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area can be used because well- watered crop ET is deter- mined mainly by energy available in the area of interest. The energy available to evaporate water is determined by the calculated reference crop ET using local climate data. Its accuracy depends on whether or not daily climate data measured by automated weather stations have been screened for data quality and consistency. Experienced engineers and scientists generally transfer existing crop coefficients by adjustment of the growing period to an area of interest either directly with reference crop ET, or as input to soil- water -crop models. The models can enable adjust- ment for level of irrigation management and soil salinity. When coupled with locally determined reference crop ET, transferred crop coefficients derived with weighing lysim- eters provide crop ET estimates with sufficient accuracy for irrigation management. Currently, an ASCE task force is refining the procedures for transferring crop coefficients. Incorporating crop coefficients into soil - water -crop models that separately estimate the components of ET, or transpira- tion and evaporation, does not always assure greater ac- curacy of seasonal ET estimates. Some of the models that I have seen overestimate the evaporation component during frequent light rains. Estimates of daily crop ET using transferred coefficients and summed for seasonal or annual totals of water con- sumption over many fields may not always have the desired accuracy because ET is also affected by the level of irriga- tion management. Under such conditions, validation by water balance for the study area is desired, but not always achievable because of complex inflow- outflow conditions and sources and sinks. Locally or Regionally Determined Crop Coefficients Having locally, or regionally determined crop coefficients will not necessarily assure greater accuracy of estimated crop ET, but may create greater confidence in the estimates of seasonal or annual water consumption. Improved ac- curacy will depend on the system and methodology used. Determining crop coefficients using precision weighing ly- simeters usually requires several years to establish the site, then one or two years of measurements for each major crop being considered. Crop coefficients can also be determined on existing fields using a Bowen Ratio (BR) energy balance approach. Most previous BR studies have provided limited data over a few days at a time. As mentioned, crop coef- ficients can also be developed using the SEBAL technology for fields of currently grown crops that represent current irrigation management and soil salinity conditions. Local Calibration of Reference Crop ET Estimates In Colorado, transfer of water rights often involves esti- mating crop ET going back many years before automated to weather stations were used and before calculations were being made with the P -M equation. In such cases, another empirical equation based mainly on temperature and ex- traterrestrial solar radiation data may need to be calibrated during recent years using modern calculation methods to provide improved estimates for earlier years. Likewise, managers operating water systems according to legal regu- lations are often reluctant to change procedures that have been used, but calibration of methods used can improve the accuracies of ET estimates. PLANNING A NEW COLORADO SYSTEM FOR ESTIMATING CROP ET Before installing and implementing a new system for refin- ing ET estimates for Colorado, research leaders should consider organizing a workshop involving scientists and engineers who have had many years of experience install- ing and operating lysimeter - systems. They should also consider individuals having experience in using remote sensing and surface energy balance modeling technolo- gies. Such a workshop would assure the selection of the most current and practical methodology for developing Colorado crop coefficients as well as provide guidelines as to land area, timetable and expertise required to achieve the desired results. The proceedings of a special ASCE session on lysimeters provide many additional guidelines (Allen et al., 1991). Equally, or more important, for improved estimates of crop ET in Colorado, climate data collected by various automated weather stations should be subjected to rigorous quality control standards. Important considerations are the site conditions surrounding the automated weather station, which should approximate reference crop conditions. Cur- rently, most Colorado engineers must screen climate data before using them for estimating crop ET. When adjust- ments are needed in some of data, different procedures may be used to make these adjustments. It would be better to have a central unit review the data and make necessary ad- justments before the data are archived. A series of papers on operational experiences with automated weather stations was presented at a recent ASCE Water Resources Engi- neering conference (Elliott, 1995). The automated climate data network operated by the Northern Colorado Conser- vancy District has a full -time person dedicated to maintain- ing the data quality from about 20 automated weather sta- tions along the South Platte River (NCWCD, 1996, 2001). This activity involves calibrating sensors annually, periodi- cally visually checking each station, and checking daily reports for abnormalities by comparison with surrounding stations. Climate data collected at the NCWCD stations back to 1996 are available via the district's web site.