Viability of karezes (ancient water supply systems in Afghanistan) in a changing world

The Afghanistan population living far from rivers relies upon groundwater delivered from karezes (sub-horizontal tunnels). Karezes exploit unconfined groundwater in alluvial fans recharged largely by snowmelt from the Hindu Kush, the central mountain range of the country. Since the multi-year drought that began in 1998, many karezes have stopped flowing. This study characterizes the hydraulics of a kariz, the potential for reduced groundwater recharge because of climate change, and the impact of increasing population on kariz water production. A typical kariz in Afghanistan is 1–2 km long with a crosssection of 1–2 m and gradient of 1 m km. MODFLOW simulations show that water delivery from a kariz can be modeled by imposing a high ratio of kariz hydraulic conductivity to aquifer hydraulic conductivity on the cells representing the kariz. The model is sensitive to hydraulic conductivity, kariz gradient, and length of the kariz in contact with the water table. Precipitation data are scarce in Afghanistan, but regional data show a long-term trend of decreased snow cover, and therefore strong likelihood of decreased aquifer recharge. Population in Afghanistan has increased at a rate of about 2.2 % over the past several decades. An assessment of a six-district region within Kandahar Province where karezes are the most likely source of water indicates that water demand could have caused water tables to decline by 0.8–5.6 m, more than enough to cause karezes to stop flowing. These results suggest that kariz water production is not sustainable under current climateand population-growth trends.