Irrigation Design Tip: Climate and Irrigation Requirements

To answer the questions “How much water has to be applied to the plant material?” and “How often and how long does the system need to run?” several factors need to be examined.

The local climate is one of the main factors that influences how much water is needed to maintain good plant growth. 

The plant water requirement includes the water lost:

• by evaporation into the atmosphere from the soil and soil surface
• by transpiration, which is the amount of water used by the plant
• the combination of these is evapotranspiration (ET)

ETo stands for reference evapotranspiration, which is the maximum average rate of water use for plants in a given climate. Reference evapotranspiration is multiplied by a crop coefficient to obtain the ET rate for a specific plant or turf. 

Although it is a rough guide to water requirements and not geared to a specific plant, the table below and in the Technical Data section of the Irrigation Design Manual will help establish a ballpark figure for the project. At the design stage, you want to provide an irrigation system that can meet peak season (summer time) ET rates.

In the table below, note the factors that affect the water use rate for a given climate type. The three categories of “Cool,” “Warm” and “Hot” indicate that temperature has an influence on water use. The hotter the climate, the more water loss is expected. Other major factors are humidity and wind speed. If the air is humid, evaporation will be lower as compared to a climate with the same average temperature but drier air.

Climate PET (potential evapotranspiration)

In the table, a “Cool Humid” climate has an ET range in inches (millimeters) of water required per day of .10 to .15 in (3 to 4 mm). At the upper end of the scale, a “Hot Dry” climate produces a requirement of .30 to .45 in (8 to 11 mm) per day. These figures are rough estimates for these types of climates for an average midsummer day.

To help determine in which climate your project is located, consult the notes on “Hot,” “Warm,” or “Cool” that are listed below the PET table. Also listed are the humidity ranges that establish the “Humid” and “Dry” classifications.

An irrigation system should always be designed to adequately water the project in the “worst case” condition. This is usually midsummer when the average daily temperature is at or near its highest for the growing season or when humidity is averaging its lowest percentages. Of course, a combination of these extremes produces the greatest water requirement. When you have determined the climate type for the area where your project is located, use the highest number, the “worst case” condition listed at the top of the ET range for that climate type, as the requirement for your project. 

Consult the Landscape Irrigation Design Manual for step-by-step process for basic design of irrigation systems.  The manual provides helpful information from understanding basic hydraulics to preparing the final irrigation plan.