Combine first four ingredients. Cut in butter until mixture resembles coarse crumbs. Press half the mixture into greased 8 inch square pan. Sprinkle rhubarb over crust. Set aside In a small saucepan, combine sugar and cornstarch. Gradually whisk in water until smooth. Bring to a boil. Cook and stir 2 minutes until thickened. Pour over rhubarb. Sprinkle with remaining crumb mixture. Bake 1 hour until bubbly and lightly browned. Serve warm with ice cream.