CKDU is considered a problem related to the environment
At present, the closer catchment is very rarely observed in small tanks. Kuluwewa, Godawala and ephemeral streams are generally located in Wew Ismaththa (Fig.04). Wew Ismaththa is protected and cutting trees and chena cultivation is prohibited. The purpose of the area was to increase the groundwater table through infiltration and thereby gradual release of water to the tank during the dry season is expected. Farmers under the headmen of the village were used to grow trees in these areas such as Weera, Kumbuk,
Palu, Mee, Thibiri, etc. and establish vines such as Kobbawel, Eraminiya and Thiththawel thinking that these would absorb more water during the rainy season and drain those absorbed water to the tank slowly. A resulting biotic environment is an ideal place for diverse macro and microfauna. These micro and macro-organisms purify water reaching from upstream tank command areas in this system. Scientists have proved that plants in the biotic-environment can absorb herbicide
molecules sorbed onto organic matter and soil microorganisms can degrade the herbicide molecules. Scientific studies showed that some
herbicides like glyphosate showed not to leach at all and remain tightly sorbed onto organic complexes. Tree roots and enhanced level of soil organic matter from tree litter improve the soil structure, aggregate stability and promote faunal activity leading to higher macroporosity so that infiltrated water passes through the soil matrix. Generally, pollutants are adsorbed to the clay and sediment particle. This environment in the Wew Ismaththa filters sediments and also pollutants adsorbed to the sediment particle may not then reach to the tank water. Thus, closer catchment had a higher potential to clean the drainage water reaching from the upstream command area where paddy and other field crops grown.
If this area is re-established and conserved as in the early days, there is still a potential to improve the tank water quality even though the upstream area is heavily cultivated with recommended doses of agrochemicals following integrated pest and nutrient management techniques.
THAULLA (UPPER PERIPHERAL GENTLE SLOPING LAND)
The area just below Wew Ismaththa and in between Wew Ismaththa and water spread area of the tank is Wew thaulla. This micro-land region is partly covered with water but only completely flooded during the rainy season. As shown in figure 04, the area where Gasgommana and Perahana are located can be considered as thaulla. Thaulla doesn’t allow water flow of the upstream area/ upper catchment to directly reaching it as a storm. Because it consists of arrays of bushes such as kooratiya, Tharanga, Maila, Pila , etc, and the trees such as Nabada, Dunuke, Mee, etc. In addition sedges such as Hambupan, Thunhiriya, Gal ehi, etc. and also different kind of climbers such as Kaila,
Katukeliya, Bokalawel (IUCN 2015)and grasses abound in these land mass. These florae have an ability to trap sediments and absorb pollutants and negate their toxicity reaching to the tank. Moreover, these strip of trees (Gasgommana) act as a wind barrier and also lower the temperature of the water in the tank.
We in the Department of Agric. Engineering and Soil Science of the Rajarata University have shown the nutrient and some metal retention
capabilities of thaulla. We also conducted a research at Ulankulama tank at Maradankadawala.
This study found a higher accumulation of Fe (1819.65 ppm) and Al (1586.75 ppm) with a considerable retention of P (74.80 ppm), Ca (163.79 ppm) and Mg (203.67 ppm) in the tested soil. Another study done in the same thaulla showed the phytoextraction behaviour of two monocot plants. These results suggest that thaulla, can act approximately as an active ‘constructed wetland’ which efficiently removes pollutants in order to sustain the system.
Scientists have shown the water quality improvement ability of constructed wetland. Since thaulla is submerged with water for about 3 to 5 month during the rainy season, and then drying steadily during the other months, the thaulla and tank water spread area its ecosystem can also be considered as a special wetland.
It has been shown that many species of plants/ trees absorb contaminants such as lead, cadmium, chromium, arsenic, and various radionuclides from soils.
Contaminants in water are reduced or removed by wetland systems by mechanisms including sedimentation, filtration, chemical precipitation and adsorption, microbial interactions and uptake by vegetation.
Ancient people knew that the lifespan of the tank depended on the functions of th a ulla. therefore, entering into this area was customarily prohibited to anybody except the Ayurveda doctor.
However, at present, it is rather difficult to find out the good extent of thaulla in most of the tanks and also conservation of this area is neglected due to overexploitation of the resources and ignorance.