity. Because of well developed structure and the occurrence of various types of macropores, most of the vertic soils had enhanced low-tension infiltration rates compared with less structured clay soils. The change in soil macroporosity in the shrink-swell clay soils when change in water content occurred had a greater impact on low-tension flow processes than the change in water content itself. Soil cracking, aggregation, biopores, and the time scale involved in shrink-swell processes were important factors that provided a physical understanding of infiltration behavior in the field.