Supply & Demand

a. There are 51 ADWR recognized groundwater basins in Arizona. These basins were designated by the director soon after the Groundwater Management Act of 1980 and each has a relatively distinct aquifer or aquifer system. 

b. Seven groundwater basins are regulated as Active Management Areas (AMAs), three are regulated as Irrigation Non-expansion Areas (INAs), and the remaining 41 basins are largely unregulated. A small portion of the Little Colorado River Plateau basin is regulated as the Joseph City INA. 

a. Although similar assessments have been completed periodically, 2023 was the first time ADWR dedicated annual funding and staff to conduct assessments of all of Arizona's groundwater basins on a recurring cycle. The SDRs may be used to inform the Water Infrastructure Finance Authority on funding decisions in the future (see A.R.S. § 49-1304(A)(14)). The SDRs may also be used as a planning tool for water resource management by ADWR, policymakers, community members, and other interested stakeholders. 

b. The SDRs are structured as water budgets, focusing on total inflows and outflows at the basin scale. The SDRs estimate the volumes of water demands from all uses (categorized into sectors of Agricultural, Industrial, Municipal, and Other) and the volumes of water supplies (Surface Water, Groundwater, Effluent, Incidental Recharge, Transportation Water, and Moved Water) available to meet those demands. The reports also include projected demands and supplies under various influences of future scenarios. The SDRs are designed to be understandable to the general public. 

c. The SDRs are not groundwater flow models with finer geographic results. The development of regional groundwater flow models for each basin is an extensive technical process and is not feasible within the time constraints of this project. 

a. The Average Well Depth is different for every basin. The Average Well Depth is determined by averaging the well depths from all exempt and non-exempt wells, excluding outliers. 

b. The Average Well Depth was chosen to better show the impact of declining water levels on existing local infrastructure. While groundwater levels may fall below the Average Well Depth, deepening or drilling new wells could be financially costly. 

c. ADWR decided that the reports needed to show the water availability for the average well within each basin. Through a robust and lengthy discussion with Director Tom Buschatzke, and our Chief Hydrologist Ryan Mitchell we came to the conclusion that the average well depth was the best measure to ensure that the reports could be as fair and equitable to rural residents within each basin.

d. All data on wells within the groundwater basin are collected internally at the agency, based on well drilling and completion reports. The data is quality controlled to remove incorrect or incomplete well data. Additionally, Monitoring, exploration, injection, capped, and any other wells not currently used for groundwater use were removed from the data set. All remaining wells were exempt, non-exempt, and domestic and non-domestic stock wells. This pruning removed the extremes within the data set, ensuring that the average well depth represented the wells in use for pumping groundwater. More on these methods can be found on page 18-19 of the methods appendix. 

e. The groundwater available in storage was also calculated for a 1000-foot depth to better compare the amount from basin to basin. These estimates, along with a range of specific yield values, can be found in the Methods Appendix. Please note, not all of the basins are thought to be 1000 feet deep. 

a. The depth of each well was determined by the bottom of the well which can extend deeper than the well casing depending on the well. This data was retrieved from well drilling reports drillers are legally required to file with the Wells section of ADWR. 

a. No one well had more determined value than another in the average depth determination. A deep municipal well supporting thousands had the same weight in the average as a shallower exempt private well used to support a single family. The purpose was to show the impact groundwater level change had on the average well, not the average water user. Specific location was also never taken into consideration as the assessment is meant to represent the basin totals of water availability. 

a. The top of the aquifer was determined from water level measurements taken by the field services team. We then used a GIS tool to interpolate (estimate) the water level throughout the basin based on those measurements. More on this can be found on page 22 of the methods appendix. 

a. Water level elevation is the distance from the water table to sea level. This represents the same location as depth to water (DTW), but measures from sea level instead of the land surface. 

b. Measurements from wells in a basin were used to estimate the water level elevation throughout the entire basin. This process is known as interpolation and it results in a surface or raster, which can be thought of as an evenly spaced grid.  

c. No measurements from wells that were actively pumping or recently pumped were used, so the water level elevation surface represents static conditions. 

a. 1990 through the year prior to the report analysis. For example, for the 2023 SDRs the baseline period ends in 2022 so the baseline period is 1990-2022; for the 2024 SDRs, the baseline period ends in 2023 so the baseline period is 1990-2023, and so on.

a. Groundwater flow models require a lot of detailed spatial information, such as where recharge and withdrawal is occurring or the properties of the subsurface. Because of this, a model can estimate where water levels are higher or lower throughout the basin and how that might change over time. 

b. The Supply and Demand reports follow a water budget approach at the basin scale. This means that we look at the total amount of water supplied by the basin (groundwater recharge, surface water, etc.) and compare it to the total amount of water demanded by the basin (agriculture, municipal, industrial, etc.). These reports give the big picture view of what is going on in the basin.  

a. The total groundwater storage volume provided in the SDRs reflects the volume of groundwater reasonably accessible at the average depth of the wells in the basin, rather than at the 1,200 feet groundwater storage depth used in previous ADWR reports.

b. The water level falling below the basin’s average well depth suggests that wells will have begun to go dry. Using this approach to estimate groundwater storage effectively illustrates the impact of declining water levels on the current existing infrastructure of property owners, residents, and other water users in each basin.

a. If there are more outflows than inflows over the baseline period, the basin is considered to have a negative balance in the Supply and Demand reports. Overdraft is a condition in which the volume of groundwater coming out of the aquifer is greater than the volume of recharge going into the aquifer over an annual or long-term time frame. Overdraft is a quantitative metric, which is one aspect used to assess safe-yield. The purposes of the Supply and Demand reports are not to assess Safe-Yield, but to examine total inflows and outflows at the basin scale, structured as water budgets. The Management Plans and the Supply and Demand reports are not directly comparable. There are different constituents that comprise the data contained in the two reports. For example, data found in the Management Plans has a groundwater focus and has significant complexity regarding the legal and physical nature of the water; whereas the Supply and Demand Reports account for all inflows and outflows from the basin in question, focusing largely on the physical nature of the water. These differences in data sets make it difficult to compare the two as they are assessing different things.