Preprints
https://doi.org/10.5194/hess-2023-308
https://doi.org/10.5194/hess-2023-308
17 Jan 2024
 | 17 Jan 2024
Status: this preprint is currently under review for the journal HESS.

Isotopic evaluation of the National Water Model reveals missing agricultural irrigation contributions to streamflow across the western United States

Annie Putman, Patrick Longley, Morgan McDonnell, James Reddy, Michelle Katoski, Olivia Miller, and J. Renee Brooks

Abstract. The National Water Model (NWM) provides critical analyses and projections of streamflow that support water management decisions. However, the NWM performs poorly in lower elevation rivers of the western United States (US). The accuracy of the NWM depends on the fidelity of the model inputs and the representation and calibration of model processes and water sources. To evaluate the NWM, we performed a water isotope (δ18O and δ2H) mass balance using long term mean summer hydrologic fluxes between 2000 and 2019, and gridded precipitation and groundwater isotope ratios. We compared the NWM-flux-estimated (‘model’) river reach isotope ratios to 4503 in-stream water isotope observations in 877 reaches across 5 basins in the western US. A simple regression between observed and mass balance estimated isotope ratios explained 57.9 % (δ18O) and 67.1 % (δ2H) of variance, though observations were 0.5 ‰ (δ18O) and 4.8 ‰ (δ2H) higher, on average, than mass balance estimates. The unexplained variance suggest that the NWM does not include all relevant water fluxes to rivers. To infer possible missing water fluxes, we evaluated patterns in observation-model differences using δ18Odiff (δ18Oobsδ18Omod) and ddiff (δ2Hdiff −8∗δ18Odiff). We detected evapoconcentration of observations relative to model estimates (negative ddiff and positive δ18Odiff) at lower elevation, higher stream order, arid sites. The catchment actual evaporation to precipitation ratio, the fraction of streamflow estimated to be derived from agricultural irrigation, and whether a site was reservoir-affected were all significant predictors of ddiff in a linear mixed effects model, with up to 15.1 % of variance explained by fixed effects. This finding is supported by patterns in groundwater levels and groundwater isotope ratios, and suggests the importance of including irrigation return flows to rivers, especially in lower elevation, higher stream order, arid rivers of the Western US.

Annie Putman, Patrick Longley, Morgan McDonnell, James Reddy, Michelle Katoski, Olivia Miller, and J. Renee Brooks

Status: open (until 13 Mar 2024)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on hess-2023-308', Anonymous Referee #1, 25 Jan 2024 reply
    • AC1: 'Reply on RC1', Annie Putman, 16 Feb 2024 reply
  • RC2: 'Comment on hess-2023-308', Anonymous Referee #2, 02 Feb 2024 reply
    • AC2: 'Reply on RC2', Annie Putman, 16 Feb 2024 reply
  • RC3: 'Comment on hess-2023-308', Anonymous Referee #3, 06 Feb 2024 reply
    • AC3: 'Reply on RC3', Annie Putman, 16 Feb 2024 reply
Annie Putman, Patrick Longley, Morgan McDonnell, James Reddy, Michelle Katoski, Olivia Miller, and J. Renee Brooks

Data sets

Hydrogen and oxygen stable isotope mass balance evaluation of the National Water Model (v2.1) streamflow, runoff and groundwater flows: U.S. Geological Survey data release J. E. Reddy et al. https://doi.org/10.5066/P9NOD5ES

Annie Putman, Patrick Longley, Morgan McDonnell, James Reddy, Michelle Katoski, Olivia Miller, and J. Renee Brooks

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Short summary
Hydrologic models are a simplified representation of water flows through the landscape. Simplification diminishes the model’s ability to produce accurate streamflows, especially where water management and use are prevalent and water resources less so. Hydrologic tracers are useful for tracking water. Thus, we equipped hydrologic model with tracers, and compared tracer estimates to observations. We found that water movement associated with irrigation may improve model performance.