how to paint this air field

Looks like they have used scatter materials for the grass and other vegetation. You can't get that effect with just paint. I would paint it brown, apart from the tarmac of course, and then use scatter materials as i would a normal base. Woodland scenics have a gret range of products that are very effective when mixed.

Thank you Bish. Helps me a lot.  heading to hobby lobby now to get some static grass, etc.

I am guessing this is 1/72nd scale. In which case, go for the finer scatters rather than flock, unless you can find some shorter flock.

Check with the Model Railroad area in your hobby shop.  The RR people are the experts in grass and ground cover.  And check out a book from your local library on model Railroading and read how they do it.

So I paint plastic base with earth color, then how do I glue the turf to the base? using white glue?

Ye, you can use white glue, watered down. Woodland does there own glue and a sprayer, but any white glue will work and you can paint it on. I generally don't like a solid cover of floiage, as many of my bases have the aircrfat on the grass, and that would leave bare areas. But for the base you have i would go for a more solid covering.

Also, i would paint the track slightly lighter that the surrounding terrain.