Fig.1
Geological: The landmass called St Vincent is 335km2 of volcanic deposit formed during the Pleistocene Era (2m years ago) form two major volcanic eruptions. The island has a central north south mountain range and several stream filled valleys draining unto black sand beaches reflective of their volcanic origin. Fifty percent of the island consists of slopes >30° with only 20% having slope below 20°. During the last three hundred years, the volcano had five major eruptions; 1718, 1812, 1902, 1971 and 1979.
The result of these eruptions is a cap of volcanic ash, at various stages of weathering, covering the hard igneous rock deposits. In the north of the island is the younger coarser texture sand soils that is free draining. Above the 600 ft contour are the so-called zonal or high-level yellow earth/brown earth soils. These are better drained and often more fertile and occur on gentler slopes. The valley floors (approximately 1800 acres) are the islands fertile alluvial deposits. The southern and western coastal belts consist of shoal clay soil of little agricultural value. The soil in the central mountain range is shallow, acidic and leached. They have high potential for serious erosion. Most of these soils are covered by vegetation.
Hydrological: Historically, St Vincent is purported to have a wet season from June to December and a dry season form January to June. The average annual rainfall is approximately 200cm and fairly well distributed as can be seen from the rainfall data for 2001 and 2002 - Fig. 1. Rainfall data for the period 1980 to 2000 indicates a shift in the rainfall pattern but no significant decrease in volume. The data points to a decrease in rainfall during September and October with some increase in February and March. This time frame is to short in geographic terms to be considered a pattern but warrant closer observation. It may be an element of the much-publicized phenomenon called Climate Change.
Fig.1
UNCCD: The United Nation Convention to Combat Desertification has its genesis in the 1992 Rio summit. The convention focuses on popular participation and "helping local people to help themselves prevent and reverse land degradation." Land degradation is defined as "reduction or loss of the biological and economic productivity and complexity of rain fed crop land" (Article 1[F]).
In Chapter one of Down to Earth Lean writes
all terrestrial life depends on the fragile, friable crust of soil that coats the continents. This precious covering, the very flesh of the planet, is painfully slow to form and can be destroy terrifyingly fast. Each year, the world watch institution established that 24 billions tons of top soil is lost from the continent.
The Drainage Basin: North River drains the Mt. St Andrews Bow Wood watershed. This area falls within the Agricultural census district 1 & 2. Typical of St Vincent, the steep slopes and rugged landscape of this river basin trails off into a relatively flat narrow coastal valley at the North end of Kingstown. Agriculture is the main economic activities in the upper reaches of this river basin occupying over 800 acres. This figure represents a significant decline from the 1750 acres reported in 1986. Deep-till rain-fed agriculture is the technology chosen to support the root crop characteristic of this area.
Expanding suburban and urban settlements flank the lower or seaward end of the river basin.
Fifty metres from the sea North River has a concrete bed and vertical stone wall banks supporting McCoy street to the east and Linley Steet to the west. At this point, the river structure is 5.8m wide and 1.6m deep.
Sampling: Grab samples were taken form mid stream of North River by lowering a measuring cylinder into the river form the bridge above. On retrieving the cylinder the volume of its contents was adjusted to 100cm3 by decanting the excess. The cylinder was allowed to stand for five hours to facilitate precipitation. Three such samples were done at time zero and every hour after that for the next four hours. In all cases for which data were selected, the rainfall exceeded 20mm as recorded by the meteorological station at the Botanic Gardens.
After five hours, the liquid was decanted and tested for phosphate and nitrate using Hach Test kits. The remaining sediments were heat dried in an oven then weighed. The data are recorded in the table below.
| Date | Flow volume | Approximate sediment flow | ||
| Base flow | Peak flow | |||
| September 2002 | 0.085 m3/s | 5.57 m3/s | 15.3% | 12240 m3 |
| October 2002 | 0.088 | 4.81 | 10.1% | 6995 m3 |
| November 2002 | 0.084 | 6.00 | 18% | 15552 m3 |
| January 2003 | 0.090 | 5.32 | 15% | 11491 m3 |
| February 2003 | 0.070 | 5.80 | 15.5% | 12946 m3 |
Flow Volume: On a normal sunny day, water runs in a V in the middle of riverbed at a discharge rate of 0.07 m3/s (measured with a hand held flow meter manufactured by General Oceanic Inc.).
On three separate occasions in September 2002, it was observed that the flow volume in North River went from base flow to maximum and back to semi-base flow in 3½ hours, 4 hours 50 minutes and 4 hours respectively. It was therefore decided to use four-hour study periods.
Sediment load: The sediment load at base flow was accepted as zero. At peak flow, sediment load varied between 10% and 18%. The sample mode was 15% by volume after precipitation. After drying, the volume mode was 12%.
Estimates based on preliminary data show North River transporting in excess of 1.2 x 105 m3 of soil each year. If this scenario is applied to all districts in St Vincent having streams/rivers with flow volumes equal to or greater than that of North River, of which there are ten, then one can conservatively say that St Vincent loses approximately 1.2 x 106 m3 of top soil annually.
After leaving the river, this sediment load exists as a brown plume along the coast extending for as much as 1500m seawards.
Nutrient Loss: Coupled with this loss of topsoil is the excessive leaching of nutrients (natural and synthetic). The data below show the increase in dissolved nitrate and phosphate at peak flow. This increase is attributed to leaching of agriculture plots and runoff from grazing lands. While only nitrate and phosphate levels were measured at this point, it is reasonable to assume that all other nutrients, soluble and insoluble, are lost from the soil in similar proportions.
| Nitrate Levels | Phosphate Levels | ||
| Base Flow | Peak Flow | Base Flow | Peak Flow |
| 8.5 | 10.0 | 6.0 | 9.1 |
| 8.3 | 9.5 | 5.8 | 9.0 |
| 8.1 | 9.0 | 4.2 | 8.9 |
| 8.8 | 11.0 | 7.1 | 10.2 |
| 9.0 | 11.2 | 7.0 | 9.5 |
The findings presented in this paper though statistically sound and scientifically authentic are incomplete and hence contain an element of inaccuracy. The fact that there is a data gap for the period March to June (the drier periods when agricultural activities tend to be minimal) calls the extrapolation into question. Additionally, data are needed for at least two other rivers (one on the east and one on the west since North River runs south from the centre of the island) for comparison and validation.
The data, though incomplete, make a case for land degradation occasioned by rain water/storm water runoff. It is obvious that there is tremendous soil and nutrient loss resulting from rainwater runoff. What is uncertain at this point is the actual amount of soil and nutrient that is lost. It is hoped that when completed, the current research would adequately answer these questions.
What is the net economic loss associated with land degradation in St Vincent? In 1995, St Vincent Banana Growers Association spent in excess of EC$2M on agrochemicals. It is estimated that approximately 37% of such input ends up in streams and rivers (Murray 1992).
How has agriculture survived in the face of such odds? Can the deep rich volcanic topsoil sustain such loss? The reduction in agricultural yields and the increased reliance on large quantities of agrochemical inputs are indicators of land degradation occasioned by runoff. These realities invariably erode the country's competitive ability in the market place. While no monetary cost has yet been put on the loss of topsoil there is surely a cost on the loss of fisheries and other coastal resources resulting from land-based pollution, in this case, sedimentation and nutrient load.
It should be noted that soil loss occasioned by rainfall is facilitated not only by poor agricultural practices but also by land clearing for road and housing construction. Construction is one of the most active sectors of the Vincentian economy and takes place on steep slopes as well as on deep rich agricultural soil. The excavation associated with this activity leaves large volumes of unsecured topsoil open to erosion by rain.
In examining the nexus between rain and land degradation in St Vincent one must go beyond rainfall and soil loss to capture the socio-economic impact. For while we have little or no control over volume and distribution of rainfall, we can adjust our life style vis-à-vis agricultural and land use practices.
Lean, Geoffrey, Down to Earth, Secretariat to Combat Desertification, Switzerland 1995.
Murray, Reynold, 'Sweet Fruit Bitter Harvest', CNN 1992.
Ministry of Agriculture Land and Fisheries, Statistical Division, Rainfall Totals and Averages 2001.
Ministry of Agriculture Land and Fisheries, Agricultural Census 2002.
United Nations Convention to Combat Desertification.
© Reynold Murray, 2003.
HTML last revised 17th October, 2003.
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