Statistical Approach
6.4 Results and Discussion .1 Descriptive Statistics
6.4.2.1 Impact on early crop
In case of early crop, the results (Table 6.5) indicated significant positive yield responses to rainfall variability (∆CvR1), number of rainy days (∆RD1), mean and intra-seasonal variability of winter temperature (∆TM1 and∆CvTM1) and negative yield responses to pre- monsoon rainfall variability (∆CvR2) and mean temperature (∆TM2). These variables together explained about 94% of the observed yield variance during 1991–2010. Among these, ∆RD1, ∆TM1 and ∆TM2 were statistically significant at 99% level of confidence (Table 6.5). Though these three factors together could explain about 64% of yield variance during the study period, inclusion of their variability terms improved the explanatory power of the model substantially. The relationship between individual climate variable and yield changes of early crop are shown in Fig. 6.8.
> 91 yrs 6%
81–90 yrs
2% 71–80 yrs 8%
61–70 yrs 5%
51–60 yrs 15%
41–50 yrs 31–40 yrs 9%
10%
21–30 yrs 19%
11–20 yrs 11%
< 10 yrs 15%
Table 6.5 Multivariate linear regression model between first differences of yield (kg/ha) and climate variables during 1992–2010 for early crop of tea
Source Estimate Std. error t-Value p-Value
Intercept 0.307 6.101 0.050 0.961
∆CvR1 1.837 0.608 3.021 0.012
∆CvR2 −1.719 0.728 −2.361 0.038
∆RD1 19.458 2.446 7.956 <0.0001
∆TM1 109.137 16.095 6.781 <0.0001
∆TM2 −10.775 14.697 −6.857 <0.0001
∆CvTM1 15.015 2.941 5.106 0.000
∆CvTM2 12.999 2.604 4.991 0.000
R2 = 0.944, Adjusted R2 = 0.908, F = < 0.0001
[R= total rainfall (mm), RD= number of rainy days (days), TM= mean temperature (°C), Cv=
intra-seasonal variability (%); 1 and 2 denote winter and pre-monsoons season respectively]
To estimate the role of individual climate variable on observed yield trend during the study period 1992–2010, the trends of climate variables were multiplied by the yield responses computed in Table 6.5 and the results are presented in Table 6.6. These impacts reflect only the climatic influences, which were captured by the empirical model.
Table 6.6 Observed trends in climatic parameters and their estimated impact on yield of early crop (1992–2010). Observed yield trend during 1992–2010 was −1.45 kg/ha/yr
Variable Trend Estimated impact (kg/ha/yr) % impact
∆CvR1 0.009 0.18 ± 0.06 12.55 ± 0.48
∆CvR2 −0.316 0.54 ± 0.23 37.45 ± 0.61
∆RD1 −0.325 −6.32 ± 0.79 −436.12 ± 0.18
∆TM1 0.068 7.42 ± 1.09 511.82 ± 0.21
∆TM2 0.032 −3.22 ± 0.47 −222.40 ± 0.21
∆CvTM1 −0.002 −0.03 ± 0.01 −2.07 ± 0.28
∆CvTM2 −0.041 −0.53 ± 0.11 −36.76 ± 0.29
The results emphasized the significant positive impact of increase in winter mean temperature during 1992–2010 by 0.068°C/yr on yield of early crop by 7.42±1.09 kg/ha/yr.
On the other hand, negative impact due to decrease of RD1 and increase in TM2 during 1992–2010 on yield was −6.32±0.79 kg/ha/yr and 3.22±0.47 kg/ha/yr respectively (Table 6.5). The estimated impact due to observed variability of winter and pre-monsoon rainfall during 1992–2010 on yield of early crop was positive. Similarly, deceased variability of
winter and pre-monsoon temperature on yield trend during the study period was negative (Table 6.5).
Fig. 6.8 Scatter plots of change in tea yield (early crop) vs. change in climatic parameters 6.4.2.2 Impact on main crop
The results of multivariate regression between the first differences of yield of main crop and climatic variables are presented in Table 6.7. In case of main crop, statistically significant negative yield responses to number of rainy-days (∆RD3) during monsoon season and sunshine hours (∆SSH1) during winter season was observed. The impact of pre- monsoon rainfall (∆R2) and variability of number of rainy-days (∆CvRD2) to the yield
-200 -100 0 100 200
-10.0 -5.0 0.0 5.0 10.0
∆ Yield (kg/ha)
∆ RD RD (DJF)
-200 -100 0 100 200
-40.0 -20.0 0.0 20.0 40.0
∆ Yield (kg/ha)
∆ CvR CvR (MAM)
-200 -100 0 100 200
-2.0 -1.0 0.0 1.0 2.0
∆ Yield (kg/ha)
∆TM TM (DJF)
-200 -100 0 100 200
-10.0 -5.0 0.0 5.0 10.0
∆ Yield (kg/ha)
∆CvTM CvTM (DJF)
-200 -100 0 100 200
-2.0 -1.0 0.0 1.0 2.0
∆ Yield (kg/ha)
∆ TM TM (MAM)
-200 -100 0 100 200
-10.0 -5.0 0.0 5.0 10.0
∆ Yield (kg/ha)
∆ CvTM CvTM(MAM)
trend was positive. Statistically significant positive yield responses to mean temperature and its variability during monsoon season were also observed. These variables together explained at least 84% of the yield variance during 1991–2010. The relationship between individual climate variable and yield changes of main crop are shown in Fig. 6.9.
Table 6.7 Multivariate linear regression model between first differences of yield (kg/ha) and climatic conditions during 1992–2010 for main crop of tea
Source Estimate Std. error t-Value p-Value
Intercept −3.759 13.276 −0.283 0.783
∆R2 0.477 0.105 4.552 0.001
∆R3 0.226 0.113 2.005 0.073
∆RD3 −10.619 2.499 −4.250 0.002
∆CvRD1 2.752 1.410 1.952 0.079
∆CvRD2 5.804 1.595 3.639 0.005
∆TM3 99.234 41.500 2.391 0.038
∆CvTM3 27.586 13.622 2.025 0.070
∆SSH1 −70.528 22.023 −3.202 0.009
R2 = 0.836, Adjusted R2 = 0.705, F = 0.004
[R= total rainfall (mm), RD= number of rainy days (days), TM= mean temperature (°C), SSH=
sunshine duration (hr), Cv= intra-seasonal variability (%); 1, 2 and 3 denote winter, pre- monsoon and monsoon seasons respectively]
The impact of individual climate variable on observed yield trend of main crop during 1992–2010 was assessed and the results are presented in Table 6.8. It has been observed that recent decreases in monsoon season rainfall (10.63 mm/yr) and increases in number of rainy days (0.13 day/yr) exerted a negative influence on tea yield (main crop). The estimated negative impact of monsoon season rainfall and that of number of rainy days on tea yield was 48.26±2.49% and 27.27±1.17%, respectively (Table 6.8). The positive impact of increased monsoon season mean temperature (0.018°C/yr) during 1992-2010 on observed yield trend was positive (35.84 ± 2.08%). The estimated impact due to increase of pre-monsoon rainfall (∆R2) on yield trend was positive (26.1±1.10%), whereas, its distribution (∆CvRD2), which exhibited an increasing trend during 1991-2010, showed a negative yield impact (−11.18±1.37%).
Fig. 6.9 Scatter plots of change in tea yield (main crop) vs. change in climatic parameters during 1991–2010
On the contrary, impact of increasing trend of winter rainfall distribution ((∆CvRD1) on main crop yield was found to be positive (Table 6.8). Correlation analysis showed that the relation between main crop yield and number of rainy days during February was positive and significant (0.661) while it was negative during January. Consequently, increased variability of rainfall distribution (∆CvRD1) (higher number of rainy days in February and less in January) during winter should lead to an increase in main crop yield, which was observed in the present study (Table 6.7).
CvRD (DJF)
-250 -150 -50 50 150 250 350
-50 -25 0 25 50
∆ Yield (kg/ha)
∆CvRD
CvRD (MAM)
-250 -150 -50 50 150 250 350
-40 -20 0 20 40
∆ Yield (kg/ha)
∆CvRD RF (MAM)
-250 -150 -50 50 150 250 350
-400 -200 0 200 400
∆ Yield (kg/ha)
∆RF
RF (JJAS)
-250 -150 -50 50 150 250 350
-400 -200 0 200 400
∆ Yield (kg/ha)
∆RF TM (JJAS)
-250 -150 -50 50 150 250 350
-1.0 -0.5 0.0 0.5 1.0
∆ Yield (kg/ha)
∆ TM3
CvTM(JJAS)
-250 -150 -50 50 150 250 350
-3 -2 -1 0 1 2 3
∆ Yield (kg/ha)
∆ CvTM3
Table 6.8 Observed trends in climatic parameters and their estimated impact on yield of main crop (1992–2010). Observed yield trend during 1992–2010 was −4.984 kg/ha/yr.
Source Trend Estimated impact (kg/ha/yr) % impact
∆R2 2.728 1.30 ± 0.29 26.09 ± 1.10
∆R3 −10.630 −2.41 ± 1.20 −48.26 ± 2.49
∆RD3 0.128 −1.36 ± 0.32 −27.27 ± 1.17
∆CvRD1 0.322 0.89 ± 0.45 17.78 ± 2.55
∆CvRD2 −0.096 −0.56 ± 0.15 −11.18 ± 1.37
∆TM3 0.018 1.79 ± 0.75 35.84 ± 2.08
∆CvTM3 0.001 0.03 ± 0.01 0.55 ± 2.46
∆SSH1 −0.014 0.99 ± 0.31 19.81 ± 1.56
Decrease of sunshine duration during winter season was found to show a positive impact (Table 6.8) on main crop yield (19.8±1.56%) possibly due to its role in decreasing diurnal temperature range (DTR) during winter season. Mean maximum temperature, DTR and sunshine duration during winter season had shown negative correlation with main crop yield during 1991–2010.