Groundwater balance estimation in karst by using simple conceptual rainfall-runoff model

The objective of this work is the study of Opacac karst spring which geographically lies in Dalmatia (Croatia). Numerous studies have been carried out in karst aiming the investigation of groundwater regime. The karst spring hydrograph can reflect the groundwater regime and consequently the analysis is based on them. A simple conceptual rainfall-runoff model is used for the estimation of groundwater balance components including the influences of time-invariant catchment boundaries (Jukic and Denic-Jukic 2009). The proposed parameter estimation procedure merges the soil-moisture balance and the groundwater balance approaches to obtain the complete groundwater budget. The effective rainfall is calculated by using mathematical model based on soil-moisture balance equations, i.e. Palmer's fluid mass balance method. The parameters of model of effective rainfall are determined by using simple conceptual rainfall-runoff model consisting of two linear reservoirs representing the fast and slow flow component of the recession. The weight coefficient between the fast and slow component is determined by using base flow index analysis of hydrograph. Recession coefficient of the slow flow component and the weight coefficient are determined from hydrograph analysis. Available data from nearby meteorological station includes daily average discharge, the amount of precipitation, the average temperature and the humidity from 1995 to 2010. The average catchment area is also estimated with the average yearly runoff deficit using Turc's method and compared with the values obtained from the application of the rainfall-runoff model. Nash-Sutcliffe model efficiency, root mean square error and Pearson's correlation coefficient for simulated hydrograph are applied to assess the predictive power of model. Calculated groundwater balance shows that the Opacac spring aquifer contains a significant storage capacity. The application of series of linear reservoirs is a classical and common technique, but the proposed simple approach enables the estimation of the components of groundwater balance in karst areas.