After a volcanic eruption, in situations where pyroclastic material generates thick ground cover, even small amounts of rainfall can trigger lahars; this effect sometimes continues for many years. For hazard mitigation against lahar disasters after an eruption, it is essential to evaluate the current risk of occurrence and estimate any possible temporal changes for the future. Putih River is one of the rivers where lahars occurred frequently after the 1984 eruption of Mt. Merapi. In this study, the characteristics of lahars and floods in the Putih River after the 1984 eruption and their change over the years were analyzed, focusing on the runoff characteristics of lahars. Irrelevant rainfall and discharge data for analyzing runoff characteristics of lahars were excluded in preprocessing. The magnitude and occurrences of lahars decreased annually during the four years following the eruption. The maximum runoff rate of lahars was approximately 12 during the 1984–1985 rainy season and decreased yearly after this. A judgement graph was employed to track the temporal changes of lahar- triggering rainfall characteristics. For the 1984–1985 and 1985–1986 rainy seasons discriminant lines, which discriminate between rainfall events triggering lahar flow with peak discharge > 900 m³/s and other rainfall events, were drawn on the judgement graph.

1. [1] Japan Int. Cooperation Agency, “Mt. Merapi & Mt. Semeru volcanic disaster countermeasures (2),” Evaluation highlights on ODA loan projects 2004, 2004.
2. [2] Y. Teramoto, T. Jitousono, E. Shimokawa, and N. Anyoji, “Temporal variation of sediment yield by debris flows at the Mizunashi River in Unzen Volcano,” J. of the Japan Society of Erosion Control Engineering, Vol.50, No.3, pp. 35-39, doi: https://doi.org/10.11475/sabo1973.50.3_35, 1997 (in Japanese).
3. [3] S. Hardjosuwarno, C. B. Sukatja, and F. T. Yunita, “Early Warning System for Lahar in Merapi: Current and Future Development,” United Nations Office for Disaster Risk Reduction: Global Assessment Report on Disaster Risk Reduction 2015, pp. 1-22, 2013.
4. [4] National Institute for Land and Infrastructure Management, “ Operating method of critical rainfall for warning and evacuation from sediment-related disaster,” Technical note of National Institute for Land and Infrastructure Management, 5, pp. 1-58, 2001 (in Japanese).
5. [5] T. Jitousono, E. Shimokawa, and S. Tsuchiya, “Debris flow following the 1984 eruption with pyroclastic flows in Merapi volcano, Indonesia,” J. of the Japan Society of Erosion Control Engineering, Vol.48, Special Issue, pp. 109-116, doi: https://doi.org/10.11475/sabo1973.48.Special_109, 1996.
6. [6] Volcanic Sabo Technical Centre, “Supporting report of technical development activities, 6, Mudflow forecasting and warning system,” pp. 1-159, 1990.
7. [7] S. Koga and S. Agus, “Provisional hydrological study on some data obtained by short range radar raingauge,” Proc. of Int. Symp. on Erosion and Volcanic Debris Flow Technology, Yogyakarta, Indonesia, July-August 1989, Vol.15, pp. 1-26, 1989.
8. [8] Y. Shuin, H. Shibano, M. Suzuki, and T. Ohta, “Temporal and spatial characteristics of rainfall on the southwest slope of Mt. Merapi in Indonesia,” J. of the Japan Society of Erosion Control Engineering, Vol.48, Special Issue, pp. 3-12, doi: https://doi.org/10.11475/sabo1973.48.Special_3, 1996.
9. [9] H. Sutikno, Sadwandbaru, and K. Sasahara, “Volcanic Hydrological Approach to Sediment Yield Assessment in the area of Mt. Merapi,” Proc. 11th Congress of the IAHR-APD September 8-10, 1998, Yogyakarta Indonesia, pp. 705-713, 1998.
10. [10] S. Uehara and T. Mizuyama, “Peak Debris Flow Discharge Data,” J. of the Japan Society of Erosion Control Engineering, Vol.37, No.3, pp. 23-24, doi: https://doi.org/10.11475/sabo1973.37.3_23, 1984 (in Japanese).
11. [11] F. Lavigne and J. Thouret, “Sediment transportation and deposition by rain-triggered lahars at Merapi Volcano, Central Java, Indonesia,” Geomorphology, Vol.49, pp. 45-69, doi: https://doi.org/10.1016/S0169-555X(02)00160-5, 2002.