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  Frequently Asked Questions
This page contains answers to frequently asked questions concerning the AgriMet system, the CIMIS system, irrigation scheduling, crop coefficients, evapotranspiration, and irrigation performance measures. If you have a question please use the Feedback form to submit it.
What is CIMIS?
How do I get CIMIS information?
What does a CIMIS weather station do?
How many CIMIS weather stations are there in California?
Where is the closest CIMIS station to me?
Evapotranspiration (ET)
What is ET ?
What is ETo, "versus Etp, versus Etpan, versus ETc?"
Irrigation Scheduling
What is irrigation scheduling?
Why is irrigation scheduling important?
What is water budget irrigation scheduling?
What is graphical irrigation scheduling
Crop Coefficients
What is a crop coefficient?
Where do I find crop coefficients?

Irrigation Performance
What is irrigation efficiency?
What is distribution uniformity?
What is emission uniformity?
How do I get more information about irrigation on the Internet?
What is CIMIS ?
CIMIS stands for California Irrigation Management Information System. It is a network of standardized weather stations scattered throughout California which report weather data on an hourly basis. More importantly, the weather stations use this data to calculate ETo , which is a reference point for evaporative demand for that micro-region.
How do I get CIMIS information ?
There are two ways to obtain CIMIS information:
   1. Click here to select the station closest to you.
   2. Go to the CIMIS web site and register with their system.
What is ET ?
ET is the abbreviated acronym for evapotranspiration, which is the combination of soil eva poration and plant transpiration . These two processes represent the water loss from the plant-soil system due to evaporative demand of the atmosphere.
What is ETo, versus ETp, versus ETpan?
ETo is simply a reference number which represents an estimate of evapotranspiration (ET) from an extended surface of 3 to 6 inch (8 - 15 cm) tall green grass cover of uniform height, actively growing, completely shading the ground, and not short on water. All of the CIMIS weather stations throughout the state are situated within a small grass field which is optimally irrigated. Thus, instruments attached to the weather station datalogger measure weather parameters that would directly affect ETo estimates such as solar radiation, air temperature, humidity, wind and rain. This data is incorporated within the weather station's database and calculates a reference evapotranspiration (ETo) number every hour.

ETp is another type of reference ET, but is the ET from a full-growth field of alfalfa.  It was one of the earliest references used for irrigation scheduling and is still used in many systems.

ETpan is another type of reference, probably the most widely used in the world.  It is the evaporation from a U.S. Weather Bureau Class A evaporation pan.  This pan, like sites used for gathering data needed to compute ETo and ETp, has standards for size, color, installation, water depth, screening for birds, etc. etc.
What is a Crop Coefficient ?
A crop coefficient (normally identified as Kc) is merely a numerical factor that relates the ET of the individual crop (ETc) to the reference ET.

ETc = Kc * reference ET
Note that there are several types of reference ET.  The ETo reported by the CIMIS weather station network is just one type.  Make sure that the Kc that you are using has been developed for the reference ET that you are using.  That is, do not use a Kc developed for use with ETp if your reference ET is ETo.
How many CIMIS weather station are there in California?
There are currently over 125 active weather stations within the CIMIS network and there is historical data from an additional 61 inactive stations.
Where is the closest CIMIS station to me ?
Refer to the state map to find your local CIMIS station.
What is irrigation scheduling and why is it important ?
Irrigation scheduling is a generic term for a number of techniques that aide the irrigator in determining when and how much to irrigate.  When to irrigate is usually an agronomic question.  That is, do you want to irrigate to avoid stress or, as is the common situation with a crop like wine grapes, irrigate to induce a certain amount of stress.  How much to irrigate generally depends on the type of irrigation system.  If it is a low-frequency flood-type system, furrows or border checks, "how much" to irrigate is generally the soil moisture deficit in the root zone plus any leaching fractions needed to maintain a salt balance.  With high-frequency, micro-irrigation systems the "how much" is a calculation of required hours of operation based on the estimated crop water use.
What is irrigation efficiency ?
Irrigation efficiency (IE) is a measure of how much applied water is used beneficially.  A general equation for irrigation efficiency would be:

IE = Beneficial Use of Applied Water / Total Applied Water
The two main beneficial uses are crop water use (evapotranspiration, ETc) and leaching to maintain a salt balance.   The problem in determining an exact number for IE is that IE involves both a question of physical boundaries and time.  That is, is IE determined for the individual irrigation on a field, for an entire season on the field, for the farm, for the irrigation district, etc.?  It is not generally possible to achieve 100% IE due to immediate evaporation losses during an irrigation.  An exception might be a sub-surface drip system that was managed to under-water the crop.
What is distribution uniformity?
Distribution uniformity (DU) is a measure of how evenly water is applied across a field.  If 4 inches of water infiltrates in one part of a field during an irrigation and 2 inches infiltrates in another, that is poor uniformity.  There are many ways to calculate DU.  One of the most common is termed the "low 1/4 DU".

DUlow 1/4 = average depth of infiltration in the 25% of the field receiving the least applied water average depth of infiltration in the entire field

You might also see a DU determined using the average depth in the least-watered 1/8 of the field, or even using the lowest measured infiltration.
What is emission uniformity?
Emission Uniformity or EU is a term strictly applied to microirrigation systems.  Emission Uniformity is a more complicated equation than distribution uniformity and involves measured emitter flows, the number of emitters per plant, and the coefficient of manufacturing variation.  Like DU, EU can be calculated using different basis, the lowest 25% of measured emitter flows, lowest 1/8 of emitter flows, absolute lowest flow, etc.  The equation is:

EU(%) = 100 * (1.0 - 1.27 / n ½ * Cv) * Qn / Qa

EU(%) = Emission Uniformity as a percent
n = number of emitters per plant
Cv = coefficient of manufacturing variability
Qn = the average (or absolute minimum) flow of the basis (i.e. if the basis was low 1/4,
then this would be the average of the lowest 25% emitters measured)
Qa = average emitter flow
Looking at the equation one can see as Cv is higher, then the EU is lower.  That is, as the variability of the manufacturing process increases, the expected uniformity in the field decreases.  Conversely, as the number of emitters per plant increases, then the EU will increase.  This is because it is expected that the effects of Cv (variability in the manufacturing process) will be random, thus some emitters will flow lower than design, some higher.  The more emitters per plant, the more this variability will be "evened out".