Use "Future" ( acrylic floor polish ) to simulate water in small depressions.
For static grass, apply a thin layer of white glue, sprinkle the static grass on top. Lightly blow the static grass from the side so that it stands up, like real grass. I use the static grass used for railway scenics, it is quite short and gives a nice effect.
Mud - I mix plaster of paris with very fine sand to a very fluid consistency. Pour it while holding the base at an angle and it would flow the same as real mud. The excess water will rise to the top, touch the water with absorbent paper to draw the water away from the mix. When dry paint with acrylics and seal with "Future" for wet mud, or keep flat and drybrush for dry mud. You can use a hairdryer to speed up the drying process and the "mud" will crack just like the real thing.
Persist and Succeed.