AbstractThe unsaturated zone plays a crucial role in the hydrological cycle and is a key factor in modeling and understanding hydrological processes. It regulates water transfer from the land surface to the groundwater while providing protection, transfer, and attenuation of potential groundwater contaminants. This research presents the first results of continuous in situ monitoring of all soil horizons of the Eutric Cambisols and the remaining unsaturated zone of the Zagreb aquifer (Velika Gorica well field). The Zagreb aquifer is the main source of potable water for the inhabitants of the City of Zagreb and Zagreb County, which is protected by the Republic of Croatia. Cross-correlation analysis and water stable isotopes (δ2H and δ18O) were used to investigate whether there is a hydraulic connection between precipitation and groundwater, the approximate duration of percolation, and whether there are indicators that suggest the presence of preferential flow or processes such as water mixing and/or evaporation. The cross-correlation analysis showed and confirmed the assumption of the presence of preferential flow with average lag times ranging from 1.59 up to 3.63 h. The results also indicate that low organic matter content may contribute to higher lag times associated with precipitation infiltration. It was found that only high-intensity precipitation events allow effective infiltration, and that the amount of infiltration depends on the water content of the soil at the beginning of each precipitation event. The isotopic results showed that the isotopic composition of the soil water in the first three soil horizons follows the one of precipitation, while deuterium excess confirmed that the sampled water is mobile water and that an evaporation process is possible in the A horizon. The results also show that suction cups can be used to determine the isotopic signature of both bulk and mobile water, depending mainly on the granulometric composition of the material in which they are installed.Practical ApplicationsIntensive and growing groundwater extraction in past decades now challenges humans to maintain the resource, especially in regions where climate is drying. Knowing which factor and to what extent drives the process of resource replenishment is the key to maintaining and securing the resource and thus sustainable water supply. Knowing what we can measure, and which methodologies we can use for processing the data we have measured, is of value to practitioners and water supply managers. An original combination of well-known methodologies (water stable isotopes analysis and cross-correlation analysis) as presented in this article suggests how can we process measured precipitation data in order to obtain the knowledge of the processes that drive resource replenishment. In addition, it suggests what kind of data we should measure and what kind of equipment we need for such a task.