ALGEBRAIC MODELS FOR DISC TENSION PERMEAMETERS

Four infiltration models for disc tension permeameters were compared to field and simulated data. Comparisons were made in order to assess the alternative values of sorptivity and steady state flow rates. Model 1 was the conventional analysis with assumptions of one-dimensional flow at small times and Wooding's steady state flow rate at large times. Model 2 was a modified Horton equation defined to fit one-dimensional infiltration at small times with an exponential decay to a steady state value for large times. Model 3 assumed linear diffusion for a soil with constant diffusivity ignoring gravity. Model 4 was based on ''linearized'' flow with gravity. The number of unknown parameters to be estimated ranged from one to four. Sorptivity data using models 2 and 3 generally agreed with the conventional method (model 1) both for the field and simulated data. For the steady state infiltration rate, all of the models gave generally consistent results. Model 2 with four parameters was the best fitting model to field and simulated data. The simulated data allowed comparisons of results based on the total time over which the parameters were evaluated. Results were sensitive to the time of evaluation up to 0.3-1.3 hours for the sandy loam soil.