Consumptive uses and losses
Knowing how much water is being used–and for what purposes–is central to water resource management. Accurately quantifying use can be challenging at the scale of the individual water user, especially in agriculture. In gathering water use data at the basin scale, these challenges are compounded by the varying uncertainties in quantification across uses, inconsistent reporting, and variable terminology.
The terminology used in reporting by Reclamation and the basin states is consumptive uses (CU) and losses. The combination of these two terms, sometimes abbreviated as CUL, is the counterpart to supply in the water-balance equation.
Consumptive uses refers to water that is diverted from a stream or river for any beneficial use (agriculture, municipal use, power-plant cooling, etc.), minus any return flows that would then be available to a downstream user on that same stream or river. When water is diverted from the Colorado River and used in another basin (e.g., Colorado’s Front Range), then the entire diverted amount is treated as consumptive use with respect to the Colorado River, since any return flows would take place in a different river system.
Losses refers to depletions of water apart from beneficial uses; in the Colorado River Basin losses come primarily from (1) reservoir evaporation and (2) transit losses. The latter is the loss of water to evaporation and phreatophytic vegetation within and adjacent to the river channel, the mainstem below Lake Mead. Seepage into, and storage within, the river banks is not treated as a loss, since the water is likely to return to the channel later in time or further downstream.
Total basinwide consumptive uses and losses, as tabulated at Imperial Dam (including consumptive use by Mexico) rose to around 15 million acre-feet (maf) per year in 2010-2016 (Figure 1) but dropped to 14.5 maf over the period 2016-2020. That average of 14.5 maf/yr of basinwide CUL can be partitioned roughly as follows, following Reclamation's schema in Figure 1 but dividing Mexico's use into agriculture and municipal components (estimated):
- 9.0 maf for agricultural water use (U.S. 8.0 maf and Mexico 1.0 maf )
- 3.1 maf for municipal water use (i.e., municipal and industrial; M&I) (U.S. 2.6 maf and Mexico 0.5 maf)
- 0.3 maf for energy production, mining, and fish, wildlife, and recreation
- 1.5 maf lost to reservoir evaporation
- 0.7 maf lost to transit losses and other losses
Thus, all beneficial uses (the first three items above) accounted for about 82% of consumptive uses and losses, with reservoir evaporation and other losses accounting for the remaining 18%. Agriculture accounted for about 74% of all beneficial uses and 62% of all consumptive uses and losses. Municipal & industrial use accounted for about 25% of all beneficial uses and 21% of all consumptive uses and losses.
It is more familiar in reporting to see consumptive uses and losses broken down by basin (within the U.S.), with Mexico treated separately. On average, in recent years (2016-2020) that 14.5 maf/yr of CUL breaks down as follows:
- 4.6 maf in the Upper Basin
- 8.4 maf in the Lower Basin
- 1.5 maf in Mexico
Note that the 8.4 maf total for the Lower Basin includes about 1.5 maf of reservoir evaporation (mainly Lake Mead) and transit losses that are not currently charged against the Lower Basin states’ Compact allocation of 7.5 maf. Not counting those losses, there was 6.9 maf of consumptive use, on average, in the Lower Basin from 2016-2020. A variant of this accounting would attribute roughly 20% of the Lower Basin’s reservoir evaporation and transit losses to Mexico, since Mexico uses about 20% of the water delivered from Lake Mead. This logic would attribute 8.1 maf of CUL to the Lower Basin, and 1.8 maf to Mexico.
Data and tools
Reclamation reporting of consumptive uses and losses (CUL) data
Consumptive uses and losses (CUL) reports are prepared by Reclamation pursuant to the Colorado River Basin Project Act of 1968 (Public Law 90 537) and provide 5-year reports covering major uses within each basin (Upper and Lower) of the Colorado River. The reports for the Upper and Lower basins are not prepared in parallel and contain different metrics for consumptive uses and losses.
Prior to 2006, 5-year CUL reports that covered both the Upper Basin and Lower Basin (including deliveries to Mexico) were made available. Since 2006, the 5-year CUL reports only cover the Upper Basin. Lower Basin consumptive uses and Mexico deliveries are summarized in the annual Water Accounting Reports (issued since 1964), but these reports do not contain estimates of Lower Basin reservoir evaporation or other losses.
Technical Report C (Water Demand Assessment) of the 2012 Basin Study, in Section 2.4 summarizes historical trends (1971-2010) in basin water use, including municipal use.
The Center for Colorado River Studies at Utah State U. published this 2019 overview of water resource modeling in the Colorado River. Page 33 lists the 2000-2017 averages for different components of basinwide CUL.