A CASE STUDY ON INFILTRATION INTO DRY CLAY SOIL I. MORPHOLOGICAL OBSERVATIONS

Abstract Infiltration patterns of water into four dry clay soils with different macrostructures were determined in the field as a function of application rate (i = 4−80 mm/h) and applied quantity (q = 2.5−40 mm), using methylene blue as a tracer. Stains on the walls of large vertical pores (dominantly planar voids) occurred as small vertical bands and were observed and counted to a depth of 1 m after excavation of the 0.5 m2 plots. Deep infiltration through large pores, as demonstrated by occurrence of stained bands, could be manipulated by varying application rates and applied quantities. The number of stained bands was always very low, occupying at the most an estimated 2% of the available vertical surface area of large prisms, and (for each stained band) a horizontal cross-sectional area of 60 cm2 or more. Infiltration was deeper, at comparable flow regimes, in two soils with strongly developed prisms, as compared with two soils with moderately developed prisms. Deep infiltration occurred along vertical prism faces where grass roots were also concentrated.