A very dry spell in the heart of the Wet Season

In the Eastern Caribbean we are familiar with a wet season beginning around June and ending in December. October is certainly the wettest month on average in Barbados, averaging 190.7 mm (1971-2000). The preceding month of September averaged 155.6 for the same thirty year period. However, this year (2005) there was a period of 31 days from 18 September to 18 October where the rainfall was far below expectation. Rainfall in this period totaled a mere 49.8 mm. This is the kind of total we would expect in the dry season months from February to April/May. The average for this same period (18 September to 18 October) over the 30 years was 183.7 mm. The lowest recorded total for the period was 62.4 mm in 1972 whilst the highest recorded was 369.7 in 1990.

When compared with the estimated potential transpiration (taken here as 0.7 x Class A Pan Evaporation) one sees a major deficit (Figures 1 and 2, the period 18 September to 18 October are represented by Julian days 262 to292). Total potential evapo-transpiration during this period was 113.8 mm. Under such conditions plants would come under unusual (for this time of the year) water stress. Stress would not necessarily begin immediately, particularly since on 17 September there was 56.1 mm of rainfall. However, as time continued the stress certainly kicked in. There were no unusually long dry spell (consecutive days with less than 1 mm of rainfall) during 18 September to 18 October. The longest dry spell was 7 days compared with an average for the same period of about 6 days. There were however 21 dry days compared with an average of 16 for the period. An estimate of what the soil moisture content during this period is shown in Figure 3. This estimate was calculated using an average available water capacity (130 mm) for soils within a 1 km radius of CIMH. The soil moisture content was well below the water capacity over the entire period and there would have been some plant stress due to the difficulty of plant to extract such low water contents.

Figure 1 Rainfall (column) and potential evapo-transpiration (red line) from 1 September (Julian day 245) to 31 October (Julian day 305) at CIMH, Barbados

Figure 2 Accumulated rainfall (red line) and potential evapo-transpiration (blue line) from 18 September to 18 October 2005 at CIMH, Barbados

Figure 3 Estimated available water from 18 September to 18 October, 2005 at CIMH, Barbados.

The fact is that looking at monthly totals alone when dealing with crops can hide much of the real story, since by the end of both months the rainfall totals were 123 and 156.5 mm for September and October respectively. These monthly totals, even though below the 1971 to 2000 averages, suggest reasonable amounts of rainfall. However, in reality there was a 31 day period within that 2 month stretch where rainfall was inadequate.