In Central Otago, which has less than 500 millimetres of rainfall annually, droughts occur every year and plant growth stops unless irrigation water is available. Irrigation is economically worthwhile on flat land with higher-value orchard or horticultural crops, or for lucerne hay or forage crop production. Large areas of pasture on Semi-arid soils are irrigated by border dyke and flooding methods. These are gradually being replaced by more efficient spray and 'K-line' methods.