Flow rate and porosity factor of Ollas
The porosity of the olla will depend on its flow rate.
Care must be taken to select a clay olla with a sufficient flow rate to meet the plant's needs under the given climatic conditions.
However, this rate is not a fixed value, as is the case for other irrigation systems. It is difficult to predict and will depend, among other things, on the evapotranspiration rate of the area to be irrigated (see section above) and the following factors:
- Self-regulation of the jar based on the dual pressure of hydraulic conductivity, inside and outside the jar.
- Hydraulic conductivity of the jar.
- Consequences of the choice of materials and manufacturing methods.
Self-regulation of ollas
Hydraulic conductivity acts according to a dual pressure:
- Water pressure inside the jar, exerted by the height of the water and the volume of the jar.
- Environmental pressure outside the jar, exerted according to the humidity and evapotranspiration rates of the soil, combined with the needs of the plants according to their nature and the climate.
We recommend a volume greater than:
- > 5 Liters for vegetable beds and shrubs.
- > 10 Liters for tree crops.
- > 20 Liters or more for specific trees and shrubs depending on technical constraints (climate, soil, species, access, desired autonomy…).
In the case of a buried jar, the external pressure will be the suction exerted by the soil. (*)
(*) “Suction, plant physiology” [archive], on www.universalis.fr
Very dry soil or air outside the jar will exert suction pressure, causing water to flow out of the jar through the walls, allowing it to adapt to external conditions.
The lower the soil moisture level, the higher the soil's suction tension. Soil moisture decreases due to evapotranspiration and the absorption of water by roots.
When soil is very dry, the suction tension is strong and increases the flow rate from the jar.
Conversely, in saturated soil, the suction pressure is zero and the flow will stop.
If the jar is empty, then the equilibrium between the different pressures is reversed and the jar will fill up (flow process from the outside in).
Regardless of internal and external pressure, the jar adjusts its water flow rate according to the plant's needs.
The plant absorbs water from the soil through its roots to meet its needs, based on evapotranspiration.
The jar therefore alters the soil moisture level, and consequently affects the water flow rate.
The suction process itself creates pressure, which is greater the closer the roots are to the sides of the jar.
Since roots can detect moisture, they grow towards it, and after a while, the sides of the jar become covered with roots drawing water directly from the source (especially for plants with extensive root systems like tomatoes). (*)
(*) AE Daka, Chapter 7 Clay pot sub-surface irrigation as water-saving technology for small-farmer irrigation in Development of a technological package for sustainable use of Dambos by small-scale farmers, PhD Thesis,, Pretoria, South Africa, University of Pretoria, 2001
Thus, a jar does not behave the same way in the atmosphere and once buried, it adapts to the conditions of its environment and the needs of the plants
Hydraulic Conductivity of Ollas
Hydraulic conductivity is the ability of a porous medium to allow a fluid to pass through it under the effect of a rate of pressure change. It is generally expressed in meters per second (m/s).
This quantity depends on the properties of the porous medium in which the flow occurs, the properties of the flowing fluid (in this case, water), and the degree of saturation of the porous medium. To simplify measurements, it is generally chosen to measure the saturated hydraulic conductivity.
Here we are interested in the properties of the porous medium, namely the walls of the jar.
The choice of materials, the preparation of the clay to be shaped, the drying, and the firing will all affect the hydraulic conductivity.
