Ratio of reference evapotranspiration calculated by Penman's equation(ETr) to reference evapotranspiration measured by Pan evapotranspiration(ETr Pan) at selected locations in New Mexico. * Penman's calculations are referneced to grass. Pan evaporation data exists for a larger number of site in New Mexico
| Month | Artesia Average(1996-1997) |
Clovis Average(1996-1997) |
Farmington Average(1996-1997) |
Las Cruces Average(1996-1997) |
Los Lunas 1997 |
| 1 | 0.79 | 1.09 | 1.13 | ||
| 2 | 0.73 | 1.31 | 0.85 | 1.16 | |
| 3 | 0.87 | 1.03 | 0.77 | 0.93 | |
| 4 | 0.93 | 1.14 | 1.00 | 0.84 | 1.05 |
| 5 | 0.88 | 1.14 | 0.80 | 0.84 | 0.93 |
| 6 | 0.78 | 0.96 | 0.74 | 0.82 | 0.93 |
| 7 | 0.81 | 0.92 | 0.78 | 0.71 | 1.01 |
| 8 | 0.76 | 0.95 | 0.79 | 0.81 | 0.99 |
| 9 | 0.77 | 0.97 | 0.90 | 0.85 | 1.01 |
| 10 | 0.92 | 1.00 | 1.13 | 0.97 | 1.10 |
| 11 | 1.08 | 1.00 | 1.06 | 1.13 | |
| 12 | 1.12 | 1.04 | 1.03 |
* missing data because the water was frozen in the pan part of the month.
Data from Sammis, T. W., E. G. Hanson, C. E. Barnes, H. D. Fuehring, E J. Gregory, R. F. Hooks, T. A. Howell, and M. D. Finkner. 1979. Consumptive use and Yields of crops in New Mexico. New Mexico Water Resources Research Institute. report 115 pp1-108
To convert Pan to Lake evaporation see:
World Meteorological Organization
Technical Note 126 "Comparison between Pan and Lake evaporation" by C E Hounam
WMO No 354 (1973).
Seasonal variations in the lake to Class A pan coefficients are usually enough to preclude the use of a constant value. The annual coefficients depend on climate and lake depth and ranges from 0.6-0.8.The average annual value of the Class A pan coefficient is 0.7. Consequently Lake Evaporation= Penman's Calculated reference Et / (Ratio Penman/Pan)*0.7. On an average during summer months Penman's refernce evapotranspiration *0.86 = small lake evaporation.
Other estimates of Class a pan coefficients for calculating lake evaporation range from 0.58- 0.76 . Also see the Utah water atlas
To calculate lake evaporation for other areas in New Mexico, calculate an approximation of Penman's Etr using Samani's equation calculated using the Pet wizard and average monthly temperature data for the site from the western regional climate center