Components of the basic water balance equation and water balance calculation for July 2011 for weighing lysimeter on water supply pumping station in Kleče, Ljubljana the presented. Lysimeter and outflow mass measured with high precision weighing cells and precipitation as determined from changes in the mass of the lysimeter were used in calculation. Precipitation measurements in the same time resolution as the lysimeter mass measurements would be needed for correct calculation of actual evapotranspiration. In time of high plant water requirements only substantial precipitation events directly contribute to immediate groundwater recharge. The low water retention of the aquifer sediments shows susceptibility of the aquifer to ground water pollution. Obravnavani so členi in izračun osnovne enačbe za vodno balance za julij 2011 za tehtalni lizimeter na črpališču pitne vode Kleče v Ljubljani. Pri izračunu so bile uporabljene meritve mase lizimetra in iztoka, merjene z natačnimi tehtalnimi celicami ter padavin, izračunane iz spremembe mase lizimetra. Za pravilen izračun dejanske evapotranspiracije bi bile potrebne meritve padavin v enakem časovnem razkoraku kot potekajo meritve na tehtalnem lizimetru. V času visoke porabe vode s strani rastlinskega pokrova k bogatitvi podtalnice prispevajo le večji, zaporedni padavinski dogodki. Nizke zadrževalne sposobnosti sedimentov vodonosnika kažejo občutljivost vodonosnika in ogroženost podzemne vode zaradi onesnaženja