WaterSim Background
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WaterSim  WaterSim Background
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There are six main categories of conditions that act as inputs to WaterSim: (1) conditions on the Colorado River; (2) conditions on the Salt and Verde Rivers; (3) climate scenarios affecting the Salt and Verde River basins; (4) population in Maricopa County; (5) agriculture retirement projections; and (6) water shortage policy scenarios.
Why does the Colorado River flow matter to central Arizona?
The Colorado River is a major source of water for the central Arizona region. The map shows the geographic extent of the river basin within the United States. Seven states and Mexico share water from this river based on the Colorado River Compact, put in place in 1922. The Central Arizona Project canal re-directs a portion of the river to Phoenix and Tucson via a canal that starts at Lake Havasu City.
The flow of the Colorado River fluctuates according to climatic conditions, seasons, and releases from dams. This flow is measured along several points on the river. WaterSim includes river flow as an input to the model and uses the flow record measured at Lee's Ferry, Arizona. The graph shows the variations in recorded river flow.
There are three main points where flow is lowest: 1934, 1954, and 1977 while the highest points occurred in 1920 and 1983. WaterSim allows users to choose a starting point for the historical river flow, so choosing a year such as 1930 will input a lower flow scenario than a year such as 1920 with greater river flow.
Why does the Phoenix metropolitan region receive water from the Salt and Verde rivers?
Salt River Project maintains the surface water rights to these rivers due to historical use. Several dams are present that allow reservoirs to act as storage facilities for the water supply. The water is distributed to the Phoenix metropolitan area via a network of canals, similar to the CAP.
What factors influence the flow of these river systems?
The flow of these rivers is impacted by precipitation, climatic conditions and snow-pack that enters the river basins as runoff. The graph below identifies the historical runoff in the Salt and Verde river basins. A historic high point occurred in the early 1990s while the lowest amount of runoff occurred in the mid-1950s and more recently, in 2002. Note the difference between the conditions in these river basins and those in the Colorado River system; the high and low periods do not necessarily occur at the same time. This suggests that climate conditions are different in each basin and there is potential for a phenomenon such as drought to only impact one of the surface water supplies at a time.
How do climate scenarios and General Circulation Models predict climate change?
General Circulation Models (GCMs) are used to predict the effects of different scenarios on Earth's atmosphere. Scenarios are storylines about the effects of future demographic, economic, political, and technological conditions on greenhouse gas concentrations. Results from these scenarios are fed into GCMs which then produce estimates of temperature and precipitation in large grids. Models produce different results because they are programmed with different equations to reproduce the dynamics of the atmosphere, use different size grids, and make different assumptions.
DCDC scientists adapted climate scenarios to predict possible temperature, precipitation, and runoff changes in the Salt and Verde River basins.
Table 1 below outlines characteristics of the 4 climate scenario families and 6 scenario groups created by IPCC. Table 2, and the accompanying graph, show the 20 scenario/GCM combinations modeled by DCDC researchers and resulting climate impacts on the Salt and Verde Rivers. Table 3 provides information on where the 6 GCMs used in WaterSim were developed.
Table 3. Sources of General Circulation Models used in WaterSim
Which model should I choose?
The point to remember is that there is no certainty as to which climate scenario will impact the region the most (or the least). In WaterSim, these models are included to allow users to consider many possibilities without recommending one over another.
Why does population matter to the water supply?
Maricopa County is one of the fastest growing counties in the United States. Currently, there are about 4 million people living in the region. All of these people rely on the same water supplies that are identified in WaterSim. The graph shows the historic and projected populations for the county. Although there is enough water to serve the existing population, future growth may be confronted with the need to change the amount of water they use to accommodate the larger population.
Historically, agriculture has been the dominant user of the regional water supply. As population increases and land uses change to accommodate this growth, agricultural fields will be retired. An acre of irrigated agriculture uses more water than an acre of residential users. As a result, this retirement is expected to provide additional water for people. The graph shows how water use is the region is shifting from agricultural to municipal (residential) uses.
Where is the agriculture now and where will it be in the future?
These maps show the location of different land uses in Maricopa County. The areas in green represent agriculture. The land uses are intended to change based on the General Plans of cities and towns. Once these future land uses are built, agriculture may only exist on Indian community properties.