USING FRACTAL DIMENSIONS OF STAINED FLOW PATTERNS IN A CLAY SOIL TO PREDICT BYPASS FLOW

Methylene blue staining patterns of five undisturbed unsaturated soil cores, 200 mm long and 200 mm in diameter, taken from a well-structured clay soil classified as a Hydric Fluvaquent, were characterized by using the fractal dimension of structure to predict measured bypass flow. Cumulative outflow curves in all cores were well described by a spherical model. Outflow in each core started after a significant time lag from the start of irrigation. Outflow rates during irrigation in all cores were almost equal to irrigation rates; nearly all the water applied, after outflow had started, contributed to bypass flow. Total outflow (O(m), mm) was regressed on the time lag (T1, min) as: O(m) = -0.181 T1 + 9.85. This time lag was caused by the effect of internal catchment of discontinuous macropores and surface storage. The three-dimensional fractal dimension of structure (D(s)3) was calculated for the upper and lower halves of the core, by using values of D(s)2 and stained area in cross-sections. A statistically significant empirical equation, relating the total amount of outflow (O(m)) to both upper and lower values of D(s)3 and to the volume fraction of stained parts (V(s)) is: O(m) = -230.6(V(s)Ds3-1) upper + 232.4(V(s)Ds3-1) lower + 12.6 Thus, a greater D(s)3 value in the upper half of the core and a lower D(s)3 value in the lower half of the core induce larger amounts of outflow: hence vertically continuous macropores, such as fragments of cracks or tubes, play a significant role in the process of bypass flow.