Dear Amit,

When precipitation is reported as a height, it refers to the depth of the sheet of water that would be standing on a waterproof surface, with no opportunity to evaporate or runoff, at the end of the rain event. Please note that 4 inches (or 101.6 mm) of water is A LOT for a daily precipitation event: are you trying to simulate a Monsoon situation, or a big storm? With that kind or water downpour, the vegetation may simply wash away and you'd expect lots of soil erosion...

Anyway, to answer your question, I'll assume that plants are contained in pots within your greenhouse, so to simulate a daily precipitation event of x mm in a day, you would have to provide a volume of water equal to the cross-section of your pots times the desired precipitation depth x.

Example: if your individual pots look like cylinders or truncated cones, with a circular shape at the top characterized by a radius of 20 cm (for instance), the surface area of the ground available for the plant would be 3.14 * 20 * 20 = 1257 square cm. If you simulated a rain input of 1 cm (for example), then your water volume would be 1257 cubic cm, which is also equivalent to 1.257 liters of water.

Lastly, please note that you should also deliver your water inside the greenhouse with a time distribution that matches that of a realistic rain event: A given amount of precipitation will have very different impacts on the vegetation and soil depending on the precipitation rate and the total time period over which it is applied.

I hope this helps. Michel.

Concur with answer of Dr, Verstraete.  I know you prefer to be as natural as possible, so I would suggest you capture rainfall as it has specific nutrients, pH, etc., and redistribute it, rather than water from other sources.  They make programmable irrigation spraying tools for laboratory work, where amount and intensity can be varied in the program.  If you have a recording raingauge or access to rainfall data, you can see how rainfall varies, and mimic with a reasonable approximation of real events.  

I however have to ask, Is it easier to try to reproduce a natural condition in lab or greenhouse, or just instrument and conduct an in situ study on a relatively natural area, which already is exposed to natural rainfall, natural plants and include some potted surrogates if desired for comparison if more control of container or individual plant measurement is desired?  Greenhouse may not have same temperature, humidity, solar radiation, condensation of morning dew, freeze-thaw, plant density, competition for site resources, or other conditions or disturbances.  

Dear Amit Raj Adhikari ,

The following links may be useful to you.

http://download.springer.com/static/pdf/145/art%253A10.1007%252FBF00209165.pdf?originUrl=http%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2FBF00209165&token2=exp=1469913137~acl=%2Fstatic%2Fpdf%2F145%2Fart%25253A10.1007%25252FBF00209165.pdf%3ForiginUrl%3Dhttp%253A%252F%252Flink.springer.com%252Farticle%252F10.1007%252FBF00209165*~hmac=e09b18922c14e1b913080420121bec62952f31cfdf9a4d2474b5be649054ddb6

http://download.springer.com/static/pdf/286/art%253A10.1007%252Fs11252-008-0064-9.pdf?originUrl=http%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2Fs11252-008-0064-9&token2=exp=1469913160~acl=%2Fstatic%2Fpdf%2F286%2Fart%25253A10.1007%25252Fs11252-008-0064-9.pdf%3ForiginUrl%3Dhttp%253A%252F%252Flink.springer.com%252Farticle%252F10.1007%252Fs11252-008-0064-9*~hmac=31d2d60df616ff074ec7a07255990c241a199a1b64d8106b935d4619fecd17e7

http://onlinelibrary.wiley.com/doi/10.1111/j.1469-8137.1987.tb00117.x/pdf

I hope this helps.

All the best!