Well... what part are you having trouble with? I can see that the road is way to "clean" and needs some snow intermixed. I would put the road texture on the bottom layer and then add the snow on top setting a layer mask and painting out bits an pieces where the road is using a grunge brush. I think you are good on the colors as snow should generally be displayed as more of a blueish white rather than pure white in most cases and it looks like you have that here. You could probably signify some field "rows" with a bit of alternate dodge/burn on an overlay layer, though the dodge won't show up that much due to the underlying white of the base texture for the snow.