Characterization of the 1991 Flood in the Helmand Basin
Flood information on the Helmand River and its tributaries in the Helmand province of Afghanistan is very important as it delivers water for irrigation and is used for drinking water. Refurbishment of Kajaki Dam on the Helmand River is one of the principal project works for which hydrologic information is needed.
The USGS is characterizing the available hydrologic data, especially the 1991 flood information. The 1991 flood will be used as the design flood. Of special interest is a tributary to the Helmand River, the Musa Qala River, because it can provide a large amount of the flow in the Helmand River at times.
Indirect determinations of discharge will be made at:
- Helmand River upstream from the Kajaki Reservoir;
- Helmand River downstream from the confluence with Musa Qala River (probably near Gereshk);
- Helmand River downstream from the confluence with Arghandab River (probably near Kalech or Darweshan);
- Helmand River near its mouth (probably near Char Burjak or the Iranian border);
- Musa Qala River near Musa Qala.
The February 1991 flood, which occurred roughly from Feb 6-17, 1991, has been used as the design flood at the five designated locations. Multiple lines of evidence are being used to best define the peak discharges and flood hydrographs. A literature search of journal articles and reports documenting historic floods in the Helmand River Basin was completed. A hydrographic comparison of streamflow data for major floods (1953-79) at selected gaging stations on the Helmand River was completed, and the streamflow data were plotted.
Anecdotal evidence and personal observations in February and March (2005) by various Ministry and Donor staff indicated a significant snowpack that provided the potential for at least moderate snowmelt runoff. In an effort to monitor streamflow during 2005 and compare the steamflow to previous flood years, USGS personnel used historic streamflow and reservoir elevation data to develop a relation (rating table) between the head on the Kajaki spillway and discharge for the Helmand River below Kajaki Reservoir. The rating table was used to estimate streamflow for February and March; streamflow estimates for 2005 and comparisons to other flood years have been supplied to several donor groups and the Ministry of Irrigation, Water Resources, and Environment (MIWRE).
As part of this effort time series of the Sistan Marshes water surface area and water volume estimates were also completed (see Figure 1).
Figure 1: Water volume for Sistan Marshes from Jan 1989 to Dec 1991 (in billion cubic meters); volume is based on the Hamun-e Saberi and Daryacheh-ye Sistan water bodies.
For 2005, water volume estimates were created for selected basins above known dam locations, including Kajaki dam on the Helmand River. This activity revealed a significant increase in snowpack water volume across the country compared to the previous two years (see Figure 2). The information served to inform flood preparedness activities as well as indicate prospects for irrigation water supply in 2005.
Figure 2: Snow water volume time series chart for the watershed above the Jalvar dam/reservoir for years 2004/05, 2003/04, and 2002/03.
