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# How to Use Crop Evapotranspiration (ETc) to Estimate Soil Water Content?

Updated: Mar 27

So, we know how to calculate the amount of water our plant uses on a daily basis, the question is how much water we should apply?

We use a simple model called “soil water budget” to account for all the inputs and outputs and calculate soil water deficit (or soil water content) which would tell us how much water we can put in the soil, or how far we are from the dry and wet thresholds.

We need a starting point for stored soil water content, and we should check our assumptions periodically. This is where the use of ET and soil water content sensors together becomes very handy. ETc tells how much water the crop has used, which tells us how much we have to return to the soil. Measuring the soil water content, we can validate when our irrigation has returned the appropriate amount to the soil to return us to field capacity.

If water lost through evapotranspiration can be determined and the application rate is known, the irrigation run time needed to replace the water lost can be calculated. Accurately calculating evapotranspiration for indoor plants is complex; however, an indoor-friendly estimation method can provide valuable information.

After calculating reference evapotranspiration, the resulting value is typically multiplied by a crop coefficient that tailors the water use to a particular species’ water use characteristics and the

specific soil or substrate evaporation of the production system and crop.

A plant species may have more than one crop coefficient curve based on developmental stage or management practices, such as pruning. Ideally, the crop coefficient would account for all of the characteristics listed above. Due to the number of necessary parameters and the complexity of the calculation, very few crop coefficients have been developed for crops grown in pots like in nurseries or for indoor plants. This concept, however, is very well developed for outdoor crops grown in fields and orchards.

Alternative method for estimating ETc of plants grown in pots: Weigh plants for a change in water content divided by the area of the top of the container.

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