@Framemaker- I live in Tempe in an area that also has flood irrigation. The reason that we have flood irrigation is because the area was once an orange grove. Agriculture in and around Phoenix was very prevalent until the city saw a development boom in the latter half of the 20th century. The agricultural areas of Phoenix were all flood irrigated, and residents were allowed to choose if they wanted flood irrigation systems or conventional irrigation once the areas had been developed.
While it may seem crazy for an area that is last in line for water from the Colorado River to use so much water, The state of Arizona built reservoirs to hold the excess water in their water rights rather than send it down river to Mexico. This allowed the Valley of the Sun to have enough water to become the once agricultural center of Arizona and the current golf capital of the country.