Forests and water

Lead author: Christopher Cox; Contributors: Jason Ernest, <name>

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Among the primary reasons for conservation of the island’s forests is for the conservation of water. Given the volcanic origin of the island, the underlying geologic formations, combined with the generally steep terrain, ground water reserves are very limited. hence the drinking water supply for the country is derived almost entirely from ‘surface sources’; streams and rivers. The largest rivers on the island originate in the high-elevation forested interior, although several smaller rivers have their headwaters at lower elevations.

The forest serves to retain rainfall that falls on the land surface, where the tree root networks and the organic matter that forms part of the forest floor, together acts like a sponge that retains moisture, and releases the moisture in a controlled and steady manner into the sub-surface of the soil and rock, that emerges as mountain springs which flows into streams and rivers, eventually draining into the sea.

As far back as the mid to late 1800s, with the destructive impacts of wanton forest clearing for agriculture, timber and fuelwood extraction becoming obvious, the colonial government at the time commissioned investigations into the state of the island’s forests to come up with recommendations for conservation.  It was a great concern that continued trends in deforestation would spell loss of the ability of the forests to yield water needed to support health and sanitation of the growing communities. In 1887, E.D.M. Hooper of the Indian Forest Department was commissioned to do such an investigation.  In his Report upon the Forests of St. Lucia he recommended that there be no more clearing of forest lands on Crown (government-owned) lands, and that boundaries of forested lands be clearly marked. He also recommended that a forest warden be appointed to manage the forest resources of the island.

Taking on board recommendations from the Hooper report and to allay the threat deforestation posed to the most important water supply area to the city of Castries, in 1916 the colonial government declared some 1,393 hectares (3,442 acres) of the expansive forests between La Sorciere and Piton Flore (that were on Crown lands) as the Castries Waterworks Reserve in accordance with Section 4 of the Castries Water Supply Ordinance (Chapter 230) of 1901 relating to designating and protecting ‘Gathering Grounds‘ for water. In those days, small water intakes were established on select forest streams at Chassin and at Forestiere, that fed water to the city mainly via gravity. In 1946 the Forest, Soil and Water Conservation Ordinance (‘Act’ since 1983 amendment) was passed into law; the naming of the legislation underscoring the importance of the inseparable link between conserving water resources through the conservation of forests. In that same year, other forest reserve areas were established, specifically the Dennery Waterworks Reserve and the Quilesse Reserve.

Saint Lucia’s watersheds

Saint Lucia’s 616 km² landscape, has been divided into 37 main watersheds based on the geography of its river networks (Migeot, J. & Hadwen, 1984).

A watershed is an area of land that drains all the streams and rainfall to a common outlet such as the outflow of a reservoir, mouth of a bay, or any point along a stream channel. The word “watershed” is sometimes used interchangeably with drainage basin or catchment. Ridges and hills that separate two watersheds are called the drainage divide. Source: US Geological Survey.

The size and the shape of the watershed depends on the terrain of the land, how the mountain ranges are arranged across the landscape; this in turn dictates how expansive the network of the river and its tributaries are within the watershed. Rivers like the Cul de Sac, Mabouya, Marquis, Roseau and Troumassee rivers have their ‘headwaters’ or sources (springs) within the deepest, high-elevation forested interior and have extensive networks of smaller streams that feed into the main river within relatively broad watershed areas. The fact that these rivers drain larger areas means that the flow volumes are higher than smaller rivers that drain smaller watershed areas. These larger rivers not only carry more water, but they also carry more natural sediments as the upper landscapes naturally erode under rainfall impact. During vary large storms the lower segments or ‘reaches’ of these rivers flood, spreading sediment over wide areas of the flood plain, gradually creating the characteristic flat valley floors that have come to support intensive cultivation.

¹Migeot, J. & Hadwen, P.  1986.  Saint Lucia Water Resources: Preliminary Assessment. Vols. 1& 2.  Ministry of Agriculture, Castries , St. Lucia
.

Click or tap on the map below for a detailed view

The Marquis Watershed – a closer look

The Marquis Watershed is the landscape that drains into the Marquis River and its network of tributaries. The Marquis Watershed is located in the northern part of the island where the Marquis River has its headwaters in the elevated ridges from La Sorciere standing at 652 metres westward to Piton Flore at 523 metres and toward the Forestriere area. The watershed (drainage basin) area is 3,059 hectares. The Marquus River eventually drains into the Atlantic Ocean at the Marquis Bay. The communities of Forestiere, Cacoa, Babonneau, Paix Bouche and Desbarras, along with the connecting road networks straddle the ridgelines or ‘watershed divides’, that separate the Marquis Watershed from the surrounding watersheds.

Click or tap on the image below for a more detailed view

The water cycle

When rain falls over the landscape, a fairly significant amount is returned to the atmosphere by a process called evapotranspiration. Trees transpire water and release to the air via their leaves as they pull moisture from the soil through the stems and to the leaves in the process of photosynthesis. Over heavily forested areas the volume that is transpired can be very significant, over half of the incoming rainfall. Over non-vegetated areas, water settled on the surface is quickly evaporated to the atmosphere by incoming solar energy. Of the water that is retained within the landscape, a portion infiltrates into the deeper layers of the soil and bedrock where it contributes to groundwater storage, while the remainder traverses through the upper layers of the soil and bedrock (sub-surface flow), and over the soil surface to enter streams and rivers generally as surface flow or river flow.  This water eventually finds its way to the sea. The graphic illustrates the water or ‘hydrological’ cycle typical for a small island like Saint Lucia, how the water components are divided.

Water supply and extraction

The Forestry Department works closely with the Water and Sewerage Company (WASCO) and the Water Resources Management Agency (WRMA) in coordinating efforts to protect lands that are catchment areas for raw water extraction. A water catchment area (in the context of water supply) is a smaller area within a wider watershed unit that drains into a water intake. Typically, the intake is an impoundment constructed in concrete across the river to create a dam from where ‘raw water’ (natural water, pre-treatment) is abstracted.  This raw water is delivered by a conveyance pipeline to a water treatment facility in the vicinity, or some distance away. WASCO’s total water production based on the water abstraction and treatment facilities it operates across the island (at year) averages XXX cubic metres (XXX gallons) per day.

All of the water that is produced comes from some 30 water intakes, generally located in the higher elevations of the island. This includes the John Compton (Roseau) Dam. These intakes are operated on a continual basis, however during heavy rains when the streams carry excessive sediment loads as a result of erosion, the intakes, depending on how they are configured, need to be closed to avoid too much silt from entering the water conveyance pipelines and the treatment infrastructure. Water intakes that receive inflow from catchments that have significant land disturbance, will typically be subject to much higher levels of sedimentation than those under forest.

During the dry season that lasts from February to May, an additional 8 intakes may be brought into production to augment the water abstractions from the other intakes, when the natural base flows in the rivers decline due to the reduced rainfall. The problem however with extracting water from intakes situated along the lower reaches of the rivers is that they are significantly more prone to pollution from the wide range of human activities that may be occurring across the landscape.  

Of all the rain that falls on the surface of the island over the course of a year, only a very miniscule amount of that water that infiltrates and ends up in the rivers is extracted for potable water. The vast majority of the water finds its way to the sea.

Click or tap on the map below for a detailed view

The water balance: ‘perspective on proportions’

The graphic below illustrates a ‘rough’ water balance for Saint Lucia. This is based on the average annual rainfall across the island, the estimated volume that is lost to evapotranspiration, the remainder that remains on the land surface to flow in rivers. Of the volume that flows in the rivers a small amount is abstracted by the Water and Sewerage Company (WASCO) for treatment and distribution as potable water.

The water balance for the island is anticipated to change under the influence of climate change where in the Caribbean it is forecast that rainfall will decline and with warmer temperatures, the rate of evapotranspiration is expected to increase [cite reference]. This will invariably mean less water to recharge rivers, with significant implications for islands like Saint Lucia that withdraws all its water supply from river sources.

How forests and landscapes are managed is therefore extremely important. Where forests are cleared and replaced by other vegetation, buildings, roads and other exposed surfaces without water and soil conservation measures, there is an immediate effect in terms of how water is ‘captured and retained’ and gradually cycle through our rivers. With loss of forests and poor land management, the rain that falls tends to run off much more rapidly, often carrying away vital soil that eventually washes out to the sea. Ultimate this means less water is retained by the vegetation and soil, that would otherwise be available for water supply.

***write-up in progress; come back later***