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. kt-elev.jpg (14019 bytes)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