How Are Watering Schedules Created?

ETwater uses the proven Water Balance Method of irrigation to calculate all watering schedules for sprinkler stations. The Water Balance Method is based on estimates of root zone depth, soil moisture holding capacity and plant water use. With the Water Balance Method, the amount of water applied, or 'run time' of each irrigation, stays the same, but the irrigation frequency varies during the year with the changes in weather. The irrigation amount is based on plant root depth and soil moisture holding capacity. Irrigation frequency is derived from estimates of the allowable soil moisture depletion and daily plant water use.

The soil moisture status and allowable depletion in inches of water are easily estimated for a given soil type by the "feel method," a simple technique involving field observations and a shovel. A general rule of thumb is to irrigate when 50% of the plant available water is depleted. ETwater uses this 50% threshold in its various algorithms that recognize soil depletion factors.

Irrigation frequency is determined by summing the daily plant water use values until the total water use equals the desired soil moisture depletion.

How the Water Balance Method Works — An Example

A tall fescue front lawn is bordered by a bed of shrubs, ivy groundcover and sweet gum trees. The lawn and the bed areas are irrigated by separate stations using pop-up sprinkler heads.

The following variables would be used to calculate irrigation schedules for the lawn area and for the bed area during typical hot July weather in Southern California.

For the lawn:

• Root depth is 6 inches
• Plant available water in the soil found on the site is about 1.5 inches of water per foot (12 inches) of soil, so the soil can provide .75 inch of water to the lawn.
• 50% depletion of plant available water is 0.375 inches of water (.75 inch of plant available water x 50%)
• ET is calculated to be 0.25 inches per day during a week in July.
• Plant water use factor is 0.9, so the lawn uses about 0.23 inches per day.
• Depletion will be reached in about 2 days: depletion allowed / plant water used, or 0.375 / (0.25 x 0.9) = 1.6 days.

Using this information, ETwater's software algorithm will program the lawn stations to receive 0.5 inches per irrigation every 1.6 days under these conditions.

For the bed area:

• Root depth is 12 inches.
• Plant available water in the soil found on the site is about 1.5 inches of water per foot (12 inches) of soil, so the soil can provide 1.5 inch of water to the lawn.
• 50% depletion of plant available water is 0.75 inches of water (1.5 inch of plant available water x 50%)
• ET is calculated to be 0.25 inches per day during a week in July.
• The maximum plant water use factor among these plant types is 0.7, so the shrubs, groundcover, and trees, use about 0.18 inches per day.
• Depletion will be reached in about 4 days: depletion allowed / plant water used, or 0.75 / (0.25 x 0.7) = 4.2 days.

Using this calculation, the bed will receive 0.75 inches of water per irrigation every 4.2 days under these conditions.