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I'm working on a project to design a self-contained plant growth environment for germinating and growing plants in space. It's important to know the absolute minimum amount of water and nutrients required to keep the plants alive and healthy over the course of 30 days, since we have very little space to store the water.

The plan is to use a single water reservoir to feed three separate growth chambers by injecting nutrient-rich water directly into a soil medium, similar to the ISS Veggie environment. We want to find out how much water we need to inject and how often.

I've looked over the literature as to what previous astrobotany projects have done. While they are typically very clear about the nutrient composition of the water, I have yet to find one that clearly states how much water the plants were given, and how often.

I focused my search on Arabidopsis thaliana to start, but ideally we would like to know water requirements for all major groups of small plants.

This seems like it shouldn't be a difficult question to answer. What am I missing?

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The plant will intake water to keep constant internal water pressure to compensate for evapotranspiration. However, the evapotranspiration will not be a simple universal number.

The evapotranspiration rate strongly depends on temperature, air humidity, water tension inside the plant you want to keep (may it wilt?) and also the radiation received by the plant (PAR - photosynthetically active radiation) and also the air flow. In longer time that will also depend on the ventilation of air in your box. That means how fast water vapour escapes the air from your box and whether any of it condensates.

Also, how much water evaporates from the soil itself? That depends on soil properties, air temperature and also air flow.

A common model for evapotranspiration is by Jarvis and Stewart but is most often used for trees. You can read about interesting measurements of evapotranspiration at Whole plant open chamber to measure gas exchange on herbaceous plants.

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