Updating the Washington Irrigation Guide

Public Involvement Timeline


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The Washington Irrigation Guide (WIG) is the standard in Washington State for estimating crop water needs.  Last revised in 1997, it is in the process of being updated through the efforts of the Department of Ecology (Ecology), Washington State University (WSU), and the United States Department of Agriculture’s Natural Resource Conservation Service (NRCS).

Ecology provided funding to WSU, with technical assistance from the NCRS, for the development of a preliminary draft report of Evapotranspiration and Consumptive Irrigation Requirements for Washington (commonly referred to as crop-water-use tables).  It is now up to the NRCS (with some help from their State Technical Advisory Committee, STAC) to determine when the draft report is sufficiently peer-reviewed by scientific professionals in the field of crop water use and evapotranspiration rates.  After a public review process, these crop-water-use tables will eventually be finalized and compiled into an accessible web database.  It will be published in the Washington Irrigation Guide through an NRCS process.

Public involvement is ongoing.  Outreach efforts began in 2012 (see timeline at right).

The old WIG data are still being used today.  The new data are provisional until approved/adopted by the NRCS.  Once the new WIG is finished Ecology can update its policy(s) on establishing the use of the new WIG.  The new WIG will govern unless there is a compelling scientific basis to use something else.

Why Update the Washington Irrigation Guide?

Good data are required to manage Washington’s water resources.  There are many significant reasons that Ecology, WSU and NRCS decided to collaborate on a WIG update:


Over the last 40 years, Ecology has collaborated with Washington State University and various state and federal agricultural agencies to produce guidance documents that estimate crop irrigation requirements. This information was used for many different purposes, including:

The first such document, published in 1969, was called “Circular 512” and provided crop irrigation requirements for 38 locations throughout Washington for 18 different crops.  These data were compiled using the modified Blaney-Criddle procedure from 30 years of weather records.

The second guidance document, Irrigation Requirements for Washington—Estimates and Methodology, was published in 1982 and is still in print today.  This guidance document estimates crop irrigation requirements for 40 locations throughout Washington for 39 different crops.  These data were compiled using the Doorenbos and Pruitt Blaney-Criddle method based on 26 years of NOAA weather data.

The most recent guidance document, the State of Washington Irrigation Guide (WIG), was published in 1985 and supplemented in 1997.  It remains the definitive irrigation guidance document in use today.  This guidance document estimates crop irrigation requirements for 90 locations throughout Washington approximately 40 different crops. These data were compiled using the SCS Blaney-Criddle Modified method and FAO 24 Blaney-Criddle method based on up to 29 years of NOAA weather data

The current update effort began in 2008:

Additional Information


Melissa Downes, LLG
Water Resources Program
Department of Ecology, Central Region Office
Phone: (509) 454-4259
E-mail: wig@ecy.wa.gov
  Troy Peters, P.E.
Irrigation Engineer, Extension Specialist
WSU Prosser
Phone: (509) 786-9247
E-mail: troy_peters@wsu.edu