Long-term changes in unconfined groundwater level in the southern Beppu volcanic fan

Transition in effective precipitation, land use change and discharge of hot spring reveals the longterm changes in the unconfined groundwater level in the southern Beppu volcanic fan from 1971 to 2019. The unconfined groundwater level was periodically fluctuated between 40 m and 50 m above sea level and indicated the maximum value in October or November and the minimum value in April or May. Annual effective precipitation was related to the maximum groundwater level in each year and it had decreased 0.7 mm every year. The urban area was slightly increased from the 1970s to 2016, but it was not a factor to decrease the unconfined groundwater level. The hot spring yield was inversely related to the maximum groundwater level in each year and it had increased 7.9 mm/y, which is approximately 11 times higher than the effective precipitation. Thus, it could conclude that the increase of the hot spring yield mainly affected the decline of the unconfined groundwater level from 1971 to 1999.