KABIR DAS RAJBHANDHARI
Water management is a very critical aspect of growth and development of any economy, more so in a developing country like Nepal which is endowed with many water resources that need to be conserved, better managed, recharged and channelized for meeting the ever increasing requirements of present trend of urban growth of Nepal which is one of the highest in the South Asian region. Groundwater is a reliable resource for drinking and production, both in terms of quantity and quality.

However, this resource is now under severe stress in Nepal, particularly in Kathmandu Valley because of the excessive groundwater abstraction in the course of socioeconomic development. The consequences of over exploitation of groundwater are either irreversible in nature or require extended periods to abate. The only source of the aquifer recharge in the Valley is rainwater but due to unplanned urbanization, the recharge area or open space in the valley is getting reduced considerably and most of the rainwaters are drained out unused.

With rapid urbanization in the Kathmandu Valley, ground water resources are unable to replenish themselves during rainfall period as surface infiltration has been vastly reduced while consumption of groundwater is ever rising. The process of urbanization in Nepal has altered the natural setting of the environment; land surface is completely disturbed and changed to either denuded or paved (developed) land. This not only reduces the groundwater recharging area but also reduces the direct infiltration of excess rainfall. Ultimately this increases the surface runoff that quickly removes the rainwater, influencing the total amount of infiltration and reducing the subsurface flow and recharge. These scenarios substantiate the urgent need to establish the recharging potential within the basins.

Suitable and applicable method of recharging groundwater through rainwater harvesting can help, more as a supplement rather than as an option, to solve present water problems in Kathmandu. Firstly, surface water is inadequate to meet our demands and we are highly dependant on ground water. Secondly due to rapid urbanization, infiltration of rain water into the sub-soil has decreased drastically and recharging of ground water has diminished. Structures to harvest rain require little space. Rain water harvesting is an old technology practiced in the valley but limited only to collecting rainwater for the household purpose only. The practice of recharging shallow groundwater aquifer through soak pits or recharge pits or directly through wells from rainwater harvesting artificially is a relatively new concept of utilizing rainwater in Kathmandu.

Simple calculations have suggested that substantial amounts of water could be made available if shallow groundwater can be recharged with the help of rainwater. With a catchment area of 656 Km2, Kathmandu Valley receives an annual rainfall of 1,500 mm on an average. In the context of accommodating growing needs of urbanization and changing lifestyles of the people there are a number of large scale housing complexes and apartments coming up in the valley. Data collected (as of 2065 BS – 2008/09) from the Department of Urban Development and Building Construction (DUDBC) shows that there are altogether 103 numbers of housing and high rise apartments, including both constructed and under-construction, that have been permitted.

Of the 103 apartments and housings, 12 were selected for a study based on their placement in three Groundwater Districts (GD); i.e. Southern GD, Central GD, Northern GD). The study found that the total theoretical volume of water available for natural recharge (due to rain fall) in the areas covered by 12 apartments and housings (before construction) is 1,251,563 m3. Considering practicality in rainfall recharging, it has been stated that only 15 – 20 percent of the total rainfall in a particular area gets recharged. According to this, the possible recharge that could take place naturally in these areas is 250,313 m3 (20 percent of 1,251,563 m3, i.e. theoretical natural recharge within these areas).

The total volume of water reduced due to construction of the buildings, roads and pavements etc in these 12 A&Hs is 773,937.77 m3 (i.e. 61.84 percent of theoretical volume of water that could be recharged naturally i.e. 1,251,563 m3). Similarly, the total volume of water consumption (by abstracting deep groundwater) per year by the people (residing in the 681 units of housings/apartments in these 12 Apartments and housings) on an average is 155,105 m3 which is 12.39 percent of 1,251,563 m3 (i.e. theoretically available volume of water for natural recharge).

The estimated cost of installation of rainwater harvesting plant in an individual household depends on the area of catchment for harvesting rainwater and the length of conveyance for the collection, storage and recharging purposes. Based on Nepal’s experience, the cost of rainwater harvesting installation is 68.8 USD per sq m of the roof catchment but this does not include the cost of preliminary treatment units.

From the analysis of 681 housings of 12 apartments and housings, the potential volume of water that can be recharged annually to shallow groundwater aquifer is 516,343 m3 (which is 77 percent of the total volume of water i.e. 671,448 m3 that can be harvested from built up areas only and is 41 percent of the theoretically available volume of water (i.e. 1,251,563 m3) that can be recharged even after deducting the yearly consumption of water by the resident people of these 12 apartments and housings.

The total potential volume of water that can be harvested from the buildings in these 12 apartments and housings is 3.33 times the total volume of water consumption (155,104 m3) in a year. This surplus volume of water can be stored and utilized for recharging shallow groundwater aquifer by providing separate storage tanks. However, if we generalize from this study conducted in 12 apartments and housings with 681 units, it can be said that 77 percent of the total volume of water can be harvested from built up areas and 41 percent of theoretically available volume of rainwater in any apartment and housings for natural recharge can be tapped for recharging groundwater.

This study clearly indicates that the rainwater harvesting in the individual housing and apartments will minimize the water scarcity problems; prevents the further abstraction of groundwater (both shallow and deep aquifer) through boring; provides opportunity to manage water resources internally by recharging shallow groundwater aquifer through simple recharging means and thereby making the apartments and housings self sufficient on their own.

Writer is an Urban Program Manager at WaterAid, Nepal

kabirdasrajbhandari@wateraid.org

source:republica(2011),"Apartments to the grounds ", republica,17 April 2011