Résumé
The understanding of groundwater circulations into a volcano-detritic edifice is difficult because of the heterogeneous geological formations and the alternating construction and dismantling phases. In Indonesia, most of the groundwater resources are located into these complex volcanic systems. Furthermore, over the world this kind of context is poorly documented. In this study, we used a multi-disciplinary method based on field observations at watershed scale (geology, geomorphology, pumping tests, physico-chemical monthly monitoring over two hydrological years, etc.) and water stable isotope analysis to built a conceptual model of the hydrogeological behaviour of the Merapi volcano located in Central Java. Temperature appears to be a relevant tracer of local and regional groundwater circulations. A numerical model (METIS code) was used to verify the hydrogeological conceptual model. The steady state flow simulations were constrained first by isotope ratios (delta (super 18) O) dispersion transport and then by heat dispersion transfer in 2D vertical cross section. The heat spatial distribution and especially the "spring temperature anomaly" have been reproduced. The numerical code allowed us to describe the relation between the geothermal gradient and the relatively cold groundwater circulations provide by the high elevation recharge. Therefore, the results of this study highlight that water temperature offers several advantages as a tracer of groundwater circulations because it is easy, quick, and inexpensive to measure accurately in the field even in volcanic systems under tropical conditions. We show also that a simple but relatively accurate numerical modelling can be achieved when it is constrained by multi-disciplinary study results.