AGRO BIODIVERSITY |
The Sharavathi river basin harbours diverse agriculture systems and the study revealed that the region has treasured 59 paddy varieties of which 29 are traditional. The genetically diverse traditional varieties well suited to the local environment are disappearing due to adoption of high yielding varieties, and greater spread of commercial and horticultural crops. The study also revealed that improper usage of modern techniques in agriculture has resulted in devastating effects on the environment. Extensive use of inorganic fertilizers with the adoption of modern agricultural practices, has resulted in the conversion of self-sustained traditional system to a system depending on external interventions. This is one of the factors contributing to non-point source of pollution in the eastern region.
Soil erosion is a natural phenomenon even in the pristine ecosystems. However, it gets accelerated due to removal of vegetation cover and disturbance to the upper soil strata. The most severe soil erosion is taking place at the ecotone of upper limit of reservoir level and the adjacent terrestrial ecosystem. Soil-separates (gravel, sand, silt, clay, etc) are transported to varied distances depending on the catchment runoff and ultimately results in silting of reservoir, which reduces its storage capacity. In western region, siltation is arrested by species such as Holigarna, Madhuca neriifolia, Elaeocarus and Pandanus etc. and by the multi-canopied evergreen forest itself.
There are two paths of escape for surplus water - through infiltration into underground aquifers, and as surface water flows. Natural land cover has various properties that help to regulate water flows both above and below ground. Forest canopy and leaf litter, for example, help to attenuate the impact of raindrops on the earth's surface, thereby reducing soil erosion. Roots hold the soil in place, especially on steeper slopes, and also absorb water. Openings in leaf litter and soil pores permit the infiltration of water, which is carried through the soil into the ground water. Where ground cover is insufficient, sheet, rill and/or gully erosion may result. Such erosion reduces the productivity of the land and may result in sedimentation of water courses down stream.
Streams eventually carry excess surface water to the ocean, though they may feed intermediate destinations such as lakes and wetlands. In their natural state, the network of streams in a catchment will slow down water flows so that there is a significant time lag between a period of peak precipitation and peak runoff further downstream. Riparian forests can serve as important buffers, reducing sediment loads and keeping runoff from moving too quickly into streams.
Growing vegetation allows retention of moisture and soil are able to absorb more solar energy than the previously sparsely vegetated surface. In addition moisture is available for evaporation, both at the soil surface and in the root zone. These changes affect the temperature and humidity of the lower atmosphere and make rainfall more likely. Conversely, in dry years soils tend to have higher albedos (absorbing less of the sun's energy) and there is less moisture available for evaporation, which can lead to a positive feedback in the opposite direction, yielding lower rainfall.
Based on composite sampling, and physico-chemical and biological analyses the river basin is categorized into most disturbed (Sharmanavathi, Haridravathi, Keshwapura, Gazni, Sampakai, Gudankattehole), moderately disturbed (Muppanae, Talakalale Dam, Reservoir, Debbe falls, Hosagadde) and least disturbed (Yennahole, Hurlihole, Nittur, Valagere, Dobbod) zones. The disturbance is due to anthropogenic activities in the catchment, mainly agriculture. Presence of coliform bacteria at Sharmanavathi, Haridravathi, Keshawapura and Nandihole indicates faecal contamination. This study also shows that salinity ingress was found to be absent at Gerusoppa and it decreases with increase in the river discharge.
Soil samples were collected from 78 locations distributed all over the upper catchment and subjected to physico-chemical analyses. Soils are rich in organic matter and low in phosphate, nitrate and sulphates concentration, while pH ranged between 5.5 - 6.8. The sediments have low sulphate (0.191 - 0.68 mg/gm), nitrate (0.0 - 0.0007 mg/gm) and phosphate (0.00024 - 0.001 mg/gm) indicating close correlation between sediment and catchment soil. The sediment samples are rich in organic carbon and the elements like Na, K, Ca, Mg are found well within the prescribed standards. Bulk density of sediments in streams of the western region indicates porous condition (0.783 - 0.983 gm/cm3) while in the eastern side they are less porous (1.23 - 1.475 gm/cm3).
