Effective Precipitation

Effective Precipitation (EP) is the amount of precipitation that is actually added and stored in the soil. During drier periods less than 5mm of daily rainfall would not be considered effective, as this amount of precipitation would likely evaporate from the surface before soaking into the ground. Effective precipitation enters the soil and becomes available to the plant.

The effective precipitation and total precipitation are both given on Farmwest. The moisture deficit is calculated by subtracting the effective precipitation from the calculated evapotranspiration.

How Effective Precipitation is Calculated

During extended warm dry periods rainfall less than 5 mm may not add any moisture to the soil reservoir as most of it is evaporated before entering the soil. Therefore, if rainfall is less than 5mm the Farmwest calculator does not enter a value for effective precipitation. In addition, only 75% of the rainfall over 5mm is considered to be effective precipitation.

The equation used in the Farmwest calculator is:

Effective Precipitation (mm) = (RAIN – 5) x 0.75

During dry periods no changes need to be considered in the Moisture Deficit data that is reported on Farmwest.com.


During extended periods of cool wet weather, less evaporation takes place and the smaller rainfall events may be effective precipitation. Farmwest users must decide when to use the total precipitation instead of the effective precipitation. Two situations may occur where the total precipitation should be used to determine the moisture deficit.

Situation 1 – Wet Periods with Small Rainfall Events

During prolonged cool wet periods precipitation in the form of daily showers can be considered to be effective. This is because the soil and air temperatures are cooler and humidity is higher allowing the rainfall to soak into the soil before it evaporates. The judgement as to how much rainfall is effective would have to be made after a number of cool days. Soil moisture monitoring could be helpful in determining how much of the rainfall is effective.

The effective precipitation shown on Farmwest is always calculated using the equation above. Therefore during cool wet periods the effective precipitation recorded could be low, resulting in a higher moisture deficit than what actually exists. Soil moisture monitoring should be done under these conditions to ensure that irrigation is applied when necessary.

The total precipitation and the calculated ET can be used to calculate a more relevant soil moisture value for wet cool period as shown in the equation below.

Moisture Deficit wet periods = ETc – Total precipitation

Situation 2 – Large Amounts or Precipitation

Very large rainfall events may apply more moisture than the soil’s water holding capacity. The following factsheet provides more information on soil water storage.

For large rainfall events, a portion of the precipitation may be lost to the crop due to moisture moving deep into the soil below the crop rooting depth, being lost by surface runoff or removed from the field through a subsurface drainage system. The effective amount of rainfall recorded on Farmwest by large rainfall events may now be much higher than what is happening in the field.

If using water budgeting and ET to schedule irrigation, the budget process must restart after large precipitation events. The following Ministry of Agriculture, Food and Fisheries Factsheets provide further information.