A prolonged period with no rain, particularly during the planting and growing season in agricultural areas. Drought can also affect urban areas particularly those dependent in reservoirs for their water. Decreased water levels due to insufficient rain can lead to the restriction of water use to certain amounts and types of uses. Droughts are difficult to predict or forecast both as to when they may begin and how long they will last. Increased pumping of groundwater and surface irrigation occurring in drought periods can result in severe land subsidence problems. Almost all areas of the country are subject to direct impact of drought as it affects their water supply systems. The most vulnerable regions are the arid southwest and the semiarid part of the mid-continent, particularly the Great Plains. During the last 100 years, major sustained droughts have occurred in these regions roughly once every 20 years. (FEMA, IEMS HICA MYDP (CPG-1-34), 1985, p. A-1)