How Long to Irrigate

Applying the right amount of water in each irrigation event is essential to improve both water and nitrogen use efficiency. When irrigation is excessive, water can be lost through deep percolation and/or runoff, carrying soluble nutrients such as nitrate with it. On the other hand, short irrigation durations usually augment the need for increased irrigation frequency, in addition to shortening the root zone depth and restricting the volume of soil a crop uses to explore water and nutrients. Increased susceptibility to drought stress and yield reduction are typical consequences of recurring short irrigation durations. Such conditions are most common during peak production, when the crop is well established and its canopy fully developed, especially if accompanied by warm and dry weather conditions. As a simple analogy, a fully established strawberry crop with most of its roots reaching down to 14 inches depth is less likely to sustain significant yield losses after a few days of limited irrigation, compared to a crop with root depth limited to 8 inches. This difference is even more important in soils of low water holding capacity (e.g. sandy, sandy loam, loamy sand) and during warm and dry weather conditions, such as the Santa Ana winds.

Determining Irrigation Duration using ET data

The most suitable and accurate method for determining irrigation duration of soil-based production of berry and vegetable crops is the ET-based method. Evapotranspiration (ET) is the sum of vaporization processes that occur in a field. If the water vaporizes from the plant surface or soil surface, the process is called evaporation. Water vaporizing inside the plant leaves and diffusing through the leaf pores to the ambient air is called transpiration.

The ET-based method consists mainly of using two parameters to determine the crop evapotranspiration (ETc): the reference evapotranspiration (ETo) and the crop coefficient (Kc).

Camarillo Weather Station #152
Camarillo Weather Station #152

ETc = ETo x Kc

ETc – crop evapotranspiration is the maximum ET of a specific crop where the crop growth and production is not limited by water availability or plant stress.

ETo - reference evapotranspiration is the ET of a reference surface (usually a grass) not short of water. The only factors affecting ETo are climatic parameters, which can be measured by weather stations. ETo data is provided by the California Irrigation Management Information System, or CIMIS. CIMIS maintains approximately 150 weather stations throughout California that provide ETo and other climatic parameters free of charge to the public.

Kc - crop coefficient values are factors that are multiplied by ETo to estimate ETc. Whereas ETo accounts for weather variation, Kc factors account for crop morphology and physiology. Usually, the bigger the canopy, the higher the Kc.


Changes in ETo over time
Changes in ETo over time


The importance of relying on weather station data for determining irrigation duration is emphasized in the graphs below, where daily ETo values vary by orders of magnitude in a matter of a couple of days. These variations are common and make it challenging to use a fixed irrigation schedule where the durations and intervals between irrigations are kept the same for weeks. 




In addition to frequent changes in weather (ETo) patterns, the crop canopy (and its Kc) also changes at an variable rate. The three images below illustrate canopy cover measurements of a celery crop taken with an NDVI camera. Multiple canopy coverage measurements taken throughout the crop cycle can provide a better understanding of how the crop canopy grows, which can be modeled and used to adjust Kc.


Example of NDVI measurement of celery canopy growth
Example of NDVI measurement of celery canopy growth

Canopy growth of two celery cultivars measured with a NDVI camera throughout a season.
Canopy growth of two celery cultivars measured with a NDVI camera throughout a season.







The photos above were collected from a celery field as part of a research project designed to assess celery water use throughout the growing season. The higher the canopy area, the higher the Kc and water use.



For more information on crop coefficients, click here



Crop ET Calculator

This calculator determines crop ET since the last irrigation based on weather data from CIMIS (station chosen automatically based on zip code input) associated with crop type and growth stage. ETo and precipitation values are updated daily from CIMIS.

Instructions: enter the zip code of where the field is located, select the day of the last irrigation in the calendar, and in the drop-down menus choose the 'Crop Type' and 'Crop Stage'. The recommended amount of water will be calculate by clicking on "Click Here to Calculate". To estimate how long to irrigate, enter the irrigation application rate.

Note that this calculator is currently under development. Please forward any feedback to .


Using CropManage for irrigation recommendations

CropManage is a free web-based software that makes irrigation recommendations based on current ETo data from CIMIS and crop parameters created with local research. Once a profile and field are setup, the software creates irrigation recommendations with only a couple clicks. For details about how CropManage works, click here.


The weather data shown below is a summary from all CIMIS stations in Ventura County. To access more detailed and daily weather data from CIMIS stations, click here.

Ventura County Stations