Wash ground cherries and place in unbaked pie shell. Mix brown sugar and 1 tablespoon flour and sprinkle over cherries. Sprinkle water over top. Mix together 3 tablespoons flour and 3 tablespoons sugar. Cut butter in until crumbly. Top cherry mixture with crumbs.
Bake in the preheated oven for 15 minutes, reduce temperature to 375 degrees F (190 degrees C) and continue to bake for 25 minutes.
