An improved analytic solution is presented to estimate water infiltration in the crop root zone by surface irrigation methods through a more rigorous determination of the subsurface shape factor, as applied in volume-balance models. Based on the results of this analysis and by averaging the shape factor of previously-published equations, a new relationship was presented, with a maximum error of 0.036%. Also, the effect of these relationships on the empirical parameters for the Kostiakov-Lewis equation and the infiltration depth were evaluated. Based on this evaluation, the relative error difference of the infiltration depth of the actual subsurface shape factor value and the new relationship were less than 0.01%. Given the high accuracy of the proposed methodology and the simplicity of its form, this relationship is recommended for use in practical work to predict surface irrigation hydraulic performance and application efficiency.
