Water Considerations
keep
Evaporation Low
Garden Tip
TB3001
Technical Bulletin Series
To understand how deep moisture avoids evaporation the issue is primarily soil temperature. A thorough and authoritative study published in 2013 by the Scientific World Journal covering soil temperature and water evaporation provides very imporatant details.

A chart from that publication is presented next along with a link to that publication.

Click to Visit Published Study
In this situation the surface soil was subjected to 65 °C (or 149 °F) for a 36 hour period. Soil at 5, 10, 15 & 20 cm depth (2 to 8 inches) were monitored for temperature.

Evaporation occurs at the surface and only when there is enough energy (heat and wind) to cause the moisture to turn to gas (water vapor). Deeper moisture is drawn through osmosis toward the surface when it dries. Moisture located less than 5 cm from the surface gains temperature rapidly for the first three hours, before leveling off. Soil moisture at twice that depth affords about 300% more time before leveling off.

There is a direct but inverse correlation between duration of moisture and the rate total moisture is evaporated. Moisture parked deeper into the soil simply evaporates much slower.

Often "Native" plants are selected in order
to reduce water consumption.
A reason these plants
can withstand drought is the depth
of their root systems.




During years that a drought prone area gets 20 or more inches of rainfall, moisture will penetrate deeply enough for many plant roots to establish as deep as 12 inches or more.


Where the soil is hard, [HCR] is slower than 0.10 in./hr. and when rainfall is low (less than 10 to 12 inches), it is essential to irrigate shrubs and trees deeply. At least during the dry season.

Run times for drip systems should be 100 minutes or more. This deep moisture is isolated from evaporation, so irrigations can, and should be less frequent. Overall, the water bill is down and the plants are healthier.

Evaporation is waste - but the other part of the water-use story is Transpiration! This is the water the plant loses to the atmosphere; mostly during the daylight hours. Without uptake of water from it's roots the plant withers and dies as it fails to be able to transpire.

True "Native" Plants have an advatage here as they have a lower requirement for transpiration. There are a number of factors making this true, but that would be for a different bulletin. There is, you might say - a battle between evaporation and transpiration for each plant. The larger and more vibrant a plants root system, the better it can uptake moisture when it's available and needed.


Short run times - shallow roots


Long & Slow Irrigations
Deep - Healthy - Roots

Act to reduce and minimize evaporation
Support Transpiration
Do both - with deep moisture!