Crop Coefficients and Plant Type

How do you know what crop coefficient should be used for each plant type? I have a 1 zone wonder with shrubs and trees on the same zone. I have my crop coefficient set at 70%?

Why is 50% recommended for shrubs and 75% for trees? They are similar plant types. There is a grey zone between tall shrubs and small trees.

Shouldn’t crop coefficient change throughout the year? I thought Deciduous plants and Evergreen plants have different water needs relative to each other based on the season.

You’ll want to look up the crop coefficients if you have the time. In Rachio they’re in Percent but everywhere else they’re not (times by 100 for input to Rachio advanced settings). You’re right deciduous trees have vastly different needs especially in the winter. The coefficients do change over the year but Rachio simplifies it with defaults to be something like annual averages for the USA. I have been known to set reminders to change crop coefficients for my stone fruit trees-spring, summer, late summer, fall, winter. Your university extension may have crop coefficients or plant factors for your local area; these can be different depending on your climate.

Here’s an example extension website with some annual average crop coefficients/plant factors:

Here’s an example of monthly crop coefficients for turf grass:

A search of the Internet will give you decent values for Crop Coefficient for most plants. The sites quoted above should help.

Basically, that just says that trees, with their deeper roots and high foliage, require 50% more water than shrubs. And yes, there’s a lot of gray area here.

Some do change throughout the year, others are fairly constant. Remember that Crop Coefficient is not an absolute value for water required; it is a multiplier of the value of FRET (Forecast Reference crop EvapoTranspiration), which basically is the amount of water evaporated out of a shallow tray per day or week which varies with the temperature, humidity, sunlight, wind speed, etc. So much more is used in summer than winter, meaning CC can be relatively constant and still result in variable rain.

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Crop Coefficient = Kc

This is a quick reference from my Cal Poly notes for PF Plant Factor which is used for a guide.

Plant Type Plant Factor
Tree, Shrubs, Vines, Groundcovers

(woody plants)|0.5|
|Herbaceous Perennial Plants|0.5|
|Desert Adapted Plants|0.3|
|Annual Flowers & Bedding Plants|0.8|
|General Turfgrass Lawns, cool-season (tall fescue, Ky. bluegrass, rye, bent)|0.8 2, 3|
|General Turfgrass Lawns, warm-season (bermuda, zoysia, St, Augustine, buffalo)|0.6 2, 3|
|Home Fruit Crops, Deciduous|0.8 2|
|Home Fruit Crops, Evergreen|1.0|
|Home Vegetable Crops|1.0 2|
|Mixed Plantings|PF of the planting is that of the plant type present with the highest PF|

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Does a crop coefficient twice a large water twice as frequent? For example will a plant at 60% Crop Coefficient need to be watered twice a often as a plant with a Crop Coefficient of 30%? The weather and soil conditions are assumed to be the same.

Not exactly but that is one approximate way to look at it. The crop coefficient scales the day’s ET0 or reference evapotranspiration, and that changes day to day.

Should I increase the crop coefficient in excessive heat?

My shrubs and trees are on the one zone wonder. I use the shrub watering depth of 16 inches and the crop coefficient of 0.75, which is the standard tree recommendation

You shouldn’t have to. The calculated ET (Evapotranspiration) value takes the heat, humidity, sun, wind, etc. into account, so that’s what does it. Now, if you find your crops drying out too much, you might increase it just because it probably needs it to fine-tune, as hot weather will bring a lack of water to light sooner than cool.

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@franz @dane

It appears crop coefficients might change throughout the year for certain plants. If so, can Rachio make it an option to have automatically adjusting crop coefficients?

@ECOBEARD I appreciate your curiosity about this topic. Good news! We are currently developing a “dynamic crop coefficient” feature we will release this spring (2022) that will automatically provide specific crop coefficients for each month of the year for all crops. We’ve combed the available research to come up with crop coefficient curves that are specific to months, plants, and regions. For example, the table you posted with the Meyer 1985 data provides coefficient values for cool and warm season grasses in a mediterranean climate, so most Rachio controllers in mediterranean parts of CA, OR, etc will get those coefficients. However, Rachio controllers in Denver, Seattle, New York, Miami etc will each get their own set of coefficient curves according to their local climate.

This should provide a more accurate watering experience for all our customers, and will hopefully solve some over-watering and under-watering issues that can occur in some parts of the country due to annual average crop coefficients not keeping up with seasonal variability.

We’ll provide in-depth background material when we release the feature (existing customers will have to opt into it, and we’ll explain how). If you want to learn more about the topic, the UC ANR sources posted in this thread are good sources of information.

For anyone curious about crop coefficients for specific agricultural crops, there is a wealth of published research, but it can be laborious to sift through. This page from the UN Food and Agriculture Organization (FAO) has a very comprehensive set of tables for crops organized around growing seasons: Chapter 6 - ETc - Single crop coefficient (Kc). We took this information into account while creating more simplified crop coefficient curves for vegetable and flower garden zones.

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I’m interested in how that will work so it won’t change the crop coefficient too much or too little. How would one change the crop coefficient up or down a little from the deafault if it’s dynamic? Would this new dynamic crop coefficient feature save more water than the static crop coefficient?

Any other interesting upcoming features coming to Rachio?

@ECOBEARD The balance between too much and too little change… therein lies the rub, as they say. We’ve made our models of dynamic crop coefficients using the best available science and erring towards the Rachio default, i.e. a conservative interpretation of the amount of variability within a year.

This feature will be available to toggle on/off in the Advanced Settings for each zone. A customer may at any time turn the “dynamic” feature off, in which case the crop coefficient will be re-set to the default, which can still be manually adjusted to whatever they choose.

While it is hard to test the real affects of these coefficient changes, simulations have convinced us that the dynamic coefficient values will yield water savings in some regions and seasons. A prime example is fall watering in a steppe/ semi-arid climate like I live in here in Denver. Daily evapotranspiration can remain relatively high due to warm, sunny, and windy fall days, but cool season grasses just don’t need nearly as much water as they did in August, due to their own biochemistry and the receding sun angle. So the Kc value throttling down over the fall will save water in this case, while also more likely providing enough water (avoiding under-watering) in the peak ET months of July, August, and September.

We will announce more information about this feature soon, stay tuned!

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