The Great Lakes Water Balance
Data Availability and Annotated Bibliography
By Brian P. Neff and Jason R. Killian

Introduction | Hydrological Information | Forecasts and Summary

Evaporation

Water loss due to evaporation is a large component of the Great Lakes water balance. Evaporation rates are difficult to estimate accurately and reliable estimation relies heavily on extensive data availability. Furthermore, no single method of determining evaporation is considered to be the best for all situations. In fact, at least 11 different equations have been proposed for determining evaporation from lakes (Winter and others 1995). The exact types of data required to estimate evaporation vary considerably and depend on the method used. The method most commonly used to calculate evaporation of the Great Lakes utilizes air and surface water temperature, wind speed and humidity data (Croley 1989, Lavender and others 1998, p.49, Quinn 1979,). The locations of weather stations that currently record air temperature, and wind speed and direction are shown in figure 2. Surface water temperatures have historically been calculated for use in the evaporation models, but the current GLERL model utilizes satellite based water temperatures. Lavender and others (1998, p.49) contains a summary of how surface water temperatures were calculated for use in evaporation models.

It should be noted that a considerable amount of buoy and some satellite data are available, but are not currently being used to calculate evaporation. NOAA and EC operate an integrated network of buoys that monitor numerous parameters such as air and water temperature and wind speed and direction (figure 2). In addition to the available buoy data, NOAA, NASA and the European Space Agency (ESA) operate satellites used to calculate surface water temperatures. The degree to which this unused data is useful is a debatable point.

Canadian Data

In Canada, weather data are collected by EC and archived by the National Climate Data Archive (NCDA). These data generally include hourly readings of temperature, humidity, wind speed and direction, atmospheric pressure, cloud types, amounts and heights, and occurrence of precipitation. There are approximately 300-400 active hourly reporting locations, most of which are located at airports. The archive maintained by NCDA includes over 800 datasets of current and discontinued weather monitoring stations. In addition to the hourly datasets, a network of almost 10,000 volunteer climate observers record daily temperature and precipitation values throughout Canada. Data made available through the volunteer network includes maximum and minimum daily temperatures, and rainfall and snowfall amounts. These data may be retrieved with the assistance of the Ontario Climate Center on a "cost recovery" basis. 

In addition to the land-based weather monitoring information, EC operates numerous buoys throughout the Great Lakes that record air and water temperature, wind speed and direction and wave frequency and height. Canada's buoys range in size from 1.5-meter to 12-meter discus-style platforms. Buoys are placed in the Great Lakes in April or May and are taken out in November or December. When weather permits, the large 12-meter platforms are left in place longer. Datasets of Canadian buoy measurements are kept by the Marine Environmental Data Service (MEDS) and can be obtained directly from them free of charge. MEDS assures the quality of the wave height and frequency data, but not the air and wind temperature or the wind speed and direction data (termed "environmental data") used in the real-time measurements. Environment Canada does assure the quality of the water and air temperature and wind speed and direction data and maintains a database of these measurements. The degree of quality assurance and backlog of each dataset varies. EC's datasets can be obtained from EC on a cost recovery basis. It should be noted that MEDS and EC maintain parallel "environmental data" datasets, but the MEDS environmental datasets are not quality assured. This system is currently under review and may change in the future. MEDS also maintains a dataset of recent NOAA buoy measurements, though this dataset is not considered to be complete.

United States and European Data

Weather data commonly used to calculate evaporation in the United States are archived by the National Climate Data Center, a division of NOAA. NCDC maintains over 500 datasets of atmospheric and precipitation data. These datasets have all been subjected to some quality assurance measures. In the interest of producing data in a timely manner, some of the most recent data are released but considered to be "preliminary" until it is reviewed and becomes part of the permanent datasets. Assistance with accessing these data is provided by the NCDC office in Asheville, NC.

The National Data Buoy Center (NDBC) maintains a network of buoys and C-MAN stations (Coastal-Marine Automated Network) in the Great Lakes. The buoys are 3 meter discus style and are installed in April and removed for the winter season in November or December. The C-MAN stations operate 12 months a year. This buoy network is integrated with Canadian Great Lakes buoys and monitors the same parameters as the Canadian buoys. NDBC is responsible for the quality control and archival of all buoy data and data inquires should be directed to their offices at the Stennis Space Center in Mississippi. NDBC is a part of the National Weather Service, which is a subsidiary of NOAA.

Satellite based measurements of Great Lakes surface water temperatures are abundant. NOAA's polar orbiting environmental satellites (POES) are equipped with the Advanced Very High Resolution Radiometer (AVHRR), which records data that is used to determine surface water temperatures of the Great Lakes. AVHRR data are available from 1978 to present. Numerous datasets of sea surface temperature (SST) based on AVHRR measurements exist. These datasets represent different interpretations of the same AVHRR measurements. Stated margins of error for these datasets range from ± 0.5° C to ± 1.0° C. Additional information on the nature of the differences between these datasets, and access to the data, is available from the Physical Oceanography Distributed Active Archive Center (PO-DAAC).

NASA operates the satellite-based Moderate Resolution Imaging Spectroradiometer (MODIS). Like the AVHRR, MODIS records information used to calculate surface water temperatures of the Great Lakes. MODIS data are available from 2000 to present. Certain design modifications were made to this apparatus, which will theoretically improve the accuracy of surface water temperature measurements when compared to AVHRR-based systems. However, this remains unproven. An account of the differences between the AVHRR and MODIS SST sensors is outlined in Esaias and others (1998). A description of the accuracy of the MODIS data is given in Brown and Minnett (1999) and Kearns and others (2000). MODIS based SST data are available from Goddard Space Flight Center.

The European Space Agency (ESA) operates the European Remote Sensing (ERS) satellite, which houses the Along Track Scanning Radiometer (ATSR). This radiometer measures sea surface temperatures to an accuracy of ± 0.3 °C. Data are available from 1991 to present. Further information on the ATSR and datasets are available from the European Space Agency at their ERS Help Desk.

References

• Bibliographies of selected references

• Full reference list

• Sources of Hydrological data and information

This report is prepared by USGS in cooperation with the Great Lakes Commission.

The Great Lakes Commission is a  multi-agency, multi-disciplinary, multi-faceted team approach to addressing the science and policy of a Great Lakes water management regime will produce such application-oriented products as a water use web site, updated annual water use inventories, ecological evaluations of the system, and an integrated binational role for future Great Lakes-St. Lawrence endeavors.