Comparison of Procedures to estimate Steady Flow Rate in Field Measurement of Saturated Hydraulic Conductivity by the Guelph Permeameter Method

Abstract

Different procedures for estimating steady flow rate during field application of the Guelph permeameter method were compared in terms of field-saturated hydraulic conductivity Kfsand equilibration timets estimates. The steady flow rate was evaluated by the slope of the linear portion of the cumulative drop in water level in the permeameter reservoir versus time plot (reference procedure) and by approximate procedures that use the first three or four consecutive values of the rate of fall of the water level equal or differing by less than a given percentage. The influence of the selected time interval between two successive readings (Δt=2 or 10 min) at the permeameter was also investigated. On average, the approximate procedures were practically equivalent and produced aKfs overestimation for the selected experiments (sample sizeN =81;Δt=2 min; 0·7 mm/h⩽Kfs⩽121·5 mm/h) which was not, however, consistent with the observed, substantial tsunderestimation. TheKfs prediction accuracy varied with soil permeability: in low permeability soil, theKfs overestimation was substantial, being also equal to more than an order of magnitude; in high or relatively high permeability soil (i.e. for reference Kfsvalues greater than about 25 mm/h), the approximate and the reference procedures produced similar results, i.e. differing by a maximum factor of 1·5. The increase in the time interval between two readings of the permeameter gave an improved accuracy of thets and Kfsestimates but also a decrease of the number of successful experiments (i.e. experiments producing estimates of Kfs) within each approximate procedure. An empirical relationship between the minimum equilibration time required to obtain accurate Kfsvalues and the early-time infiltration amount was derived. Using this relationship effectively allowed improvement inKfs estimation accuracy compared to the considered approximate procedures.