Estimation of the maximum amount of water available for infiltration from precipitation, Velika Gorica well field

Zagreb aquifer presents one of the main sources of potable water for the inhabitants of the City of Zagreb and Zagreb County. It presents strategic water reserves protected by the Croatian State. In the first sanitary protection zone of the Velika Gorica well field, which is one of the major well fields, pedological pit was excavated and scientific research polygon was established. Long-term goals of the research are related to the modelling of groundwater flow and transport through unsaturated zone and quantification of effective infiltration. In this study, various equations for estimating evaporation and evapotranspiration were evaluated in detail to determine the maximum amount of water available for infiltration in 2019. The modified Penman- Monteith (CIMIS), Meyer, Turc, Hargreaves and Jensen-Haise equations were used. Due to different input parameters, linear regression was used to test the relationship between the most important parameters. In general, reference evapotranspiration estimated with different equations showed differences up to ±30%. Correlation and regression analysis between the evapotranspiration, evaporation, temperature, and solar radiation showed statistically significant positive correlations, with the highest correlation coefficient and coefficient of determination observed for solar radiation and reference evapotranspiration. However, only the equations of Meyer and Turc gave smaller values of evaporation and evapotranspiration than precipitation. The equations used for reference evapotranspiration gave higher values that ultimately resulted in unrealistic values for water available for infiltration. The annual Turc equation was chosen as the most appropriate equation for the investigated area, estimating the maximum available water for infiltration in 2019 to be about 269 mm.