Effects of gypsum slotting on the hydraulic properties of a red-brown earth

In situ measurements of unsaturated hydraulic conductivity (k) and sorptivity (S) were made on a transitional red-brown earth at two sites. The first site was a non-ameliorated soil (control). At the second site the soil had been modified by mixing the soil with gypsum to a depth of 0.4 m using a rotary digger (gypsum slot). Measurements were made using a disc permeameter (radius 100 mm) at the surface of both sites and at depths of 0.15, 0.4 and 0.6 m at the control site, at various potentials.

At the control site, the values of k and S decreased with depth and potential, except at the depth of 0.6 m. The range of S measured was within one order of magnitude. The values of k ranged over two orders of magnitude. The values of k and S measured for the surface horizon were similar to previous measurements by other workers. However, the values of k for the subsoil (0.4 and 0.6 m depth) were greater than previously published values. The values of k and S for the slot soil were intermediate between values for the surface horizon and subsoil of the control soil, and similar to the values at 0.15 m depth of the control soil.

The steady state infiltration rate into a slot was calculated using a model developed for vertical slot mulches and compared with values from previous infiltration experiments. The calculated and measured values of the steady state infiltration rate were in agreement. An equation was derived for predicting the increase in steady state infiltration rate, of a soil containing slots, from the slot depth and spacing. The slot width has only a small effect on the steady state infiltration rate. The maximum steady state infiltration rate occurs when the depth of the slot is 0.837 m. The relationship between the increase in the maximum steady state infiltration rate and the slot spacing is presented. This and the relationship between the increase in steady state infiltration rate, slot spacing and depth were used in examples to predict the increase in steady state infiltration rate for various slot spacings and dimensions.