Dear Weng,

It depends on which ecosystem you are talking about.

1st example: if it is a forest ecosystem with rather thick moss layer you can easily measure it with any soil moisture sensor (depending on sensor-type either put it horizontally inside the moss layer (e.g. ECH2O) or just put in on the top (e.g. three-pod T-Delta)).

2nd example: it is open area with very thin moss layer (or sporadic moss spots) you can use leaf wetness sensors. There are different designs of such sensors available in the market. E.g. plate-like sensor to cover the leaf/ moss in your case (or to be placed at the same level to detect any water presence on it) [see http://pinova-meteo.com/en_UK/pinova-meteo-products-and-services/pinova-meteo-agriculture-weather-station/sensors/leaf-wetnes-sensor-plws; https://www.campbellsci.com/lws; https://www.campbellsci.com/237-l ]. There are also clip-like leaf moisture sensors, which can allow you to measure real leaf wetness (as a result of both climatic and physiological processes) [I used to work with them; see attached photos].

3rd example: if you use humidity sensor on moss surface (as you were planning to do) you will likely measure the mixture of moss wetness and air humidity. If thickness of moss is enough you can put your humidity sensor into the moss and than probably you will measure the humidity in the moss layer (%). However before do that you have to check if you humidity sensor is precise enough to measure variation inside the moss layer (just look at the results over a couple of days) and of course you have to calibrate your humidity sensor for 100% of humidity (at least) in advance to carefully interpret results.

Good luck with your study and hope it helps somehow.

Cheers,

Serge.