Anomalous behavior of soil mantle groundwater demonstrates the major effects of bedrock groundwater on surface hydrological processes

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[1] We investigated processes of soil mantle groundwater generation in a granitic headwater catchment in central Japan. Two types of groundwater were observed: ephemeral-type groundwater (EG), which developed in response to rainfall events and disappeared rapidly after the events ceased, and semiperennial-type groundwater (SPG), which remained formed for more than several months. The groundwater level, chemistry, and temperature within the soil and bedrock layers indicated that the source of EG was rain or soil water, whereas the source of SPG was deep bedrock groundwater. The generation processes of soil mantle groundwater varied both spatially and temporally under the influence of the underlying bedrock. Whereas only EG was generated in upslope areas, bedrock groundwater continuously seeped into the soil layers in downslope areas to generate SPG. In middle-slope areas, an increase in the bedrock groundwater level generated SPG in soil layers, but the SPG disappeared when the bedrock groundwater level fell. Our results indicate that bedrock is important in controlling soil mantle groundwater generation and water flow processes in headwater catchments and that direct measurements of bedrock conditions are vital for clarifying the roles of bedrock in these processes.Citation: Katsura, S., K. Kosugi, T. Mizutani, S. Okunaka, and T. Mizuyama (2008), Effects of bedrock groundwater on spatial and temporal variations in soil mantle groundwater in a steep granitic headwater catchment, Water Resour. Res., 44, W09430,

Section: Study Sitementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

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Effects of bedrock groundwater on spatial and temporal variations in soil mantle groundwater in a steep granitic headwater catchment

Katsura

Kosugi

Mizutani

et al. 2008

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Dynamic, structured heterogeneity of water isotopes inside hillslopes

Oshun

Dietrich

Dawson

et al. 2016

100

121

Use of the stable isotopes of water (dD, d 18 O) to determine vegetative water sources, runoff paths, and residence times generally assumes that, other than shallow evaporative enrichment, the isotopic composition of precipitation is conserved as it travels through the subsurface to the stream channel. Here we follow rainfall through a thick (up to 25m) vadose zone of soil, saprolite, and weathered bedrock mostly composed of argillite, and underlying a steep (328) forested hillslope. We discover a persistent structured heterogeneity of water isotopes inside the hillslope. Summer dry season causes evaporative enrichment of the soil, but not in the saprolite and weathered bedrock. In winter, the mobile water, generated by successive rainstorms with widely varying isotopic composition, mixes in the vadose zone, elevating soil and rock moisture content, and eventually recharging the groundwater with isotopically invariant water similar to the seasonally averaged rainfall. Yet throughout the winter the less mobile winter soil and rock moisture remains relatively light, and water extracted from the interior of argillite lies well to the left of the local meteoric water line. This persistently light composition of soil and rock moisture and the deviation from average meteoric values suggest that subsurface fractionation, or the inheritance of paleo-meteoric rock moisture associated with rock uplift may lead to large enduring isotopic differences between high and low mobility water. These differences suggest that the use of water isotopes as tracers must consider the possibility of subsurface isotopic evolution and the influence of exchange with more tightly held water.

Groundwater similarity across a watershed derived from time‐warped and flow‐corrected time series

Rinderer

McGlynn

Meerveld

2017

Information about catchment‐scale groundwater dynamics is necessary to understand how catchments store and release water and why water quantity and quality varies in streams. However, groundwater level monitoring is often restricted to a limited number of sites. Knowledge of the factors that determine similarity between monitoring sites can be used to predict catchment‐scale groundwater storage and connectivity of different runoff source areas. We used distance‐based and correlation‐based similarity measures to quantify the spatial and temporal differences in shallow groundwater similarity for 51 monitoring sites in a Swiss prealpine catchment. The 41 months long time series were preprocessed using Dynamic Time‐Warping and a Flow‐corrected Time Transformation to account for small timing differences and bias toward low‐flow periods. The mean distance‐based groundwater similarity was correlated to topographic indices, such as upslope contributing area, topographic wetness index, and local slope. Correlation‐based similarity was less related to landscape position but instead revealed differences between seasons. Analysis of variance and partial Mantel tests showed that landscape position, represented by the topographic wetness index, explained 52% of the variability in mean distance‐based groundwater similarity, while spatial distance, represented by the Euclidean distance, explained only 5%. The variability in distance‐based similarity and correlation‐based similarity between groundwater and streamflow time series was significantly larger for midslope locations than for other landscape positions. This suggests that groundwater dynamics at these midslope sites, which are important to understand runoff source areas and hydrological connectivity at the catchment scale, are most difficult to predict.