| Summary: | This study aims at furthering our understanding of the Modified Philip−Dunne Infiltrometer (MPDI), which is used to determine the saturated hydraulic conductivity <i>K<sub>s</sub></i> and the Green−Ampt suction head Ψ at the wetting front. We have developed a forward-modeling algorithm that can be used to simulate water level changes inside the infiltrometer with time when the soil hydraulic properties <i>K<sub>s</sub></i> and Ψ are known. The forward model was used to generate 30,000 water level datasets using randomly generated values of <i>K<sub>s</sub></i> and Ψ values. These model data were then compared against field-measured water level drawdown data collected for three types of soil. The Nash−Sutcliffe efficiency (NSE) was used to assess the quality of the fit. Results show that multiple sets of the model parameters <i>K<sub>s</sub></i> and Ψ can yield drawdown curves that can fit the field-measured data equally well. Interestingly, all the successful sets of parameters (delineated by NSE ≥ the threshold value) give <i>K<sub>s</sub></i> values converged to a valid range that is fully consistent with the tested soil texture class. However, Ψ values varied significantly and did not converge to a valid range. Based on these results, we conclude that the MPDI is a useful field method to estimate <i>K<sub>s</sub></i> values, but it is not a robust method to estimate Ψ values. Further studies are needed to improve the experimental procedures that can yield more sensitive data that can help uniquely identify <i>K<sub>s</sub></i> and Ψ values.
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