(Article V(A) of the Yellowstone River Compact of 1951 incorporates ordinary doctrine of appropriation without significant qualification; as doctrine applied in Montana and Wyoming appropriators can improve their irrigation systems to detriment of downstream appropriators; Montana's challenge to Wyoming's more efficient sprinkler irrigation process (to which pre-1950 users may switch) not challengeable even though increased consumption in Wyoming deprived Montana of typical water allotment; Compact did not guarantee Montana set amount of water).