Evapotranspiration and Net Irrigation Needs Introduction


Evapotranspiration or ET is a measurement (usually in inches) of the amount of water required for plant growth. ET measures the quantity of water transpired from plant tissues and evaporated from the surface of surrounding soil, expressed as a depth (usually in inches).  ET is based on a number of factors that can include: local temperature, precipitation, cloud cover, solar radiation, and the type of plants you are growing. Most of the ET calculations done in urban settings are for turf grass.

The American Society of Civil Engineers (ASCE)  has attempted to standardize the calculation of ET by establishing the modified Penman-Monteith equation as the preferred method for determining  ET.  A detailed explanation of the ASCE-Penman-Monteith formula and methodology is available here.

The general form of the ASCE-standardized equation is :

ET_eq copy

The publication from ASCE explains each factor in the equation.

Other ET calculation methods, such as Blaney-Criddle, are available and may be perfectly appropriate.

Know Your ET

ETo is the reference ET for a standard crop of grass 4 inches to 7 inches tall (10.2 cm to 17.8 cm).

ET Factor is used to set a landscape water efficiency goal.  Also known as an “adjustment factor”.

Net ET is ETo with the effective rainfall depth deducted.

NIR is the net irrigation requirement – which is often less than ETo or Net ET. 

How to calculate the water requirements for a landscape

Intrepid irrigators and internet users find it fairly easy to calculate the theoretical irrigation requirement for a landscape. There are two key pieces of information you need to obtain:

1) The area (in square feet) of your lawn; and

2) the evapotranspiration (ET) rate for the irrigation season in your area. Don’t worry, both of these items should be fairly easy to obtain.

Lawn Area

If your lawn isn’t too big you could simply go outside with a tape measure and physically measure the area. Divide the yard into a series of rectangles and triangles and sum up the areas. Recall that the area of a rectangle is the base length ´ height length. The area of a triangle is ½ ´ the base length ´ height length.

If you’re not in the mood to measure your yard you can calculate the lawn area in another way. Start with the total lot size. If you only know the lot size in acres you can convert to square feet by knowing that 1 acre = 43,560 square feet (4,047 square meters). From the total lot size subtract the footprint of your house and the area of your driveway and sidewalks. If you don’t know these exact areas make an educated guess. Finally, subtract any other areas on your lot that are not irrigated (swimming pools, patios, bare patches, ponds, etc.). The result will be an estimate of the lawn area at your house. This calculation is summarized below.

Total lot area (sf)
- building footprint (sf)
- driveway area (sf)
- sidewalk area (sf)
- all other non-irrigated areas (sf)
= Total irrigated area (sf)

Evapotranspiration (ET) Rate

To find the ET rate for your area you will need to do a little searching on the world wide web. Using google or your favorite search engine simply search for “evapotranspiration” followed by your city and state. You should turn up a number of possibilities. Usually ET is calculated by a local university or weather service. It is also used frequently in agriculture.

Once you have a measurement of the annual ET rate in inches you are ready to go!

Calculate the Water Requirement for Your Lawn

Use the following equation to calculate the water requirement for your lawn:

Irrigated area (sf) x ET rate (inches) x 0.6233 = Water requirement in gallons

This calculation will give you a rough estimate of the amount of water your landscape needs over the course of the entire irrigation season.