2025-10-06 – Weekly Hydrology News : Why is Ia = 0.2S still used?

Last week centered on practical modeling choices and day-to-day QA. Members unpacked the 0.2S initial abstraction in SCS-CN—where it came from, when to calibrate it, and how it affects small-storm runoff. Field workflows got a lot of attention, from conductivity probe calibration to stream level logger checks, alongside compliance threads on PFAS MCLs and CEUs for lab QA. Career posts rounded it out with advice on moving into project management, making clean networking asks, and a fresh list of senior openings.


This Week’s Hot Topics

  • Why is initial abstraction 0.2S
    A back-to-basics look at the 0.2S assumption: origins, calibration strategies, and how Ia/S choices shift CN runoff, especially for small events.

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  • Training that accelerates the PM leap
    Members compare courses and playbooks that actually help technical hydrologists step into PM roles—scope, schedule, risk, and stakeholder skills included.

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  • My repeatable conductivity calibration
    A simple, repeatable calibration routine with temperature compensation and verification steps to cut drift and pass audits.

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  • Keeping hydrographs flat on solar sites
    Practical stormwater design to limit peaks on PV arrays—distributed storage, level spreaders, hydrologic soil group tweaks, and maintenance realities.

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  • Best way to ask for a 20‑min chat
    A straightforward outreach script that gets responses without being pushy, plus what to include (and avoid) in the first message.

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  • 2025-10-02 – Weekly Hydrology Jobs: Senior roles
    This week’s openings skew senior—flood modeling, stormwater PMs, and water quality leads—with notes on required software experience.

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  • Field check routine for stream level loggers
    A field checklist that covers baro comp, staff gauge ties, fouling, desiccant, and time sync to keep records clean through storm season.

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  • PFAS compliance templates for 2024 MCLs
    Ready-to-use sampling plans, COC language, and reporting tables tailored to the new MCLs—useful for utilities and consultants alike.

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  • CEU recommendations for lab QA compliance
    Course suggestions that actually count toward QA/QC requirements—method validation, MDLs, uncertainty, and data integrity.

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  • How far does 10 mm of water move
    A sanity-check thread translating rainfall depth to movement and storage—soil intake, depression storage, slope, and roughness all in play.

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Thanks for keeping the discussions practical and generous. See you next week with more to learn from each other.

, 0.2S hangs on because TR‑55 and most software bake it in, but if small‑storm bias is biting you, calibrate Ia/S from a few clean <1" events using your logger‑checked hydrographs and then adjust CN. On pervious urban sites I’ve landed closer to 0.05–0.12, not 0.2. Are you keeping AMC‑II and tweaking CN, or letting Ia float per NRCS TR‑55 guidance (https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs144p2_030915.pdf)?

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