ABSTRACT
A comprehensive analysis of daily streamflow records available over the period 1961-2001 at the seven very important flow gauging stations across the Karkheh River and its major tributaries revealed that the streamflows have large intra-annual and inter-annual variability across the basin. The high flows are observed from November to May, with peak flows occurring in March-April. The high flood events (1-day maximum) can occur anytime from November to April, though most often they occur in February and March. The low flow period corresponds to June through October. There are large differences between the amount of water available during high and low flow periods. For example, at the Jelogir station at the Karkheh River, mean monthly streamflow in April (386 m3/s) is nearly ten times higher than in September (41 m3/s). The observed spatio-temporal variations could be substantiated by the values of Coefficient of Variation (CV). Monthly CV values range from 0.4 to 1.77 across the examined streamflow gauges. In temporal terms, the minimum and maximum CV values correspond to February and November, respectively, whereas, in spatial terms, the Gamasiab River indicated higher variability and the Kashkan River the lowest variability. The mean annual streamflow indicated CV values in the range of 0.41 to 0.54, indicating marked differences in water available in the long-term perspective. For example, the mean and median surface water availability at the Paye Pole station at the Karkheh River was estimated as 5.83 × 109 m3/yr. and 5.59 × 109 m3/yr. As in all other examined stations, the minimum and maximum had a wide range at Paye Pole, with values of 1.916 × 109 m3/yr. observed during 1999-2000 and 12.60 × 109 m3/yr. observed during 1968-69. Under such highly variable conditions, the understanding of the reliability of the water availability becomes more meaningful for better resources use and allocation planning. The flow duration analysis conducted in this study provides such estimates of streamflow reliability for the Karkheh Basin at daily, monthly and annual time resolutions. For instance, the value of mean annual streamflow with a reliability level of 75% (indicated by 75th percentile of streamflow derived from the flow duration analysis) at Paye Pole was 4.10 × 109 m3/yr., which is about 30% lower than the mean annual flow estimated for this location.
