Ecology, Environment and Conservation Paper

ESTIMATING REFERENCE CROP EVAPOTRANSPIRATION- INFLUENCE OF ANGSTROM VALUES

A. Murugappan and K. Kannappan

Abstract

This paper brings out the effect of the Angstrom values, a and b in estimating reference crop evapotranspiration by the FAO Penman-Monteith method for two different locations namely,Annamalainagar (I I °24'N and 79°44'E) and Coimbatore (I I °00'N and 77°00' E) in Tamil Nadu State. Daily reference crop evapotranspiration were computed for the period 1977 to 1992, for each of these two locations, using the FAO Penman-Monteith method with the Angstrom values a and b derived from actual solar radiation presented in Sarker (5). Also, daily reference crop evapotranspiration were computed with general values of a = 0.25 and b = 0.50 for the same period for each of the two locations.The use of general values of a and b in computation of ETo resulted in values that were different from those obtained using the derived values of a and b. For each of the two locations considered in the study, the significance or otherwise of the differences in daily, weekly, seasonal (summer, winter and monsoon seasons) and annual reference ET estimates when using the derived values and general values of a and b were tested statisitcally.Based on the statistical test, it was found that the differences in dialy, weekly, seasonal and annual reference ET estimates are insignificant for both the locations. Hence, it can be stated that the Angstrom values do not have any sig-nificant influence in estimation of reference crop evaportranspiration.