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Updated: May 2, 2026

Continuous Instream Monitoring of Nutrients and Sediment in Agricultural Watersheds
Published on: September 26, 2017
Isotope-enabled hydrograph separation reveals groundwater-dominated streamflow in a tropical monsoon basin: The Upper
Nantawoot Inseeyong1, Kiattipong Kamdee2, Pavisorn Chuenchum3
1Department of Water Resources Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok, 10300, Thailand.
Abstract:
This study investigates seasonal rainfall-runoff dynamics and groundwater-surface water interactions in the Upper Chao Phraya River Basin (UCPRB), Thailand, using stable isotopes (δ18O and δ2H), hydrochemical tracers, and hydrograph separation techniques. Rainfall exhibited wide isotopic variability (mean δ18O = -7.20‰, SD = 4.41‰), with more depleted signatures in the wet season (mean = -7.97‰) and enriched values during the dry season (mean = -3.86‰). River water showed moderated seasonal variation (δ18O mean = -7.38‰), while groundwater remained relatively stable (mean = -6.65‰, SD = 1.72‰), indicating its buffering role. A three-component end-member mixing analysis (EMMA) revealed that the Nan River contributes the largest portion of discharge to the Chao Phraya River, 66% in the wet season and 63% in the dry season. Two-component isotope hydrograph separation showed that groundwater sustains 61-65% of dry-season river flow, while event water dominates during the monsoon (55-70%). Hydrochemical interpretation using Durov and Stiff diagrams identified dominant HCO₃-Ca and mixed water types, along with ion exchange and calcite dissolution. Strong inter-basin δ18O correlations (r > 0.75) suggest coherent monsoonal recharge across the region. These findings indicate the critical role of groundwater in maintaining river discharge and demonstrate the value of isotope-enabled hydrograph separation for supporting integrated water resources management in tropical agricultural basins facing climate and land-use pressures.
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