Improved Estimation of Reservoir Evaporation

Project ID: 7662
Principal Investigator: Mark Spears
Research Topic: Water Operation Models and Decision Support Systems
Funded Fiscal Years: 2013, 2014 and 2015
Keywords: reservoir, evaporation, consumptive use, demands, operations

Research Question

Effective and accurate management of surface water storage and the development of new storage and water accounting policies and procedures are impeded by lack of accurate estimates of historical and current evaporation. In addition, accurate evaporation estimates are essential for long-term water resources operations and planning under a changing climate. Historical estimation of evaporation using pan evaporation data is known to have significant uncertainty both in magnitude and timing. It has been found that evaporation pans can over estimate lake or reservoir evaporation by 25 to 100% when compared to water or energy balance estimates of evaporation. Heat storage in open water bodies can alter both the rate and timing of evaporation depending on the volume, geometry, clarity, and the surrounding environment of the water body. An important weather data requirement for accurately estimating evaporation is the collection of weather data over the water surface. Air temperature is lower, relative humidity is higher, and wind speed is increased when weather data is collected over water opposed to land. Having long-term weather data readily available that is representative of water surface conditions is critical for accurately estimating evaporation.

Heat storage in many Reclamation reservoirs is believed to alter the rate and timing of evaporation, and a proper accounting of heat storage is required to accurately estimate seasonal and annual evaporation rates. We seek to answer specific questions regarding the reliability and validitiy of various evaporation rates that are based on pan evaporation, land based weather, and remote sensing data. These questions will be evaluated by developing a long-term reservoir meterological network that consists of weather station bouys, measuring all required weather variables and reservoir themal properties to accurately estimate evaporation with near real time data transmission.

Need and Benefit

Quantifying evaporation from lakes and reservoirs is required for operations and modeling, basin water balance estimates, irrigation water management, and review and litigation of water right applications and disputes; all of which are receiving more and more high priority attention. Evaporation under future climates is largely unknown, however, it is essential for long-term water resources planning in the Western U.S. Providing a scientifically sound and widely accepted basis that can be consistently applied across all 17 Western states for estimating historical and future evaporation will be essential for addressing this directive and the overall mission of Reclamation. Developing and testing bulk transfer-aerodynamic, energy balance, CRLE, and Landsat remote sensing approaches at buoy weather stations will allow for broad applications of CRLE and Landsat remote sensing approaches over all Reclamation regions. In addition, showing that buoy weather stations can be easily installed, operated, and maintained with near real time data transfer that can be used to accurately estimate evaporation, will spur broad interest and funding opportunities.

Current methods for estimating evaporation within Reclamation are typically based on evaporation pan or temperature based methods, are not consistent among Reclamation regions, nor can they be considered to be consistent with what is generally considered to be state-of-the-art. Pan and temperature based methods are known to have significant uncertainty and error in estimating magnitude and timing of evaporation. Providing the ability to estimate evaporation for operational purposes requires a long-term monitoring network consisting of weather stations that are operational beyond the typical 2-3 year study, where over water and land based weather variables are recorded, and evaporation estimates are based on methods that have been well tested and are accepted by the scientific community. Furthermore, estimating accurate monthly and annual evaporation climatologies, and long-term impacts of climate on evaporation requires a long-term monitoring network of over water weather stations across multiple climatic regions. Currently, no such reservoir meteorological networks focused on reservoir evaporation exist in the U.S.

By investing in a long-term evaporation monitoring network and research program that leverages a wealth of design information, software development, and remote sensing and modeling products, costs are significantly reduced. For example, research partner DRI has made significant investments developing buoy weather station designs and software programming. We will leverage buoy designs, remote sensing algorithms, and software developments, and NICE database and website developments to construct buoy weather stations, provide remotely sensed evaporation estimates using Landsat, house and disseminate buoy weather station data in near real time, and provide users with robust capabilities for data analysis and downloading options.

If robust evaporation estimates are not available using state of the art and widely accepted methods, Reclamation is effectively forced to use pan evaporation data and temperature based methods that are known to produce significant uncertainty and error. This puts Reclamation in a situation where water right applications and protests, policies, litigation, and the accuracy of operations and modeling can be questioned due known errors in evaporation. Having accurate data and defensible approaches to present to customers, and local, state, federal agencies, is critical for Reclamation to avoid conflicts and questions centered on evaporation.

Contributing Partners


Research Products

Bureau of Reclamation Review

The following documents were reviewed by experts in fields relating to this project's study and findings. The results were determined to be achieved using valid means.

Improved Estimation of Reservoir Evaporation (final, PDF, 2.4MB)
By Mark Spears
Publication completed on September 30, 2015

In summary, Science and Technology grant funding that made this project possible has been extremely useful for obtaining benchmark datasets to develop and compare and contrast multiple methods for operationally estimating evaporation. DRI, Reclamation's Technical Service Center, and Cal DWR collaboration has been very successful, and we plan to continue this collaboration.

Last Updated: June 29, 2015