Drought Monitoring in the West Indies

Agricultural drought monitoring in the West Indies is largely restricted to determining the presence of ongoing agricultural drought. It is generally the responsibility of the Ministries of Agriculture in the individual territories or other appointed bodies. In all cases monitoring of agricultural drought by these bodies consists of monitoring two sets of indices: a set of simple rainfall indices to determine when meteorological drought has set in, and a set of equally simple agricultural production indices to determine when production levels are abnormal with reference to a base year.

The island of Jamaica is divided into 14 parishes, which are further subdivided into 4-5 agricultural extension areas each. It is the responsibility of an agricultural extension officer to collect both the rainfall indices and the agricultural production indices. The subset of rainfall indices are reported by an agricultural extension officer to the parish supervisor on a monthly basis. The indices include the number of rainfall days per month, the total rainfall received for the month, the highest rainfall received, and the days on which it was received. These are supplemented with similar data representing the means of the same indices for the same region and for the same time of year as calculated from data collected over a number of years. A comparison of the collected and mean values is used to determine if a meteorological drought occurred during the month.

Similarly, a subset of production indices is also collated by crop type for the region. These include hectares planted during the month and to date, hectares harvested during the month and to date, hectares currently growing, yields per acre, and total production in tons. As with the meteorological data, these are also supplemented with similar data for both the previous year and for a reference year which was deemed a good produc tion year for the region on the basis of yields. The comparison between the collected and mean values determines anomalies in the agricultural production levels.

It is on the basis of both the reported monthly values of the rainfall and production indices that the ministry determines whether an agricultural drought is in the making. If the meteorological drought has continued (on the basis of previous month's reports) and there is at least a 5% reduction in production levels, then a drought alert is issued for the region. If the drought condition persists for consecutive months, it leads to a severe drought.

Agricultural drought is monitored in the other islands of the West Indies by monitoring both rainfall and production indices and by comparing these indices. This is true of both government-designated institutions that monitor domestic crop production and the privately funded institutes or large-scale producers of export crops. In some cases, however, the Jamaican subset of indices is augmented by one or more additional and easily measured indices as deemed relevant by the particular territory's monitoring institute.

It is generally true that for the West Indian region, the use of more complex drought indices such as the Palmer drought severity index, the standardized precipitation index (chapter 9), rainfall percentiles, or the crop moisture index, is not common in agricultural drought monitoring and management. Departures from the norm (calculated or intuitively deduced) are largely used to determine the presence of ongoing drought and to develop mitigation strategies.

Regional Drought Monitoring

In 1991, by an Agreement of Heads of Government of the Caribbean Community, the Caribbean Disaster Emergency Response Agency (CDERA; www.cdera.org) was established. The primary function of the CDERA is to make an immediate and coordinated response to any disastrous event affecting any participating state, once the state requests such an assistance.

Drought is one of the disasters to which CDERA reacts. Using the knowledge of the impact of El Niño and La Niña in the region, CDERA (in conjunction with other institutions) issues to the heads of government of its 16 participating territories information regarding the likelihood of drought conditions for specific periods of the El Niño event. The information also includes the potential impacts of the drought on various sectors including agriculture and ways to mitigate them.

Drought Monitoring for Sugarcane

Figure 11.3 shows the major sites and ecological regions in which sugar is produced in Jamaica. Roughly a third of the industry occupies the relatively arid and southern coastal lowlands, where irrigation is a necessity, and where average annual rainfall is just > 800 mm (table 11.1). The rest of the industry, however, is rain-fed and typically receives between 1300 and 1600 mm of rainfall per annum. Even in the irrigated areas there is a dependence on seasonal rainfall, which is estimated to be responsible for up to 50% of yields in good rainfall years and 30% in poor rainfall years (SIRI, 1973).

A boost in early growth of sugarcane usually occurs with the AprilJune (early season) rains. The July-September period is regarded as the "boom period of growth," because during this period the cane grows at its fastest rate (Shaw, 1963). Lack of rains during this period has a telling effect on sugarcane growth. In contrast, good rainfall during this period can dramatically compensate for severe growth retardation from drought in the earlier part of the year, as happened in 1998 at the Worthy Park estate, when the cane yield of 96.81 tons per hectare (tc/ha) was one of the highest ever recorded. The Frome cane-growing area, which lies in the wet west and receives nearly 2000 mm of rainfall per annum, shows a tendency toward bumper crops in years following below normal April-June rainfall (Felix, 1977).

The industry attempts to harvest as much of the crop as possible between December and April, taking advantage of the cool and dry period (figure 11.2), which favors sugarcane ripening (Biswas, 1988). The onset of rains in April-May usually triggers a sharp deterioration in sugar content. Often, when the dry period is too prolonged due to drought in the early rainfall season, cane yield begins to be adversely affected.

Four climate-related stresses lead to a poor harvest of sugarcane in Jamaica: (1) below-normal July-September rainfall, (2) below normal November-May rainfall, resulting in poor establishment of plantings and retardation of early growth, (3) above-normal temperatures and excessive rainfall during November-March (i.e., four to six weeks before harvest, and (4) excessive spring rains in poorly drained, flood-prone areas. These anomalies in seasonal rainfall and temperature are attributed to El Niño or La Niña events (Spence and Taylor, 2002; Stephenson and Chen, 2002). To illustrate this we briefly consider the production records from two sugar estates in Jamaica: Worthy Park, which is situated in the rain-fed area of St. Catherine in the Central Uplands of Jamaica (table 11.1 and figure 11.3).

An examination of the last 50 years of yield data for sugarcane at Worthy Park revealed that El Niño and La Niña years reduced yields in subsequent years. This happened in 10 different years, and the yield reduction was about 8% from the normal yield of 86 tons/ha (table 11.2). For example, the June-October rainfall in 1991 was only 46% of normal rainfall, and the December 1991 to February 1992 rainfall was only 61% of normal rainfall, and this significantly reduced yields in 1992. In addition, the minimum temperature was 2.8% above normal. All these weather anomalies were due to the 1991-92 El Niño. The 1965 El Niño caused

Table 11.1 Quarterly and annual rainfall (mm), 30-year means, in the major ecological sugarcane-growing areas of Jamaica

Ecological area

Jan.-March

Apr.-June

July-Aug.

Sept.-Dec.

Total

% Cane

Irrigated area

93

246

213

276

828

33.5

Wet west

210

514

540

385

1649

39.4

Dry north

198

335

315

359

1207

12.3

Wet east

261

409

484

637

1791

6.2

Central uplands

192

480

442

414

1528

8.6

268% of normal rainfall during the spring of 1996. In the following year, the sugarcane yield declined significantly.

In Trinidad, sugarcane is grown mainly on the drier western and central plains of the island, with a growing cycle similar to that of Jamaica. Pulwarty et al. (2001) found a correlation between ENSO values and sugarcane yield.

Monitoring Agricultural Drought in the Sugar Industry

It is evident that the effects of a drought on sugarcane may either be positive or negative, depending on the stage of the growth cycle at which it occurs. The Sugar Industry Research Institute (SIRI) in Jamaica collects and collates data on production and productivity factors, including rainfall, and other biophysical factors and management practices. A careful watch is kept on the development of drought, and attempts are made to predict its likely effect on sugar production.

SIRI (1976) reported that disastrous droughts occurred in 1976, when an average of 26.2 mm of rainfall fell on the irrigated area (and in some farms as little as 17.8 mm) against a long-term annual total of 115.9 mm for the region.

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