An important effect of temperature increase due to global warming is the possibility of decreasing agricultural efficiency of atmospheric precipitation as well as the change in the fundamental components of water balance due to increase in evaporation, which is accompanied by the absence of clear precipitation trends in the temperate European zones. The subject of this article is to determine the values of atmospheric precipitation, which during the drought phase should be supplemented with irrigation values so that it does not lower the moisture of the active soil layer in terms of the recent mean levels. We should also take into account different temperature scenarios (1.0, 1.5 and 2.0°C). The multiple-step regression model that has been calculated taking into account the level of soil moisture, air temperature and precipitation indicates that for a mean 10-day period (decade) during the growing season (April-October) the values obtained are 2.2, 3.6 and 5.2 mm, as well as 46, 76 and 109 mm in the entire growing season respectively. This particular model was devised for the city of Poznan and its vicinities, which is situated in north-western Poland.