Dear Sir, I am having some doubts over thermal time (growing degree days or heat unit) concept. General equation for thermal time calculation is ((Tmax+Tmin)/2)-Tbase, Where Tmax, Tmin are maximum and minimum temperatures, respectively and Tbase is the base temperature below which there will be no growth of plant. Most of the process oriented crop growth models viz., DSSAT series, ORYZA, SPAW, SWAP models use thermal concept for defining different phonological stages as well as for quantifying the developmental rates. I am herewith listing different combinations of maximum and minimum temperatures, for which same thermal time occurs. All are these temperature values are hypothetical values, but will represent the thermal regime of tropical environmental scenario. ------------------------------------------------------ Tmax Tmin Tmean Tbase Thermal time (0 C) 0 C) (0 C) (0 C) ------------------------------------------------------ 30 25 27.5 10 17.5 32 23 27.5 10 17.5 28 27 27.5 10 17.5 34 21 27.5 10 17.5 36 19 27.5 10 17.5 38 17 27.5 10 17.5 40 15 27.5 10 17.5 42 13 27.5 10 17.5 ----------------------------------------------------- From the above data set, it is clear that there is wide variation of 28 to 42 0 C of maximum temperature and a variation of 13 and 27 0 C of minimum temperature. But thermal time is same for all the data sets. My doubt is how this will be interpreted for phenology and developmental rate? It is possible to modify the thermal time concept with the inclusion of solar radiation or any other way. Thank you very much for spending your valuable time for me. Have a nice and good day. Thanking you, With kind regards, Sincerely yours, N. SUBASH Scientist (Agricultural Meteorology) ICAR Research Complex for Eastern Region WALMI Complex, Phulwari Shariff P.O PATNA 801 505, BIHAR Email address : [log in to unmask]