Urban lake: A reflection of its watershed

The urban lake is not a stand-alone entity but is a reflection of its watershed and there is a direct linkage between them. Study of this linkage is important as the condition of the lake is directly related to the condition of its watershed.

Land-use along a lake's shorelines and within its basin (watershed or catchment) affects its characteristics. The condition of a lake is directly related to the condition of the lake's watershed.

Even upstream areas can affect the lake through the water flow, which in turn, channels pollutants, sediments and nutrients into the lake. With urbanisation in the catchment area, urban lake can suffer from large cumulative impacts.

The relationship between land development and pollutant transport is one of the reasons for this direct linkage. Critical to any lake ecosystem, the urban lake's watershed is the surrounding land area that drains into that particular lake. Watersheds are defined by topography, in which the high areas drain into the low areas. Usually in urban lakes run-off either runs through direct runoff from the land, or through drains.

Storm water that runs off from roads, pavements, rooftops, parking lots, and driveways flows to the streets where it drains into underground catch basins and into the storm sewers that flow into lakes.

Watershed size, along with land use / land cover, soil types, topography and geology, all influence the quantity of water, its temporal distribution and the pollutant load associated with inflows to a lake.

The inter connectivity between lakes and their watersheds is a central analysis in lake management. This also holds true with respect to the assessment and quantification of pollutant loading. Although water bodies play multi-functional role in urban India, a majority of these are in a bad state due to over exploitation and pollution. For example, Guwahati's Deepor Beel lake has been used to dump solid waste by Municipal Corporation since 2006.

A case of Bhalswa lake, in north-west Delhi is strongly affected in its shape and size by a landfill site standing next to it, releasing toxic gases. Even the Pallikarni lake in Chennai is being used for dumping of solid waste. Urban run-off, industrial effluent load, domestic wastes, soap detergents and sewage into lake releasing nutrients are also prevalent issues of eutrophication of lakes in urban areas.

Sankey lake, Bengaluru, has become a stagnant water body with high eutrophication due to sewage inflow from seven points, storm water choked drains with sewage and garbage and leakage of sewage pipes.

The conversion of natural green area to residential, commercial and industrial land use brings about an increase in impervious surface, which serves to alter the hydrological cycle through increasing surface water runoff while decreasing groundwater infiltration. Associated with increased surface water runoff is increasing amounts of phosphorus, nitrogen and sediments.

These pollutants stimulate excessive algal and weed growth within the receiving lake. Most of these pollutants originate from diffused, ubiquitous sources and enter the lake on a landscape scale.

The input of pollutants in this manner from diffuse sources is referred to as non-point source.

Point sources of pollution for lakes observed in Indian cities: Wastewater effluent, both municipal and industrial; outlets from storm water drain; outlets from combined storm and wastewater drains.

Non-point sources of pollution for lakes observed in Indian cities: Run-off from roads, pavements, rooftops, parking lots, and driveways; septic tank effluent and runoff from failed septic tanks; run-off from construction sites; overflows of combined storm and wastewater drains.

In the last two decades, urban water bodies have also been a victim of unplanned development with increased pace of built up areas.

This has led to declining numbers of urban water bodies rapidly. For example, Bengaluru, known as a 'city of 1,000 lakes' in a cascade system of reservoirs for its supply demand, in 1960s it had 280 lakes, which reduced to 80 in numbers in 1993 and are now only 10 of them hold water. In Delhi, a survey conducted by the Delhi Parks and Gardens Society in 2015, revealed that 274 water bodies out of 611 have already dried up; around 190 of them are in critical condition.

In Chennai, only 30 lakes of 650 of lakes are left and the area of 19 major lakes has shrunk to 645 hectares from 1,130 ha. Hyderabad has also lost 3,245 ha area of its lake area in last 15 years. In Ahmedabad, 137 lakes were listed in 2001 but by 2012, 65 were already destroyed and built upon.

Encroachment, heavy concretisation and haphazard legal or illegal construction activities are a few of the major reasons for loss of these lakes as these water bodies are more recognized for their real estate value than for their ecosystem services.

In Bengaluru, 14 layouts with government buildings have been developed on lakes, according to a report by Bengaluru Development Authority. In Guwahati, Assam's revenue department allotted the Sola Beel lakebed for construction.

Other lakes in Indian cities such as Ousteri lake in Puducherry, Charkop lake in Maharashtra are known for encroachment. In India, encroachment by private is common and overlooked. Size of several lakes has reduced rapidly because of dumping of solid waste, sewage discharge and the construction of railway stations, a new road and buildings.

Poorly managed activities near shoreline areas also leads to influx of pollutants and loss of the ecological values associated with a well-functioning lake ecosystem. Take the case of associated shoreline land uses (eg, by washer folk) or any other social and cultural misuses by local communities (eg, immersion of idols).

Heavy metal concentration has been found to increase after idol immersion in water in lakes in Nagpur, Bhopal and the Hussain Sagar in Hyderabad. Another example of over-exploitation is by the Hanjis, who run boats on the Dal lake in Srinagar. The lake has suffered from dense mushrooming of settlements around the lake, land use/ land cover change in the catchment, floating gardens and sanitation and drainage problems polluting the lake.

In short, the anthropogenic alteration of a lake's watershed hastens eutrophication that leads to overall shrinkage of water bodies. In addition to an increase in autotrophic productivity, there are additional water quality impairments such as increased sediment infilling, declining species diversity, and declines in water clarity.

The water quality problems that then arise reduce the lake's ability to satisfy its ecological, recreational and aesthetic functions. It is often at this point that it is realized that there is a problem and initiates actions are intended to remediate the lake and restore its functions.

Unfortunately, the lake rejuvenation projects which are developed in response to declining recreational use and aesthetics tend to be very reactive and focused on addressing only the symptoms of eutrophication.

Such projects are never successful over the long-term. Despite various acts, policies and programmes the status of urban water bodies are not very clear, they need to be addressed with effective urban lake management plans.

Courtesy: Down to Earth