EVAPOTRANSPIRATION AND CANOPY TEMPERATURE OF RICE AND GROUNDNUT IN SOUTHEAST COASTAL INDIA - CROP COEFFICIENT APPROACH AND RELATIONSHIP BETWEEN EVAPOTRANSPIRATION AND CANOPY TEMPERATURE

Actual evapotranspiration (ET) from rice, groundnut and bare soil was measured by micrometeorological methods along with surface temperature measurements on two experimental plots in southeast coastal India. Hourly energy balances are presented here for different stages of crop growth as well as the daily and seasonal values of ET. The following expression for estimating ET is proposed: ET = C{s(Ta)/[s(Ta) + gamma]} (RN-G) where s(Ta) is the slope of the water vapour pressure curve, gamma is the psychrometric constant, RN is the net radiation, G is the soil heat flux and C is a crop coefficient. In the case of well-irrigated crops like rice, the interest of well-calibrated C values to estimate ET is shown, but for non-permanently irrigated crops, like groundnut, C values mainly depend on the particular soil moisture conditions and, therefore, can not be proposed for general use. Finally, relationships between daily values of sensible heat and instantaneous values of canopy to air temperature differences for rice, groundnut and bare soil are presented. Experimental results were justified by an analytical interpretation of the relationship between evapotranspiration and canopy temperature, in conjunction with a succinct error analysis. The agreement between experimentally derived relationships and theoretical expectations was acceptable.