Water-balance simulations of runoff and reservoir storage for the Upper Helmand watershed and Kajakai Reservoir, central Afghanistan

Abstract

A study was performed to provide information on monthly historical and hypothetical future runoff for the Upper Helmand watershed and reservoir storage in Kajakai Reservoir that could be used by Afghanistan authorities to make economic and demographic decisions concerning reservoir design and operation, reservoir sedimentation, and development along the Helmand River. Estimated reservoir volume at the current spillway elevation of 1,033.5 meters decreased by about 365 million cubic meters from 1968 to 2006 because of sedimentation. Water-balance simulations indicated a good fit between modeled and recorded monthly runoff at the two gaging stations in the watershed for water years 1956-79 and indicated an excellent fit between modeled and recorded monthly changes in Kajakai Reservoir storage for water years 1956-79. Future simulations, which included low starting reservoir water levels and a spillway raised to an elevation of 1,045 meters, indicated that the reservoir is likely to fill within 2 years. Although Kajakai Reservoir is likely to fill quickly, multiyear deficits may still occur. If future downstream irrigation demand doubles but future precipitation, temperature, and reservoir sedimentation remain similar to historical conditions, the reservoir would have more than a 50-percent chance of being full during April or May of a typical year. Future simulations with a 10-percent reduction in precipitation indicated that supply deficits would occur more than 1 in 4 years, on average, during August, September, or October. The reservoir would be full during April or May fewer than 1 in 2 years, on average, and multiyear supply deficits could occur. Increased sedimentation had little effect on reservoir levels during April through July, but the frequency of deficits increased substantially during September and October.

Historical and hypothetical future sedimentation and water storage in Kajakai Reservoir, central Afghanistan

“…Investigations of hypothetical future water storage in Kajakai Reservoir included a spillway elevation of 1,045 meters, continued sedimentation, climate change, and irrigation development (Vining and Vecchia, 2007). Some studies suggest that precipitation decreases of about 10 percent (Milly and others, 2005) and temperature increases of about 2 degrees Celsius (Vining and Vecchia, 2007) could occur in central Afghanistan by about 2057.…”

“…Some studies suggest that precipitation decreases of about 10 percent (Milly and others, 2005) and temperature increases of about 2 degrees Celsius (Vining and Vecchia, 2007) could occur in central Afghanistan by about 2057. In addition, irrigated area below Kajakai Reservoir could increase from about 234,000 hectares (Favre and Kamal, 2004) to about 420,000 hectares (U.S. Army Corps of Engineers, written commun., 2005), resulting in a possible near doubling of irrigation demand.…”

“…In addition, irrigated area below Kajakai Reservoir could increase from about 234,000 hectares (Favre and Kamal, 2004) to about 420,000 hectares (U.S. Army Corps of Engineers, written commun., 2005), resulting in a possible near doubling of irrigation demand. The effects of doubled downstream irrigation demand, hypothetical monthly temperature increases of 2 degrees Celsius, reservoir sedimentation estimated at 2057 levels, and 10-percent decreases in monthly precipitation on hypothetical future water levels in Kajakai Reservoir indicate reservoir levels are likely to decline and supply deficits are more likely to occur (Vining and Vecchia, 2007). For example, with current sedimentation and climate conditions and doubled downstream irrigation demand, water-supply deficits in July would occur 10 percent of the time (once every 10 years, on average) compared to 28 percent of the time (more than once in 4 years, on average) with future sedimentation and potential climate change ( fig.…”

“…However, increased water volumes and water levels in Kajakai Reservoir could necessitate the resettlement of people who live near the reservoir. This fact sheet, which is based on information from Vining and Vecchia (2007), is intended to provide Afghanistan authorities with information on historical and hypothetical future sedimentation and water storage in Kajakai Reservoir that could be used to make decisions concerning reservoir operation and economic development along the Helmand River.…”

Water resources activities of the U.S. Geological Survey in Afghanistan from 2004 through 2014

“…3). The Kajaki Reservoir is a major source for hydropower and irrigation in the Helmand River Basin; the storage capacity at the reservoir has been reduced because of sedimentation and likely will be further reduced in the future because of continued problems with sedimentation (Vining and Vecchia, 2007).…”