Certified Landscape Irrigation Auditor

How is turf irrigation needs commonly calculated based on ET?

1. It is multiplied by 0.5

2. It is multiplied by 1.0

3. It is multiplied by 0.8

4. It is added to rainfall data

The correct answer is: It is multiplied by 0.8

Turf irrigation needs are often calculated based on the concept of evapotranspiration (ET), which quantifies the amount of water lost from the soil and plants through evaporation and plant transpiration. The correct approach to determining irrigation requirements for turf typically involves factoring in how efficiently the irrigation system can replace the moisture lost due to ET. When using a multiplier like 0.8, it reflects the efficiency of the system and helps account for the fact that not all applied water will effectively contribute to plant growth. Irrigation systems often have losses through evaporation, wind drift, and runoff, and a multiplier of 0.8 helps adjust for these inefficiencies, thus providing a more accurate estimate of the actual water needed for turf health. In contrast, multiplying by 1.0 would imply that all the water lost through ET is needed, which doesn't account for system inefficiencies. A multiplier of 0.5 would suggest that only half of the ET loss needs to be replaced, which is generally not sufficient for healthy turf growth. Simply adding rainfall data without factoring in ET can lead to over or under-watering, as it does not accurately reflect the actual water needs of the grass during different weather conditions.