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Volumetric Water Concentration (VWC) and Irrigation

Updated: Jan 26, 2023

Volumetric Water Concentration (VWC) and Irrigation

In large-scale farming understanding, VWC has a huge environmental impact on water usage and production of food crops. In the US only 16 percent of cultivated cropland is irrigated, that same portion however makes up for 50 percent of all crop revenues. It is therefore imperative that advanced irrigation management be utilized to maximize water and crop productivity.

We also want to manage our water usage and maximize growth in our home gardens. Several methods can be implemented to achieve improved irrigation management. These methods include maximizing canopy, creating enclosed spaces, and most important; soil water management.

Volumetric water content is the ratio of the volume of water to the unit volume of soil. It can be expressed as a percentage or as a depth of water per depth of soil such as inches of rain per foot of soil. In container gardening it is the percentage of water per volume of the container.

Soil Matrix Potential (SMP) is the soil mix's ability to retain water. Plants must apply a force greater than the SMP in order to extract the water particles from the soil matrix where they are bound. As water is removed from the soil the remaining moisture is held more strongly making it harder for the plant to extract soil using its roots. SMP increases as water is removed from the root zone of the plant. SMP is expressed in two equal units Kilopascal (kPa) and Centibar (cb). One kPa is equal to one cb.

Soil Water Thresholds are specific values of Soil Water Content (SWC) that indicate the water availability to your plants.

Saturation is the threshold at which all cavities of the soil mix are completely filled with water. The VWC ranges from 30 percent in sandy soils all the way up to 60 percent in clay. The SMP at saturation is much less dependent of soil texture at this point and would equal close to zero. This means that there is minimal restriction to water absorption in the plant.

Field Capacity is the point at which all larger cavities have be drained by the force of gravity. For outdoor applications, irrigation SWC that goes above field capacity turns to wastewater as drained water is not available to the plants and cannot be reclaimed.

Permanent Wilting Point is the threshold at which the plant is unable to extract water fast enough to keep up with their demand. At this point transpiration in the plant comes to a near halt, causing a significant reduction in growth. Extended exposure to wilting and the plant will die. Irrigation should be applied before this point. The value of this wilting point can vary between plant species and soil type. Ranging from 7 percent in sandy soil to 24 percent VWC in clay.

Total Available Water is the amount of water available to the plant estimated by subtracting the water content at wilting point from Field Capacity. Sandy soil cannot hold as much water and have the lowest available wat

er whereas medium texture loam and silty clay loam have more available. Sandy soil will therefore need to be irrigated more often.

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