Contrasting Contribution of Surface and Groundwater Discharge to the Water Balance of a Lake and a 10 km2-scale Volcanic Catchment: Lake Yamanaka Catchment, Japan

概要

Because volcanic rocks have high permeability, rainfall quickly infiltrates into the ground.
Perennial rivers often do not exist in volcanic areas. This results in a different degree of
contribution of surface runoff and groundwater to hydrological processes in volcanic areas than
in non-volcanic areas. However, available studies elucidating hydrologic characteristics and the
mechanisms of stream formation in volcanic areas are pretty limited in number, extent, and
coverage prohibiting our comprehensive understanding of hydrological process formation. Also,
contrasting analyses of the hydrological components in volcanic and non-volcanic areas are
also not sufficiently systematic. Based on the above considerations, this study conducted field
monitoring and modeling analysis in a 10km2-scale volcanic lake catchment in Japan. In Lake
Yamanaka catchment, field monitoring to measure stream runoff and groundwater level was
implemented from June 2017 to July 2020. We present a simple coupled modeling framework
for studying hydrological processes in the study area. The coupled model consists of the surface
runoff model (Tank model), the comprehensive evapotranspiration model, and the groundwater
model (Visual Modflow flex). As a validation of this modeling framework, we performed a 2year simulation for the surface runoff model and a 1-year simulation for the groundwater model
using field measurement datasets of the study areas. Our coupled modeling approach estimated
hydrological components well which can be used for past estimations in this region. A 5-year
hydrological process is reproduced through the models. The 5-year average is used as a
representation of the water balance of lakes and catchments in the study area.
The no significant change in meteorological elements and land use over time provides the
confidence to conclude that 5-year average results are representative of a pattern that has
persisted in the area. Compared with the eastern catchments, western catchments with higher
hydraulic conductivity and deeper water tables show smaller runoff coefficients, larger
thresholds, smaller lag times, and shorter runoff duration. Therefore, the eastern catchments
contribute 77% of the total surface runoff per year, while the western catchments contribute
only 23% of the total. The runoff coefficient affected by snowmelt (0.083) is greater than that
of rainfall-runoff events (0.016), and this difference is caused by the thick accumulation of
snow in the channels. The contribution of groundwater (33×106 m3/year) is significantly greater
than that of surface water (1×106 m3/year) in the lake. ...