PRECIPITATION: Any and all forms of water, liquid or solid, that falls from clouds and reaches the ground. This includes drizzle, freezing drizzle, freezing rain, hail, ice crystals, ice pellets, rain, snow, snow pellets, and snow grains. The amount of fall is usually expressed in inches of liquid water depth of the substance that has fallen at a given point over a specified time period.

Source: http://www.weather.com/glossary/p.html#precip

A combination of Harry and Hikmat would produce the best result:
"very low, unpredictable and sporadic precipitation" (or "precipitation that is very low, unpredictable and sporadic") Although--if you can take "tak merata" as applicable to geography, not time, you might use "uneven" instead of "sporadic."

NOT LOOKING FOR KUDOS, my colleagues did the heavy lifting!

This answer is also not meant for the sake of KudoZ point; but meant more to give a different viewpoint.

Actually, I'd like to support available answers from Harry, Hikmat, and Julia. But, I have my own view on these:

tidak menentu: this refers to temporal sense as commented by Julia. So, this must be "sporadic" or "irregular".

tidak merata: this surely refers to spatial/geographical sense. So, this is "unbalanced" or "imbalance" or "uneven".

So, ..The precipitation is very low, sporadic, and imbalance..