Water Year Type

The Napa River Sediment TMDL Implementation Tracking and Accounting System (ITAS) provides climatic context for monitored years by classifying each water year into one of the 5 water year types. Providing a climatic context for monitored years will assist future interpretation of the ITAS metric results and detection of trends above the variability associated with climatic differences.

Water Year Categories | Water Year Type Definition Method | Recent Water Year Types

Water Year Categories

A "water year" runs from Oct. 1st - Sept. 30th and long-term local precipitation data are used to define 5 water types using the relative amount of total precipitation: very dry, dry, average, wet, and very wet. 

The Napa State Hospital precipitation gage located on the valley floor provides the longest period of record for the Napa River Watershed (over 100 yrs). While this gage may not reflect measured total precipitation in the higher elevation areas of the watershed, the data does represent the watershed wide precipitation patterns. This approach is assumed to be reasonable for the purposes of evaluating the potential climatic influence on the ITAS metric resluts year after year.

Water Year Typer Definition Method

A statistical precipitation frequency analysis was conducted on 104 years of Napa State Hospital precipitation data to determine the water year precipitation thresholds that define 5 water year types. The water year type definitions were created using reasonable exceedance propabilities, ensuring the average precipitation totals were bracketed and the extremes (very wet and very dry) have less that a 10% probibility of occurring any one year. 

Year Type

WY Precipitation

Lower (in/yr)

WY Precipitation

Upper (in/yr)

 WY Type Recurrence

Interval (yrs)

Annual PPT Exceedence

Probability (%)


 Very Dry

   13.8  11  ~91  9 


 13.8  19.7  4  ~67  26 


 19.7  27.8  3  ~33  35 


 27.8  36.0  4  ~10 25

 Very Wet

 36.0    11  <10 9

Recent Water Year Types

The total water year precipitation and water year type over the past 8 years (WY08-16) is summaraized based on the frequency analysis. See chart bleow.


WY08  WY09   WY10   WY11  WY12  WY13   WY14   WY15 WY16

Napa State Hospital WY

Precipitation (in)

20.8 21.3 28.9 30.0 21.0 19.89 19.4 20.71 24.42

Water Year Type

Average Average Wet Wet Average Dry Dry Average Average








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