El Dorado Irrigation District
Serving people, agriculture, business, and the environment in California's El Dorado County since 1925

Irrigation Management Service

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Total Rain-fall*: 39.69
 *July 1 through Today
Previous Year Total: 32.75"
25-year Average: 37.14"

Rainfall Data Analysis Skip Navigation Links
CIMIS Rainfall and ETo
CIMIS/Mt Demo Compare

Weather Station Info**
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Camino, CIMIS #13
Madrona Weather & Webcam
Mt. Democrat Rainfall

Irrigation Scheduling Basics Skip Navigation Links
Irrigation Overview
Irrigation Scheduling
Crop Coefficients
Crop Curve
Water Budget
Root Zone
The Soil Triangle
The Grower Report

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Data Loggers, Feb. 28 2008
Grower Reports, Feb. 21, 2008

Useful Ag Links**
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IMS Newsletter
IMS Photo Gallery
Ag Water Management Council
California Irrigation Institute
ITRC- Cal Poly
CIT-Fresno State

El Dorado County Ag Links**
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Ag Commission
Ag Grading Permits and BMPs
Annual Crop Report
Farm Bureau
Right to Farm Ordinance
Small Farm Irrigation Rate Application
UC Cooperative Extension
Water Agency

Crop Coefficient

Crop coefficients (Kc) are used with reference evapotranspiration (ETo) rate to estimate specific crop water use and needs.  These coefficients are dimensionless numbers (usually between 0.1 and 1.2) that are multiplied by the ETo value to arrive at a crop ET (ETc).  The resulting ETc can be used to help an irrigation manager schedule when irrigations should occur and how much water should be put back in the soil.  An example of how quickly the ETo can change and affect the ETc is seen below.

This is a graph of nut crop water use during a specific time period during the 2007 growing season.  This crop has a Kc of 0.8.  The ETc is represented by the blue line while the ETo is the top line.  Therefore, on any given day the crop uses 80% of the water that the refernence crop uses during the height of the growing season.  This graph shows that the ETo varies from 0.12 inches per day to over 0.33 inches per day in just a two week period near the end of July.  If a grower is using an irrigation controller that is set to deliver a certain amount of water on a daily schedule independent of weather parameters then the grower probably would have over irrigated on July 22 and under watered on August 3 due to changes in crop water use casued by the weather.

Crop coefficients vary by crop, stage of growth of the crop, by some cultural practice, and the elevation the crop is being grown.  Citrus trees have smaller coefficients than peach trees when the peach tree is in full leaf. Coefficients for annual crops will vary widely through the season with a small coefficient in the seedling stage to large coefficients when the crop is at full cover. Pear orchards with cover crops between the tree rows will have larger coefficients than orchards without cover crops since these cover crops also utilize soil moisture. Apples grown on a south facing slope at 1,500’ will use about twice as much water as apples grown on a north facing slope at 3,000’ due to the changes in temperature and humidity of these two setting, and this is reflect in the Kc values for the different locations.

The crop coefficients tend to be constant over a wide range of growing areas with extremes in weather conditions. This will not be a problem for the irrigation manager if he knows what the coefficients are for his crops and he has access to regional and/or specific ETo information. With these two pieces of information the manager can calculate the water use on any given day during the growing season.

Crop coefficients have their roots in the agricultural setting but the same principles can be applied in the urban landscape setting.These coefficients have been published and are free to the public. Use the following links to find the appropriate coefficient:

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