Short communication Agroclimatic study of rainfed sorghum grown in lysimeter K.VASU, M. K.CHANDINI, R.JAGANNATHAN 1 and T.N. BALASUBRAMANIAN 1 India Meteorological Department, 1 Agroclimatic Research Centre, Tamilnadu Agricultural University, Coimbatore-641003. Journal of Agrometeorology 10(1) : 97- 98 (June 2008) Evapotranspiration is the vital process in plant system. It is proved in research that the ratio between actual yield and potential yield is directly proportional to the ratio between the actual evapotranspiration and potential evapotranspiration. It is understood that the actual evapotranspiration is greatly influenced by atmospheric, soil and plant factors. Considering this, a study was made to investigate the relationship between (i) grain yield (Y) and seasonal rainfall (RF), effective rainfall (ERF), growing degree days (GDD), relative temperature disparity (RTD), relative humidity disparity (RHD), helio-thermal unit (HTU), and change in soil moisture ΔS (ii) lysimeter evapotranspiration (ET) and RF and ERF, and (iii) between duration of sorghum crop (D) and GDD, RTD and HTU. The study was undertaken under dry land situation for seven years (1997-98 to 2003- 04). The test crop sorghum (variety: COS-28) was sown in the 3 lysimeter at a spacing of 30x15 cm with 45 plants maintained in each lysimeter. In and around lysimeter up to a distance of 15 meters the same sorghum crop was raised and recommended cultural practices were adapted in both the lysimeter and outside. Evapotranspiration (ET) was recorded daily from the lysimeters and the average ET was summed up for the crop duration. Effective rainfall was computed as reported by Gupta et al (1972). Growing degree days (GDD) were calculated for sorghum crop based on the base temperature of 18 0 C as reported by Karthikeyan (2002). Similarly RTD and RHD were calculated following Karthikeyan (2002). Statistical relationships were developed through correlation and step down regression. The mean data on crop yield and weather parameters for different years are presented in Table 1. Five years (1999-2000 to 2003- 04) data were used for development of model and 2 years (1997-98 to 1998-99) data were used for validation. In general when rainfall was more, the sorghum grain yield also enhanced. The simple correlation between grain yield (Y) and weather parameters did not indicate any significant relationship. Hence multiple regressions were attempted between Y and ERF; GDD, and between Y and. RHD and RTD, which explained 38 to 59% of grain yield variation. The equations are; Table 1: Dependent and independent variables data set. Year Dependant variables Independent variable Grain yield kg ha -1 Evapotranspiration mm Duration of crop days Rainfall (mm) Effective rainfall (mm) GDD ( 0 C) RTD RHD HTU 0 Chr 1999-2000 4230 322 119 463 280 805 36 43 4848 2000-2001 1634 277 106 135 130 629 35 45 4560 2001-2002 2564 393 114 535 338 862 34 41 5030 2002-2003 2209 343 120 349 258 894 40 53 5585 2003-2004 1331 380 123 315 274 896 41 53 6252 Validation data 1997-1998 1390 401 128 557 308 981 38 47 --- 1998-1999 1500 429 119 620 451 923 34 38 ---