3.4. Influence of monthly weather conditions on rubber yields
To identify the monthly climatic conditions associated with yield variability in a rubber plantation, mean meteorological conditions during the 30 days prior to tapping (192 cases) are summarised in Table 3. Cumulative rainfall showed highest variability, from no-rain to 789 mm during the 30-day period. A total rainfall of 50 mm or less was received during the previous 30-day period on 60 occasions (31%), exceeding 350 mm on 52 occasions (27%). Thus, the data represents prolonged moisture stress and heavy rainfall conditions associated with rubber productivity.
The 30-day mean meteorological conditions before the latex harvest are obtained as R of 237 mm, Tx of 31.4°C, Tn of 22.7°C and S of 6.9 h. The mean Tx varied between 29°C and 33°C. The mean Tn primarily (74%) varied between 22°C and 24°C. Thus, in over a month's period, thermal conditions exhibited less variability (1°C) than cumulative rainfall. The mean S varied between 4 and 9 h/day during 76% of the occasions. The mean Ep was found to be 3.9 mm/day.
The relationship between latex yield and mean meteorological variables during the 30-day period is shown in Fig. 3. The moisture as represented by cumulative R is positively associated with the variability in day-to-day yields of rubber trees. Whereas, thermal conditions showed negative correlation with yields. The multiple regression equations obtained are as follows:
where R30 is cumulative rainfall, Tx30 is mean maximum temperature, etc., during the antecedent 30-day period. The changes in Tx30 or Ep30 alone could explain about 64% of yield variability. Inclusion of other variables like R30 or Ep30 in the multiple regression analysis ( and ) has not shown any improvement over the Tx30. As discussed earlier, this reflects the indirect influence of tropical rainfall in regulating the changes in plant–atmospheric system.