Consumptive uses and losses
Overview
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 typically used in reporting by Reclamation, the basin states, and other analysts is consumptive uses (CU) and losses. The combination of these two terms, often 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 exported for use in another basin (e.g., the South Platte River in Colorado), then the entire diverted amount is treated as consumptive use with respect to the Colorado River, since any return flows would take place in that other river system.
Losses refers to depletions of water apart from beneficial uses. In the Colorado River Basin, the largest category of "losses" is reservoir evaporation; in Reclamation's current reporting of CUL (see below), reservoir evaporation is the only type of "losses" that are included. Some sources also include in CUL the "transit losses" along the mainstem below Lake Mead to evaporation from the river channel and transpiration from phreatophytic vegetation adjacent to the river channel. Seepage into, and storage within, reservoir banks, river banks, and river beds, is not treated as a loss, since the water is likely to return to the channel later in time or further downstream.
Reclamation reporting of consumptive uses and losses (CUL) data
Consumptive uses and losses (CUL) reports and data are prepared by Reclamation pursuant to the Colorado River Basin Project Act of 1968 (Public Law 90-537). The reports and data present estimates of anthropogenic consumptive uses and losses for the Colorado River System for each calendar year from 1971 and the data are categorized by type of use, reporting area, and by individual state.
In 2024 and 2025 Reclamation made significant changes in how Lower Basin CUL data are calculated and reported, and has recalculated historical CUL data for the Lower Basin. Over the same period, Reclamation implemented an improved method for estimating agricultural consumptive use in the Upper Basin, and likewise recalculated historical CUL data for the Upper Basin.
Annual data for the Upper and Lower Colorado River Systems are calculated and reported by Reclamation’s respective regional offices, and joint reports covering five-year periods are published with system-wide summaries. Methodologies between the two basins are generally consistent, with any differences highlighted in either the Upper Colorado Basin Region’s Methodology Manual, or the Lower Colorado Basin Region’s Methodology Manual.
Important considerations for interpreting Reclamation’s CUL Data:
- Only anthropogenic uses and losses are reported. Natural evapotranspiration, (e.g., from riparian areas) and losses (e.g., natural lakes/river evaporation) are not reported in Reclamation's CUL data.
- Lower Basin CUL data include mainstream and tributary uses (see Figure 1). Mainstream data are derived from Reclamation’s annual Colorado River Water Accounting Report and do not use the same methods as the tributary areas.
- Use categories (agriculture, municipal and industrial, thermal electric power, etc.) are assigned only to water used within the boundary of the hydrologic Colorado River Basin. Water that is exported out of the basin, e.g., to Colorado's Front Range, southern California urban areas, and the Imperial and Coachella valleys, is reported as an export and not assigned a more specific use category (Figure 2).
Data and tools
Reclamation CUL reports and data
Reclamation's CUL data for the Upper and Lower basins are now available as Excel files containing monthly data aggregated at the HUC8 watershed level within a given state, and via visualization tools that display annual data aggregated at the state level, as well as the traditional 5-year reports.
Also, since 1964, Lower Basin diversions and consumptive uses, and deliveries to Mexico, have been summarized in the annual Water Accounting Reports, but these reports do not contain estimates of Lower Basin reservoir evaporation or other losses.
Additional resources
Colorado River Basin Water Supply and Demand Study - Technical Report C
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.
Water Resource Modeling of the Colorado River: Present and Future Strategies
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.
Comparison of U.S. Geological Survey and Bureau of Reclamation Water-Use Reporting in the Colorado River Basin
This 2019 report compares the two agencies’ water-use information programs in the Colorado River Basin, describing the legal drivers for each program, clarifying confusing terminology, comparing the methods used, and contrasting the information reported by each agency.