Monthly and annual effective infiltration coefficients in Dinaric karst: example of the Gradole karst spring catchment

The problems and present methods for the calculation of monthly and annual effective infiltration coefficients in Dinaric karst catchments which cover areas smaller than 200 km are discussed. An example is given of the catchment of the Gradole karst spring, which covers 114 km. The climate on the catchment is North Mediterranean, with average annual rainfall of 986 mm, average air temperature of 11.4°C, and average discharge from the Gradole Spring of 1.98 m s". All the data refer to a period from 1987 to 1998. An auto-correlation analysis is carried out of daily and monthly discharges from the spring and monthly rainfall on the catchment. Special attention has been given to determination of monthly effective infiltration coefficients. It was found that the time-scale effect makes it infeasible to use the equation for effective infiltration coefficient, because it gives values greater than 1 in 25% of cases, which is theoretically impossible. Therefore, two different procedures for calculation of monthly effective infiltration coefficients are given, out of which one uses the master depletion curve. This procedure gives acceptable and physically well-established values of mean long-term monthly effective infiltration coefficients. Obtained values can be used for regional analyses as well as for water resources management in karst regions. There was also a strong relationship between mean monthly air temperatures of the catchment and mean monthly effective infiltration coefficients. It is found that rainfall distribution during the year significantly influences the annual effective infiltration coefficients, which for the Gradole catchment ranges between 0.356 and 0.763 with the mean value of 0.57.