Bias in ponded infiltration estimates due to sample volume and shape

Estimates of saturated and unsaturated water flow in soil are important for predictions of infiltration, runoff, and solute transport. Previous research indicates that ponded infiltration estimates are influenced by the volume or cross-sectional area of the measurement. Our study quasi-steady infiltration measurements made using 20-, 30-, 45-cm-diameter cylinders driven 25 cm deep into 56 field plots under diverse agricultural management practices. Mean infiltration rate increased from 50, to 81, to 95 mm h(-1) as diameter increased. Standard deviation and range also increased with diameter. All three diameters produced lognormal data distributions. These results indicate that increasing the sample area is not equivalent to pooling of many smaller samples, which would have produced the same mean but with a lower variance. Follow-up experiments with a double-ring configuration or a divider placed in the center of a 45-cm cylinder demonstrated that adding vertical barriers reduced infiltration even when the total infiltration area was unchanged. A pulse of dye introduced 10 min before removing the ponded water showed an extensive network of dyed flow pathways in all but the slowest infiltration situations. The pathways were not associated with visible macropores. Careful consideration should be given to the dimensions of samples used to estimate saturated and possibly unsaturated flow from infiltration experiments.