Ring diameter effects on determination of field-saturated hydraulic conductivity of different loam soils

Establishing ring diameter effects on the field-saturated soil hydraulic conductivity, Kfs, determined with ponding infiltrometer methods can help to find a compromise between the need to sample a large area with an individual measurement and the impracticality of using large rings in the field. Five ring sizes (diameter, D = 5.5, 10.9, 16.0, 27.8 and 31.8 cm) were used to determine Kfs by the simplified falling head (SFH) technique in four loamy soils with different salinity (electrical conductivity of saturated extract, ECe = 0.9-29.4 dS/m) and sodicity (exchangeable sodium percentage, ESP = 2.7-81.3%) levels. According to USDA classification, two soils were non-saline, non-sodic (NN1 and NN2), a soil was saline-sodic (SS) and another soil was non-saline and sodic (NS). Ring diameter did not have in general a statistically detectable influence on the mean Kfs of a given soil. The only exception was for the NS soil but also in this case the effect was negligible for many practical applications since Kfs increased by 2.3 times as D increased from 5.5 to 31.8 cm. However, smaller rings implied either higher or lower estimates of Kfs variability as compared with larger rings, depending on the soil. The former result probably occurred when only a part of total heterogeneity was sampled with a small ring. The latter result was probably obtained when insertion of small rings altered or even destroyed the fragile macroporosity and also when using small rings increased the probability to only sample relatively homogeneous soil volumes. As compared with the largest rings, those with a diameter in the range 10.9 to 27.8 cm yielded a similar information on the differences between the soil hydraulic conductivity of the four considered soils. More discrepancies were detected for the smallest rings (D = 5.5 cm). Even small rings appear usable to obtain a mean value of Kfs for the sampled soils, perhaps with the exception of the smallest ones. However, as large as possible rings should be used if Kfs variability has also to be determined.