Influences of a dual monsoon system on River Water Recharge in the Japanese Southern Alps

Mountain regions are major recharge areas that sustain downstream groundwater and river systems, yet they are among the least observed parts of catchments. In dual-monsoon climates, the proportion of precipitation falling as pre-monsoon snow and monsoon rain varies greatly between regions. However, harsh environmental conditions and limited accessibility have restricted observations in mountain areas. As a result, it remains unclear how much different types of precipitation, such as snow and rain, contribute to groundwater recharge and which controlling factors, such as geology and slope, regulate these contributions.

Stable water isotopes and d-excess are powerful tracers of the seasonality of recharge precipitation and subsurface groundwater mixing processes. For example, Nakamura (2017) clarified seasonal local meteoric water lines (LMWLs) in the Kofu Basin of central Japan and demonstrated that groundwater in the alluvial fan is strongly dominated by pre-monsoon (snow-season) precipitation, even though pre-monsoon precipitation accounts for only about 25% of the annual precipitation in the lowlands. This apparent paradox has been explained by recharge from snowmelt originating in the surrounding mountains. However, due to limited accessibility, precipitation and river water have not been sufficiently observed in mountain regions, and large-scale, observation-based verification from the mountain side remains limited.

In central Japan, mountain ranges with elevations of approximately 3,000 m extend across the main island of Honshu from the Sea of Japan to the Pacific Ocean. These ranges are collectively referred to as the Japanese Alps and are subdivided into the Northern, Central, and Southern Alps. Several alluvial fans have developed along their foothills, forming important recharge areas for downstream water resources.

In this study, we focused on the Japanese Southern Alps, located on the Pacific side of the Japanese Alps. The Japanese Southern Alps are characterized by steep topography, multiple geological units, and a dual-monsoon climate. River water was sampled at 48 sites across an elevation range of 392–1,556 m (the elevation of the downstream urban area of Kofu City is approximately 200 m).

River water samples were collected during both the pre-monsoon and monsoon seasons, and δ¹⁸O, δD, and d-excess were analyzed. In both seasons, river water d-excess values were close to those of pre-monsoon precipitation, indicating that winter-origin water dominates streamflow even during the monsoon period. Mass-balance analysis further confirmed that pre-monsoon precipitation makes a dominant contribution to river water in both seasons.

Furthermore, significant differences in d-excess were observed among geological units. These differences followed a consistent ranking, with higher d-excess associated with higher hydraulic conductivity. This suggests that, in steep and geologically complex mountain regions under a dual-monsoon climate, permeability contrasts among geological units regulate the infiltration of pre-monsoon precipitation and thereby influence the relative contributions of pre-monsoon and monsoon precipitation to river water.

These results demonstrate that differences in geological structure within mountain blocks influence the d-excess values of downstream alluvial-fan groundwater and river water. This finding has important implications for identifying recharge areas and for understanding mountain–lowland hydrological connectivity in dual-monsoon regions.