Irrigated agriculture remains the dominant use of freshwater in the United States, although its share of use is declining. Irrigated cropland area has expanded over 40 percent since 1969, while water application rates have declined about 20 percent. The total quantity of irrigation water applied increased about 10 percent since 1969. Nationally, the average variable cost of supplying water for irrigation was about $50 per acre in 2003; however, that amount does not reflect the full value of water. This book presents leading-edge research from around the world on this topic.