Gabilan Watershed: Flow duration analysis

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This is an example of a flow duration analysis, provided for students in ENVS 560. There are more examples, produced by students.

GabilanWatershed FlowDurationAnalysis.png

Hydrologic setting

Gabilan Creek flows from steep headwaters in the Gabilan Range, down to a broad flood plain a few miles before reaching the Hebert Road USGS gage. Its course then runs over several miles of flat Salinas Valley land before reaching the City of Salinas. There, in Carr Lake in the middle of the city, Gabilan Creek is joined by Alisal Creek and Natividad Creek. The combined watercourse is called the "reclamation ditch". A few miles below the City of Salinas, it passes the San Jon Road USGS gage. Several miles later, it becomes Tembladero Slough, confluences with the Old Salinas River Channel, and drains into Moss Landing Harbor.


The flow duration curve (FDC) for Gabilan Creek indicates that at Hebert Road, the creek flows only about 30% of the time. The general smoothness of the FDC is indicative of an unregulated flow regime i.e. there does not appear to any regulation system upstream that would cause any particular flows rates to be more common than slightly higher or lower flow rates.

The FDC for the Reclamation Ditch at San Jon Rd indicates perennial or near-perennial flow. There are inflection points in the curve, indicating some non-natural intervention in the flow regime. Relative to the smooth curve expected of an unregulated stream, steeper parts of the curve correspond to relatively uncommon flows, and flatter parts of the curve correspond to relatively common flows. The curve is steeper between the 20% and 5% quantiles, corresponding to flow rates between about 0.2 and 2 CMS (about 7 and 70 CFS). Another way of putting this is that the Reclamation Ditch FDC is relatively flat out during the driest 80% of the record, indicating that flows during the driest 80% of the record are higher than would be expected if the site were more similar to the Gabilan Creek site.

Comparing the two curves, Gabilan Creek's highest flows (>10 CMS) are higher than the highest flows downstream in the Reclamation Ditch. Vice versa, during the lower flow periods, the Reclamation Ditch carries more flow than Gabilan Creek, which doesn't flow at all during dry weather. These two observations in combination indicate either:

  1. diffusion of storm hydrographs i.e. a lowering of storm peak flows and extension of duration of sub-peak flows as one moves downstream
  2. and/or, loss of flow due to diversion or infiltration

A combination of both is likely. For example, Carr Lake as able to temporarily store a large volume of storm flow, which is then gradually drained through a culvert. This amounts to diffusion of storm hydrographs. At the same time, the sand- and gravel-bed streams overlying the Salinas Valley aquifer are well known to lose a great deal of water to infiltration.

If only these two mechanisms occurred, we would expect the Reclamation Ditch FDC to dive to zero at some point to the left of the of the Gabilan Creek FDC. However, it stays non-zero through even the driest periods. This suggests a source of flow that is unrelated to direct storm runoff and in-stream flow routing. Potential sources include irrigation tailwater from domestic and agricultural sources, industrial discharges, and basement sump pumping.


Based on visual comparison of flow duration curves, the Gabilan Creek / Reclamation Ditch system appears to exhibit watershed processes such as:

  1. Storm hydrograph diffusion
  2. Streambed infiltration
  3. Managed streamflow sources (e.g. residential, agricultural, industrial)


  1. An analysis similar to this one was published in the Reclamation Ditch Watershed Assessment and Management Strategy: