The aim of the study is the quantification of deviations that occur at local contours approximation of drip soil moistening. The actual contours of drip moistening (made by test data) are asymmetric to varying degrees. The derivation of the dependencies describing them can’t be carried out without approximation of the actually recorded contour lines by data measuring soil moisture levels in the in-loop and contour spaces. By approximation, such contours bring to a symmetric mode about the trickler axis. In the course of investigation, the accuracy of the isopleths approximation of the moisture contour was determined by the results of comparing the values of the left and right parts contours radii with their mean value. In conducting the analytical study, materials and data given in well-known publications were used. As a result of the research it was found out that the deviations of the boundary contour lines and the in-loop isopleths from the vertical symmetry axis vary in the range from 0 to 15 and more percent of the mean value of the contours radii and in some cases reach 50 %. The significant deviations in the left and right radii values in relation to the dripping axis do not have explanations in most of the considered studies, so as the high degree of actual moisture contours asymmetry in others. This circumstance and averaging of the analytical study results presented in the article make it possible to recommend the tolerance of isopleths approximation of the local drip moistening contours at the level of 12 % for practical use. Local moisture contours, approximated with an error of 12 or less percent, can be considered both qualitatively and quantitatively identical to the actual (local) contours of soil drip irrigation.
Key words: drip irrigation, irrigation rate, local moisture contour, isopleth, approximation, acceptable deviation.
