Designing of underground constructions, such as irrigation and drainage systems, requires engineering surveys. Such surveys include the study of the hydrological conditions of the territory, which are determined by the hydrophysical properties of soils, such as their water-retention capacity. The formation of a hysteresis loop for the soil water-retention capacity occurs because of the variability of meteorological conditions. It is almost impossible to measure all possible scanning branches that fill the gap between the main branches of the hysteresis loop. A mathematical model of the hysteretic soil water-retention capacity is proposed. The model is based on physical concepts of the structure and capillary properties of the soil pore space. Model parameters are identified by dot fitting using data on the main (boundary) hysteresis branches. Scanning branches starting with pre-calculated reversal points are evaluated. Suggested model has a quite low error to predict the scanning branches of soil water-retention capacity. The use of this model ensures reliable estimates of the hydrological conditions of the territory for underground construction. Also it gives precision irrigation rates which result in reduction of gravitational runoff of excessive moisture, preventing pollution of groundwater with agrochemicals.