By Ranil Senanayake –
Preparing for the future by looking back.
Understanding the issues and options before us.
It has been calculated that a one-degree increase in global temperature would eliminate fresh water from a third of the world’s land surface by 2100, mostly in the tropics. This is supposed to happen incrementally, as the number of record hot days increase so does the water evaporated from a hot land surface. Satellite imagery has long demonstrated that the heat signature from open or exposed lands is much higher than tree covered lands. Keeping the land cool, is another confirmation of the value of our mountain forests.
Recent studies demonstrate a temperature difference between cleared and non-cleared regions with similar incident solar radiation to be as high as 15 degrees centigrade. As the CO2 concentrations are the same in both regions the cooling effect is directly associated with evapotranspiration from leaves, changed water and cloud dynamics.
The cooling effect of 1 large tree is about the same obtained by running 10 room sized AC units during daytime, which is equivalent to 1,200,000 BTU day or 94,000,000,000 BTU/day per hectare of forest. As an example of this type of loss, when we consider that, between 1990 and 2000 Sri Lanka lost an average of 26,800 ha of forests per year, it translates to a loss of daily cooling by factor of 5,267,000,000,000,000 BTU as a consequence.
If designed well, this cooling effect can be harnessed in the design of agricultural lands, especially for crops sensitive to high temperature stress. The traditional pattern of rice fields that follow a contour bordered by tall trees is a design feature that might lend to modern application. Tree boundaries have a cooling effect and help to maintain a lower ambient temperature over the agricultural field.
The other advantage of developing forested hillsides is their role in cloud creation. Clouds are formed by water vapour condensing around some microscopic nuclei, the greatest contribution to Cloud Condensation Nuclei (CCN) coming from aerosols of biological origin. The first understanding of this phenomenon came from the study of Di-Methyl Sulfide (DMS). DMS comes in huge quantities from the oceans of the world and gives oceans their characteristics smell. It is produced by a variety of marine organisms from phytoplankton to coral reefs. DMS influences the global climate by producing the particles that promote cloud formation, and increases cloud albedo reflecting much of solar radiation back into space and thereby controlling the rate of planetary heating.
The cloud condensing nuclei in terrestrial ecosystems are produced by the forests. Forests represent some 48% of all terrestrial evapotranspiration, where groundwater cleaned by a tree is released into the atmosphere. This massive amount of water release is accompanied by huge quantities of plant aerosols and bacteria such as Pseudomonas and Areogenes. These bacteria live on the leaves of plants and their stomatal cavities and are convected into the atmosphere with the water vapour and aerosols released by these leaves. Once in the atmosphere they to act as cloud condensation nuclei (CCN) around which water condenses as droplets which go on to form clouds. Recent studies estimate that over one billion tonnes of such organic nuclei are released into the upper atmosphere annually. The effect is easily seen by the increase of cloud cover, cloud albedo and rainfall over forested regions. For tropical regions, the effect is significant, the forested areas being up to 15 degrees cooler than in cleared areas.
But all tree cover is not equal, given the ecological and biodiversity differences between plantations and natural forests, the capacity of forests to generate and release quantities of cloud forming nuclei is directly related to the maturity and identity of the forest. The older and more natural forests host vast quantities of vines, epiphytes, etc. and can increase the release of CCN by over 500% more than a monoculture tree plantation of equal age.
Once many of the streams on the island had well demarcated ‘river reservations’ containing mature natural vegetation. Although these reservations have now disappeared, they still exist in law and on the statutes. The restoration of the stream reservations is another action that needs to be implemented in designing a sustainable landscape. The stream reservations also act to trap sediment and reduce the sediment load reaching our reservoirs.
While planning to optimize the retention of rainwater through the agency of the mountain forests, the quality of the water that we produce must also be addressed. Sri Lanka had many streams with drinkable water till less than fifty years ago. The onset of agriculture using toxic chemicals and the dumping of untreated wastewater from industries and local municipal bodies changed the water quality to such an extent that over 90 % of our watersheds is polluted today.
The estimated groundwater potential of the country is 780,000-hectare meters per annum. Rainfall is the primary source of groundwater and its contribution to the groundwater recharge is estimated to be 7-30% or 200 – 600 mm/year. Although the majority of Sri Lankans rely on surface water or shallow aquifers for their supply of drinking water, both the quantity and quality of this resource is being rapidly eroded.
The water sources from the very tops of the mountains are being contaminated with large volumes of toxins and leached nutrients, the consequence of a socially irresponsible vision of agriculture that accepts no responsibility beyond production goals. One consequence of this view and practice of agriculture is that toxic chemicals are often applied at rates exceeding eighty times the manufacturers recommended dose. While the slow poisoning of the population through its food intake is an item that needs urgent address, the poisoning of the headwaters of our rivers bespeaks of an incredible lethargy in the monitoring of water quality for public health.
The high loads of nutrient dumped into the river systems by wasteful applications of fertilizer, a consequence of a poorly informed agricultural sector, are rendering rivers eutrophic, full of algae and silt. The problem is compounded by the high loads of garbage and sewage dumped into the river systems by riverside towns, communities and industries that contribute towards creating algal and bacterial blooms of unprecedented magnitude. A season of low flow and drought can now render many of these waters toxic. Further, the contamination of the shallow aquifer by industrial chemicals, currently tapped by a majority of domestic wells, has now become a common phenomenon in most industrial areas.
In cleaning our rivers and waterways another traditional design could be employed. The reservoir system of tradition operated on a “cascade system’ where reservoirs were constructed at the end of the micro watersheds leading to bigger tanks as the inflow increased (fig 1). If the new science of ‘constructed wetlands’ can be employed (fig 2) to plant the water inflow areas, with vegetation designed to remove pollutants, the surface waters can become safe again.
Constructed Wetlands are artificial wetlands created for the purpose of treating anthropogenic discharge such as municipal or industrial wastewater, or stormwater runoff. The enormous number of small reservoirs that we posses can be designed to use natural functions of vegetation, soil, and organisms to treat different water streams. Constructed wetlands can also be designed to emulate the features of natural wetlands, such as acting as a biofilter or removing sediments and pollutants such as heavy metals from the water. If the science of constructed wetlands is used in conjunction with the reservoir system of the country, the quality of much of the surface water could be improved and the sediment load contained.
To be continued…….