Thanks a lot for your interest in Instant Terra.
We plan to implement a feature that may fulfill your needs: the ability to paint a mask and use this mask to specify the areas where you want the rain to fall down for the erosion simulation.
That way if you want a single water spring, you put a single point on the mask and the terrain will be eroded using only the water coming from this point.
If you want several water sources or areas with rain and areas with less rain or no rain, you just paint the mask as you want.
This feature won’t be available in version 0.6 in November, it will come in a future version.
Edit 2018 : painted mask is available since early 2018