Groundwater, a reliable resource for drinking and production, is under severe stress in Kathmandu Valley because of the excessive groundwater. The process of urbanisation in Nepal has altered the natural setting of the environment; land surface is completely disturbed reducing groundwater recharging area and direct infiltration of excess rainfall. Ultimately this increases the surface runoff that quickly removes the rainwater, influencing the total amount of infiltration, reducing the subsurface flow and recharge.
A suitable and applicable method of recharging groundwater is through rainwater harvesting. 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. 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.
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). 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.
The total potential volume of water that can be harvested from these 12 apartments and housings is 3.33 times the total volume of water consumption (155,104 m3) in a year. When we generalise 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.
The post is written by Mr Kabir Das Rajbhandari, Programme Manager – Urban, WaterAid in Nepal.