Evapotranspiration & Water Use Calculator Information

   A Primer on Evapotranspiration (ETo)


Simply put, Evapotranspiration (ETo) is the total evapotranspiration graphicamount of water lost through evaporation in the soil and transpiration ("breathing") of the plants.  In a drought-prone state, it is of utmost importance to understand this concept and its implications.


Reference Evapotranspiration, as displayed in the Local Environment Bar on the left hand side of the page once a zip code is entered, shows how much water evaporates and transpires from a 4-7" tall cool season grass growing in an open-field condition in a given month.  Because ETo values can range wildly throughout a zip code, we have chosen to display the average County ETo rate.  In most cases, your actual reference ETo will be slightly higher or lower.  For specifics, you may visit www.cimis.water.ca.gov.


Landscape Resource has chosen to display reference ETo for a number of reasons.  First, it is a number and concept that many people are unfamiliar with.  By displaying this information, we hope to encourage the public to learn more about ETo and how it relates to landscape.  Secondly, we feel it is important to respect and honor our local environment by selecting plant material that is appropriate to our climate.  Each month, the reference ETo on your Local Environment Bar will update and you may track the variations over the year to better understand how weather affects the ETo.  Third, we use the reference ETo as a variable in the equation for the Water Usage Calculator.  And lastly, a quick calculation with your reference ETo and the WUCOLS ratings of a specific plant located in the Plant Library yields powerful and specific water needs information.


For more information on ETo, we recommend visiting  http://www.water.ca.gov/wateruseefficiency/landscape/ and downloading a pdf copy of WUCOLS. 

As landscape enthusiasts, we are in indebted to the team of professionals and academics that have prepared this document in the spirit of conserving resources, saving money, and improving the quality of our landscapes.



   Using WUCOLS data for Estimating Water Needs


Thanks to the creation of the WUCOLS (acronym for Water Use Classifications of Landscape Series) document, those in the landscape trade have access to an incredible wealth of information regarding water needs for plants in California.  Created by the University of California Cooperative Extension Publication and funded by Department of Water Resources, "the purpose and intended use is to provide guidance to landscape professionals when selecting plant material, while taking into consideration water needs. It also serves as a guide to assist in developing irrigation schedules for existing landscapes." (Department of Water Resources)


In the Landscape Resource Plant Library, hundreds of plant listings have been assigned values of VL, L, M, H, or /.  This data is directly referenced from the Species Evaluation in the WUCOLS document.  Based on your region, which corresponds to your zip code, a value for a specific plant is given regarding its category of water needs.








70-90% ETo



40-90% ETo



10-30% ETo


Very Low

<10% ETo





WUCOLS does not define the above values in terms of number of times a plant should be irrigated in a given length of time.  However, it does provide numerical values for each rating that can then be used in an equation to create quick, and fairly accurate*, water needs calculations.


With your reference ETo in hand, you can create your own calculations:


Water Demand in a given month = Reference ETo x WUCOLS Value




Equation:  Water Demand in a given month = Reference ETo x WUCOLS Value

Example:  Reference ETo for month of August in Santa Barbara= 2.7".  You are trying to determine which of the below plants is better suited to your landscape.


Sample One:  Euphorbia rigida

Sample Two:  Imperata cylindrical ‘Rubra'

WUCOLS Rating:  (L) Low = 10-30% ETo

WUCOLS Rating:  (H) High = 70-90% ETo



Water Demand = (.1) / (.3) x (2.7")

Water Demand = (.7) / (.9) x (2.7")

Water Demand = .24 - .81" per month


Water Demand = 1.89 - 2.43" per month


As you see, with quick calculations and readily accessible data, powerful information can be obtained regarding the water needs of specific plants.


*For even more accurate and involved water calculations, reference the WUCOLS document.