Dear Enayet,

There is no single or best answer to your question, because there may be a number of constraints or limiting factors that may affect the frequency of observation but are not described in your question.

Your primary concerns should be to determine (1) what is the main purpose of your measurements (agriculture, meteorology, climatology, aviation safety?) and (2) whether you are interested in rain accumulation or rain rate. In either case, you will want to make your observations at a frequency that is coherent with your goal. Of course you will need very frequent observations (during rain events) if you wish to determine the precipitation rate.

The second essential concern is to assess the accuracy required by your application: What is the maximum error you can tolerate before your observations become useless for your purpose? If the basic measurement is the total volume accumulated in the rain gauge, then remember that measuring small volumes (or masses) are likely to be less precise than larger values.

There may be practical limits, such as the accessibility of your rain gauge, or its capacity (to hold water). Remember that water that has fallen into the measuring device can and does evaporate from it, especially when the temperature rises: to limit this type of effect, it is recommended to fill your rain gauge with a thin film of oil. Rain water would quickly sink below the oil film, and then not evaporate. Of course, you need to account for the amount of oil you inserted in the first place, or that you add each time you empty your rain gauge.

In short, your observation protocol should depend on your goal, on the required accuracy, on the characteristics of the instrument, on the practicality of actually acquiring the measurements, and on the seasonality and intensity of the precipitation.

Good luck with your investigations. Michel.

Thanks a lot sir for your answer. We have an artificial rainfall simulator, we need to measure both the rain rate and rain accumulation . It's for a geotechnical engineering project to assess the slope stability of a vegetated slope under rainfall.

As Michel indicated in his answer, there are many aspects to be considered. You have clarified that you are wanting to use an artificial rainfall simulator to measure both the rain rate and the rain accumulation. This narrows some of the considerations. First off, you should know before hand what size rainfall event you plan to simulate. That value should be determined with some knowledge of rainfall intensities normal for the area. Do you want to simulate a 1-h, 10-year return interval or some other intensity that is reasonable for your area. Secondly, the artificial rainfall simulator should be equipped with a nozzle with a known maximum flow rate. Actual flow rate should be somewhat adjustable based on water pressure (flow) to the nozzle. Rainfall intensity will likely be controlled through a solenoid device and electrical flow controller. Careful calibration off-site must be undertaken prior to actual test run until the desired rainfall rate can be achieved, but measurement will still be necessary throughout the run. We have calibrated using about 20-30 small collection vessels scattered throughout the area covered by the simulator spray pattern. We have used three (4") manual rain cans to measure rain accumulation throughout the run. Your question of "How much time the rain gauge should be exposed to the rainfall" might be simply answered as: as long as you have the rainfall simulator spraying rain onto the slope. You might want to refer to our publication where we give details of our rainfall simulation methods as well as runoff and soil moisture measurements and results of sediment transport.

Pyric-herbivory and Hydrological Responses in Tallgrass Prairie

West, Amanda L; Zou, Chris B; Stebler, Elaine; Fuhlendorf, Samuel D; Allred, Brady. Rangeland Ecology and Management; Vol. 69, Iss. 1,  (Jan 2016): 20-27.

" Your question of "How much time the rain gauge should be exposed to the rainfall" might be simply answered as: as long as you have the rainfall simulator spraying rain onto the slope."

--That simply clarifies the whole thing, Thanks you very much. This conversation was amazing.