Estimation of cotton yield based on weather parameters of Junagadh district in Gujarat state

The study was carried out to find the quantitative relationship between weather parameters and district level yield of cotton. For this purpose 32 years weather and crop yield records of Junagadh district (India) were collected. A twenty six week crop period model was recommended for pre harvest forecast due to higher R 2 value and lower simulated forecast deviation. The time trend, maximum temperature, morning and evening relative humidity significantly affected crop yield.


MATERIALS AND METHODS
To meet the objective, the yield data of cotton crop for the period of recent 32 years ( 1975-76 to 2006-07) of Junagadh district were collected from Season and Crop Report published by Directorate of Agriculture, Gujarat State (Anonymous, 2006-07). Corresponding data on weather parameters were collected from meteorological observatory situated in Junagadh and Anand Agricultural Universities. While data for the period from 22 nd meteorological standard week ( MSW) to 47 th week of each year were collected. For selecting best regression equation with significant weather variables, the stepwise regression procedure was adopted for 28 years data i.e. 1075-76 to 2002-03. The simulated forecasts based on these selected variables for next four years i.e. 2003-04 to 2006-07 and deviation from the actual yields were worked out.
To achieve earliest forecasts four models were fitted by using original weather variables, week wise considering up to 17, 20, 23 and 26 weeks of crop period. The time trend variable was included in this analysis as an explanatory variable.

RESULTS AND DISCUSSION
The results are presented in two parts for different models. First part deals with fitted regression equations.  Second part deals with their corresponding simulated forecasts for subsequent years not included for obtaining the regression equations (Varmola et al., 2004).
The results related to 17 weeks crop period, (Table 2) indicated that there is a positive and significant influence of time trend (T), maximum temperature of 8 th week (X 208 ) and afternoon relative humidity of 10 th week (X 510 ). A negative In case of 20 and 23 weeks duration models, the fitted equations followed the same trend as the one for 17 weeks crop period model.
The results, obtained in case of 26 weeks model ( Table  4), revealed that in addition to 17 weeks model, morning relative humidity of 8 th week ( X 408 ) negatively influenced and morning relative humidity of 25 th week (X 425 ) , positively influenced the yield of cotton. Influence of other variables X 208 , X 217 and X 510 remained same as per 17 th weeks model. These variables explained 93 to 98 per cent variations in the yield of cotton crop. The simulated forecasts (Table 5) showed 1 to 9 percent deviations from recorded yield of cotton in Junagadh district.
Comparison of the models, with respect to their predictability and the deviations of simulated forecasts from the actual yields, revealed that for both the models fitted in this approach, R 2 was very high ( > 98% ) in case of 26 weeks crop model and deviations of simulated forecasts from observed yields were less than 10 percent. Therefore recommended forecast model of cotton yield for Junagadh district is, Y = 205.93 + 8.93** T + 24.90** X 208 -22.94** X 217 -9.19** X 408 + 7.36** X 425 + 4.73** X 510 .

CONCLUSION
The bumper yield of cotton during the crop year of 2005-06 in the state as well as in the district ( 1171 Kg ha -1 ) was observed due to introducing B.T. cotton and favorable weather for crop conditions. The data of crop season for the 2005-06 were not involved in constructing the models. Therefore relatively higher deviation during crop year 2005-06 was observed.  The fitted model suggested that the yield of cotton in Junagadh district was observed to be increase nearly 9 kg / ha. per annum due to technological advancement during the study period. The morning relative humidity of 25 th week ( 46 th msw) and after noon relative humidity of 10 th week ( 31 st msw) were also beneficial to improve the productivity of cotton in Junagadh district. The maximum temperature of 17 th week ( 38 th msw) and minimum relative humidity of 8 th week ( 29 th msw) were found to adversely affect on average cotton yield in the district.