Estimation of sorptivity from disc-permeameter measurements

The use of disc permeameter in the field often requires the placement of a layer of sand in order to provide good contact between the soil surface and disc supply membrane. The effect of the contact sand on water infiltration into the soil and the estimate of sorptivity (S) was investigated. A numerical study using a loam and clay, and a field experiment on heavy clay was conducted. Placement of the sand significantly increased the cumulative infiltration but showed small differences in the final infiltration rate. Estimation of S based on Philipʹs two-term algebraic model using different methods was also examined. Conventionally S is estimated from the early-time infiltration rate when the capillary force is dominating. But results showed that this method always overestimated S and give the largest error. A numerical study indicated that differentiating the cumulative infiltration with respect to the square-root of time provided the closest estimate of S compared to the true value, and furthermore showed small variations between treatments with contact sand. Field experiment revealed that the error in infiltration measurements followed the pattern of the infiltration curve. The absolute rate of infiltration without placement of sand was considerably lower because of the poor contact between the disc and soil surface. Thus contact sand is recommended, although care must be exercised with the interpretation of the data.