Simulated Flows ≈30× Lower Than Observed at Outlet
atlas probest
Dear SWAT Team,
I’m reaching out for help with a persistent magnitude problem in my SWAT+ project (Bartın–Arit basin, Turkey). Despite many careful checks, my simulated monthly discharge at the gauged outlet is ≈30 times lower than observations, and I have not been able to identify the root cause.
What I see
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Observed monthly flows (DSİ gauge at the basin outlet) are in the multi-m³/s range, but simulated “Channel 37 – Monthly River Flow” remains mostly 0.01–0.7 m³/s (median ~0.02 m³/s).
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Basin water-balance outputs (basin_wb_mon.txt) frequently show tens of mm/month of water yield (surq_cha/latq_cha present), yet routed outlet flows remain very small.
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Channel bookkeeping indicates large flo_in relative to flo_out for the outlet link in several months.
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Changing PET method (Penman–Monteith ↔ Hargreaves) and warm-up (2 → 5 years) does not affect the magnitude gap.
What I’ve tried (brief)
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Rebuilt the project with consistent CRS and clean delineation; outlet matches the DSİ gauge location and drainage area.
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HRUs created with realistic thresholds; climate (MGM daily PCP/TMP) loaded and assigned; first baseline run produced consistent basin precip and ET.
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Multiple parameter passes (CN2/Ksat/SURLAG/latq_co/ALPHA_BF/RCHRG_DP/GWQMN/REVAP*; channel K) tested in SWAT+ Toolbox Manual/Automatic; NSE remains ~–0.9 and the hydrograph stays far below the observed series.
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Verified date/units and conversions (m³/s ↔ mm/month) outside SWAT using the correct basin area.
Where I’m stuck
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The magnitude deficit persists across scenarios and runs. Basin files suggest sufficient water is generated, but the outlet series remains an order of magnitude low, and I cannot tell whether the loss is due to routing/transmission in channels, deep leakage settings, or a wiring mismatch (object/scenario).
I would greatly appreciate your guidance on a systematic way to diagnose this kind of “flows >> into channels but << out at the outlet” situation in SWAT+. If there are specific reach/groundwater diagnostics, channel loss flags, or known pitfalls between land-to-reach routing and outlet flow that I should check, I’d be grateful for your pointers.
Thank you very much for your time and support.
Kind regards,
HAMED MARDAN
SWAT+ Project: Bartın–Arit (Turkey)
Email: hamedmardan91@gmailcom
