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This study analyzed long-term fluctuations of groundwater levels in six shallow observation wells in the Nasunogahara alluvial fan, Japan’s second largest source of agricultural irrigation groundwater, and presented a simple method for predicting groundwater levels in April prior to the annual planting of paddy rice. The 22-year time-series of groundwater levels (1998–2019) clearly showed seasonal periodicity, with higher levels in summer than in winter. In particular, groundwater levels were lowest in April when groundwater demand was greatest. Groundwater levels in two wells at the beginning of the April irrigation period showed long-term declining trends that can be attributed more to changes in land use than to changes in precipitation or air temperature. A simple linear regression of mean groundwater level in April to antecedent precipitation provided reasonable predictions of April groundwater levels, which were significantly influenced by precipitation in the preceding 3–5 months. Further modeling after subtraction of long-term seasonal trends (detrending) improved these estimates. The performance of the linear regression model for prediction of April groundwater levels is comparable to that of the statistical benchmark model. Using long-term monthly or seasonal weather forecasts, the modeling presented here can be applied to inform appropriate changes of water use practices, such as decreasing groundwater extraction by implementing rotational water supply, changing rice-cropping seasons, or targeting deeper aquifers. The identification of the critical period of antecedent precipitation that affected April groundwater levels in the Nasunogahara alluvial fan is also important for understanding appropriate precipitation periods to be targeted in modeling for future drought risk assessments under climate change.

Groundwater flow and the geochemical evolution of a freshwater lens in an aquifer on a Pacific atoll were investigated by hydrogeochemical surveys. Sulfur hexafluoride measurements showed that deeper groundwater and groundwater at the periphery of the lens are older, consistent with a downward and outward groundwater flow scheme. This is the typical flow scheme on Pacific atolls where a Holocene–Pleistocene unconformity restricts the shape of the freshwater lens. Enrichment of Mg/Ca in the groundwater is another indicator of a longer residence time, because contact between the groundwater and the carbonate sediments composing the aquifer leads to the release of Mg from high-Mg calcite and the precipitation of Ca as low-Mg calcite. Groundwater quality was also affected by anthropogenic nitrogen loading and aboveground organic matter, which were altered by denitrification and sulfate reduction in the aquifer, especially in the older groundwater. The chemical composition of the groundwater in the center of the island, where saline water is up-coning, implies that freshwater recharge dilutes the older saline water, which as time passes will eventually be replaced by newly recharged freshwater.

Snowmelt infiltration is one of the key factors that trigger landslides. This study focused on characterization of snowmelt infiltration in the Shimekake landslide block in northeastern Japan via long-term monitoring of major ion concentrations and stable isotope ratios of groundwater and precipitation. Results revealed seasonal variations of the d-excess values of precipitation, which were significantly higher in winter. Likewise, the d-excess values of the groundwater increased during the melting season. Groundwaters collected from 17 sites in the landslide blocks were classified into five types based on a comparison of hydrochemical and isotopic compositions between melting and non-melting seasons. Groundwater with significantly higher d-excess values during the melting season was diluted by infiltration of snowmelt, which resulted in a decrease in ion concentrations. Three groundwater types underwent large changes in electrical conductivity and/or ion concentrations during the melting season, even though the d-excess values did not change significantly. These groundwaters were inferred to have been affected by the arrival of groundwater with different chemical compositions as a result of elevation of the water table by infiltration of snowmelt. The fifth type of groundwater was less affected by snowmelt infiltration. The results indicated the existence of preferential subsurface flows and a heterogeneous flow system in this landslide block.

Paddy rice fields on an alluvial fan not only use groundwater for irrigation but also play an important role as groundwater recharge sources. In this study, we investigated the spatial distribution of isotopic and hydrochemical compositions of groundwater in the Nasunogahara alluvial fan in Japan and applied a self-organizing map (SOM) to characterize the groundwater. The SOM assisted with the hydrochemical and isotopic interpretation of the groundwater in the fan, and clearly classified the groundwater into four groups reflecting the different origins. Two groundwater groups with lower isotopic ratios of water than the mean precipitation values in the fan were influenced by the infiltration of river water flowing from higher areas in the catchments and were differentiated from each other by their Na+ and Cl− concentrations. A groundwater group with higher isotopic ratios was influenced by the infiltration of paddy irrigation water that had experienced evaporative isotopic enrichment. Groundwater in the fourth group, which was distributed in the upstream area of the fan where dairy farms dominated, showed little influence of recharge waters from paddy rice fields. The findings of this study will contribute to proper management of the groundwater resources in the fan.

Tidal response method is an efficient technique for investigating hydraulic properties of an aquifer in insular and coastal areas of highly permeable geological settings. The present study extends a simple and straightforward harmonic-analysis technique recently introduced as part of a tidal response method applied to a freshwater-lens aquifer. This simple analysis technique was examined with artificially synthesized time series composed of multiple realistic tidal components. The analysis outputs of major diurnal and semidiurnal components are sufficiently accurate if the analyzed time-series length is appropriately restricted. Limitations of the simple harmonic analysis in the applicable time-series length arise from tidal-component interference that occurs in analyses over a finite length. Definitively recommended simple harmonic-analysis technique with appropriate combinations of time-series lengths and extractable tidal components are convenient for investigating hydraulic properties of an aquifer, such as on a remote island where the freshwater lens is the only freshwater resource.

The hydraulic properties of an island’s aquifers were studied using a tidal response method. Groundwater is the principal water resource on the studied island, and the investigations of the aquifer for future groundwater development are ongoing. The method consists of three parts: observations of groundwater levels at paired near-shore and relatively inland sites affected by the same ocean tide, decompositions of the collected groundwater time series to yield isolated tidal components, and application of basic formulas describing propagation of groundwater-level oscillations in aquifers for determining hydraulic parameters. The observations require only auto-logging groundwater-level meters that are commonly used these days. Employed were two decomposition techniques that are easy to implement using only the built-in functions of spreadsheet software. Digital high-pass filtering was used to separate the tidal components of semidiurnal and diurnal periods from longer-period components prior to the following isolation of tidal constituents. Basic formulas for the Fourier transform were used to isolate one tidal constituent with a specific period. The isolation of one tidal constituent helps to reduce errors in the hydraulic parameter calculations based on formulas using the amplitude attenuation and phase shift of sinusoidal oscillations of the groundwater level. The calculation of aquifer hydraulic parameters can also be done directly on a spreadsheet. The derived aquifer hydraulic parameters compared well with those that were previously reported based on a pumping test conducted on the same island. The results are now used as a basis for further investigation of sustainable groundwater development on the island.

The national-scale variations and regional characteristics of the stable isotopic compositions of irrigation ponds, which are small reservoirs for irrigating paddy rice fields and play an important role in supporting rice production in Japan, and spring waters were investigated during irrigation periods for paddy rice. The isotopic compositions of spring waters are mainly affected by geographical factors (latitude and altitude) and meteorological factors (annual precipitation and ratio of winter precipitation to annual precipitation). In particular, the ratio of winter precipitation characterizes the spatial variations of the isotopic compositions on different ocean sides of the archipelago. The isotopic compositions of irrigation-pond waters are affected by meteorological factors (air temperature, ratio of winter precipitation to annual precipitation and wind speed) and geometrical feature (water depth). The isotopic compositions of irrigation-pond waters with relatively short residence times show seasonal variations, reflecting seasonal differences in isotopic compositions of precipitation, whereas spring waters show temporally invariant isotopic compositions. In addition to the regional differences of this seasonal isotopic variation, irrigation-pond waters were affected by evaporative isotopic enrichment. These two elements influence the isotopic compositions of irrigation-pond waters across Japan, resulting in different values from spring waters.

Two simple frequency decomposition techniques were used as part of a tidal response method to derive the hydraulic diffusivities of a freshwater-lens aquifer. Digital high-pass filtering can separate the tidal components of diurnal and shorter periods from longer-period components. Discrete Fourier transform can be used to isolate a specific tidal component. These techniques are easy to practice using the built-in functions of spreadsheet software. The applied techniques were each optimized for the frequencies of known major tidal components. Isolation of the specific tidal signals helps to reduce the errors of a basic tidal response method that uses in its calculations the amplitude attenuation and phase lag of a simple sinusoidal wave of groundwater fluctuations. Another advantage of the present tidal method is the utilization of two groundwater time series collected from near-shore and relatively inland sites affected by the same ocean tide. The method does not use surface-water observation, thus avoiding errors derived from generally possible surface-water/groundwater boundary effects.The tidal response method with simple decomposition techniques was used to investigate the aquifer properties of an uplifted limestone island located in a subtropical region of Japan. A freshwater lens is the principal water resource for this island and its sustainable development is desired. Significant hydraulic layering has not been reported in the limestone aquifer. Pairs of groundwater-level time-series data collected by simultaneous observations at near-shore and inland sites were analysed by the tidal response method. The results demonstrated heterogeneous aquifer diffusivity on the island, typically with larger values in the southeastern coastal part than in the northwestern coastal part, which is consistent with the planar distribution of the entire freshwater lens and the position of its maximum thickness that are slightly biased toward the northwestern side.