How to prevent particle system from running through terrain?

For the project I am working on I am interested in having a particle system, which is supposed to be representative of water, fall from the sky and run along my terrain. The issue I am having is that once my particle system makes contact with my terrain, it lingers momentarily and then falls through the landscape. I am wondering if anyone has a suggestion as to how I can make my particle system interact with my landscape?

I discovered that with the new version of Unity 3D particle systems only interact with planes. When I imbedded a plane within my terrain, this prevented the particles from falling through the landscape, however the way in which the particle system interacted with the landscape was unrealistic (they more or less floated about the terrain). Therefore, I didn’t use the particle system at all but derived a script instead which instructed a multitude of small spheres to fall from the sky and change colour upon interacting with a specific object on my landscape.