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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 22 Nov 2018

Research article | 22 Nov 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Hydrology and Earth System Sciences (HESS).

Quantification of seasonal variabilities in groundwater discharge in an extensive irrigation watershed using H, O, and Sr isotopes

Takeo Yoshida1, Takanori Nakano2,3, Ki-cheol Shin3, Takeo Tsuchihara1, Hiroki Minakawa1, Susumu Miyazu1, and Tomijiro Kubota1 Takeo Yoshida et al.
  • 1Institute for Rural Engineering, National Agriculture and Food Research Organization, Tsukuba, Japan
  • 2Faculty of Science and Engineering, Waseda University, Tokyo, Japan
  • 3Research Institute for Humanity and Nature, Kyoto, Japan

Abstract. Numerous studies have quantified stream–groundwater interactions using geochemical or environmental tracers. However, in watersheds where water is extensively used for rice paddy irrigation, uncertainties in estimation remain due to kinetic fractionation of stable isotopes during evaporation from ponded paddies and seasonal variations of the isotopic composition of recharged water. In this study, we used three different methods (streamflow observation, stable isotopes of water, and Sr isotopes) to quantify groundwater discharge to streams in a watershed substantially impacted by rice paddy irrigation in central Japan. We conducted point- and watershed-scale observations of surface water, soil water, groundwater, and ponded water in rice paddies and examined changes in these isotopic compositions. Point-scale observations revealed that Sr isotopes were more appropriate for quantification because the Sr isotopes in groundwater was significantly different from surface water and less variable in time compared to water isotopes. At watershed-scale, isotopic compositions of stream water changed linearly from upstream end to downstream end, suggesting streamflow consisted of two endmembers. We then quantified groundwater discharge to the stream based on measurement of streamflow and surface lateral inflow/outflow during both irrigation and non-irrigation periods. This water balance method yielded large uncertainties in the estimation due to errors in streamflow measurement, while Sr isotopes provided well constrained estimates during both irrigation and non-irrigation periods. The ratios of groundwater to the stream, estimated from Sr isotopes, was in the range 7–86% during the irrigation period and 38–66% during the non-irrigation period. Stable isotopes of water also provided good estimates during the non-irrigation period but underestimated groundwater discharge during the irrigation period due to the ill-defined groundwater end member. The use of Sr isotopes has the potential to aid in quantification of temporal variations in groundwater discharge and to provide important information for water resource managers.

Takeo Yoshida et al.
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Takeo Yoshida et al.
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Short summary
Irrigation for rice paddy accounts a large part of water use in watersheds, especially in the Asian countries. Although irrigation water is lost through vaporization by plants, some portion of the irrigation water recharge local aquifers and return to the original streams through subsurface pathways. However, quantifying the amount of water returning to the stream is not an easy task. We quantified the amount of return flow using colors of water, or environmental tracers dissolved in water.
Irrigation for rice paddy accounts a large part of water use in watersheds, especially in the...