Hydrogeologicd Processes in Karst Termites (Proceedings of the Antalya Symposium and Field Seminar, October 1990). IAHSPubl. no. 207, 1993. ' 225 A RAINFALL-RUNOFF MODEL FOR LARGE KARSTIC AREAS ALPARSLAN ARIKAN & LEVENT TEZCAN International Research and Application Center for Karst Water Resources, Hacettepe University, 06532 Beytepe, Ankara, Turkey ABSTRACT The enormous development in computer technology has made it possible to construct more realistic physically-based mathematical models of hydrological processes. The definition of the groundwater circulation system mainly based upon the spring hydrographs in karst areas and, a rainfall-spring flow relationship can be used in modelling the behaviour of karst aquifers in which the surface and subsurface catchment boundaries do not coincide. In this study, surface and subsurface catchments are simulated in two different catchment systems, one represents the surface catchment system and simulates the peak flow, and the other simulates the contribution from adjacent aquifer systems. Each catchment has two reservoirs in order to represent the saturated and unsaturated zones. The parameters of the model are the catchment areas, the minimum and maximum levels in the reservoirs, the initial heads and the discharge coefficients that decrease exponentially with depth between maximum and minimum values given as parameters. These parameters are adjusted by using generalized least squares approximation in order to match the computed and observed flows, and applied to the Manavgat River basin in southern Turkey. After calibration, a correlation coefficient of 0.97 was obtained. INTRODUCTION The modelling of karst systems can be taken in the quantitative sense of reconstructing the input-output of the system. The rapid and huge development of computer technology allows the construction and use of more realistic mathematical models for catchment processes. The hydrograph of karst springs or streamflow in karst basins is the overall continuing response of the karst system to the previous precipitation and evaporation events over the catchment. In this study these events are modelled in two catchment systems, since the surface catchment boundary does not coincide with the subsurface catchment boundary. Thus, one of the catchment systems used in this model simulates the output of the surface catchment, i.e. peak flow of the hydrograph, the other simulates the contribution from adjacent catchments through the karstic features. Each system consists of two reservoir systems (Fig. 1). The first reservoir represents the behaviour of the unsaturated zone. In this zone, the rate of infiltration is evaluated by taking into account soil moisture and evaporation and the excess water transferred to the second reservoir system, representing the saturated zone. In this zone, there may be a number of reservoirs connected with each other in a parallel fashion or serially with different hydraulic characteristics. Reservoirs with rapid discharge form peak flows, while those involving slower discharge rates contribute to the baseflow. In this model, the saturated zone is handled as a single reservoir with variable discharge coefficients decreases exponentially with depth. Thus, it can be thought that this zone is simulated with an infinite number of reservoirs. The parameters of the model are the areas of the catchments, the minimum levels of the first reservoirs, the maximum levels of the second reservoirs, initial heads of both reservoirs, the discharge coefficients of the first reservoirs and the maximum and