Table 1. Evaluation of the Coefficient Kt for Various Locations in the United Stated
for months of mean temperature of 10 C or above (From Z. A. Samani and M.Pessarakli, 1986:
Estimating Potential Crop Evapotranspiration with Minimum Data in Arizona, Transactions of
the ASAE Vol. 29, No. 2, pp. 522-524)
The default value in the program of 0.185-0.220 is appropriate for Las Cruces. The
range depends on the time of period. The 0.185 applies for the March through Oct. The
0.220 applies for Nov. to March 1. For the year use a Kt=0.188 for Albuquerque and 0.220
for Farmington and Clovis to calibrate to Penman's equation for grass calculated on this
web server. The Kt factor for Costilla at 6500 ft during April to Sept. is
0.23. A plot of the Kt values vs. elevation shows as the elevation of the site increases,
the Kt factor must increase. 
The table below from Samani and Pessaraki is
referenced to Alfalfa and appears to be low for New Mexico. The Albuquerque value needs to
be increased by 5% and then the number is appropriate for reference Et to grass not
alfalfa. Dr. Samani , personal communication, suggested that Kt= 0.00185 TD^2 -0.0433 TD
+0.4024 where TD is degree C and is the temperature difference between max and min
temperature averaged for a month . This equation also appears to be low by (10 percent)
for estimating the appropriate Kt referenced to grass in New Mexico but does explain the
differences in Kt between Las Cruces and Farmington and Albuquerque.
| Station |
Kt |
Station |
Kt |
| Montgomery, AL |
0.149 |
North Omaha, Ne |
0.161 |
| Fairbanks, AK |
0.130 |
Las Vegas, NV |
0.182 |
| Phoenix, AZ |
0.170 |
Concord, NH |
0.119 |
| Little Rock, AR |
0.148 |
Newark, NJ |
0.149 |
| Los Angeles, CA |
0.212 |
Albuquerque, NM |
0.178 |
| Sacramento, CA |
0.168 |
Albany, NY |
0.127 |
| Denver, CO |
0.158 |
New York (C. Park) |
0.148 |
| Hartford, CT |
0.125 |
Raleigh/Durham, NC |
0.141 |
| Wilmington, DE |
0.146 |
Bismarck, ND |
0.150 |
| Wa./Sterling, DC |
0.137 |
Columbus, OH |
0.133 |
| Miami, FL |
0.177 |
Tulsa, OK |
0.154 |
| Atlanta, GA |
0.157 |
Portland, OR |
0.144 |
| Honolulu, HI |
0.200 |
Koror Island, PI |
0.184 |
| Boise, ID |
0.169 |
Philadelphia, PA |
0.141 |
| Chicago, IL |
0.158 |
San Juan, PR |
0.199 |
| Indianapolis, IN |
0.141 |
Providence, RI |
0.144 |
| Burlington, IA |
0.158 |
Charleston, SC |
0.148 |
| Wichita, KS |
0.169 |
Sioux Falls, SD |
0.156 |
| Lexington, KT |
0.145 |
Memphis, TN |
0.156 |
| New Orleans, LA |
0.161 |
Dallas, TX |
0.168 |
| Portland, ME |
0.130 |
Austin, TX |
0.156 |
| Baltimore, MD |
0.142 |
Salt Lake City, UT |
0.168 |
| Boston, MA |
0.157 |
Burlington, VT |
0.131 |
| Detroit, MI |
0.149 |
Richmond, VA |
0.160 |
| Minn./St. Paul, MN |
0.152 |
Seattle/Tacoma, WA |
0.160 |
| Jackson, MS |
0.144 |
Charleston, WV |
0.130 |
| St. Louis, MO |
0.161 |
Madison, WI |
0.148 |
| Billings, MT |
0.164 |
Cheyenne, WY |
0.154 |
Mean Kt = 0.154, standard deviation of Kt = 0.019, coefficient of variation of Kt=
0.122