Agriculture contributes 24 percent to the GDP of the country, absorbs about half of the total labour force and is the base of about three fourths of our exports. The Indus Basin Irrigation System is the major source and user of river water. Direct rainfall contributes only 15percent of crop water requirement. The loss in storage capacity due to silting up at Mangla, Tarbela and Chashma dams is 1.0 million acre feet every 6 years. These storages have lost 5.9 maf of their capacity by now, equal to the loss of one mega dam. The surplus water in the river system is available only for about 70 to 100 day period of summer high river flows. To conserve this potential, construction of additional storage reservoirs is essential to make it useable for hydropower generation and for sustainable irrigated agriculture. ENGR KHURSHID ANWER, Lahore, November 28.