Phoenix currently has a relatively diverse array of water resources available. The valley has access to 2.8×109m3 of water from the Salt and Verde rivers, groundwater, and the Colorado River. Sources comprise approximately 44% surface water (Salt and Verde Rivers), 39% groundwater, 12% Colorado River water, and 5% treated wastewater effluent (ADWR 1999). The Colorado River water is delivered by the Central Arizona Project canal (CAP canal), which moves water from the Colorado River eastward across the state some 450 km and uphill more than 700 m, to the cities of Phoenix and Tucson. The Phoenix valley has several hundreds of metres deep groundwater sub-basins that have been used to supplement surface water supply since the early 1900s. Declining water table levels have been occurring in some places since the 1940s, although early legislation (the 1948 Critical Area Groundwater Code) proved insufficient to slow the trend of increased well drilling. The state created the Groundwater Management Act (GMA) in 1980, a complex and ambitious regulatory plan to achieve safe-yield by 2025. As an Active Management Area (AMA), the Phoenix metropolitan area is subject to several management approaches (Jacobs and Holway, 2004). To date the GMA has had mixed success across the state. In the Phoenix AMA, the groundwater overdraft was reduced by approximately 40% between 1985 and 1995 (ADWR 1999), but 44 million m3 per year are still overdrafted today (Baker et al., 2004). A major state programme now allows housing developers to purchase canal water for recharge into aquifers at a distance quite removed from their groundwater pumping, creating new ponds, which are continuously refilled to make up for infiltration and evaporation losses.