An overview of the hydrology of the Zayandeh Rud Basin

This paper provides an overview of the hydrology and water use in the Zayandeh Rud basin based on the data available over the 11-year period 1988-1998. The inflows into Chadegan reservoir, the releases from the reservoir, and the extractions along the river for irrigation and other purposes are considered, and a rapid water balance of the basin is performed. Inflows to the Chadegan reservoir, which serves to collect and regulate the runoff from the upper catchment of the basin to better meet the downstream water requirements for irrigation, urban and industrial uses, follow a regular pattern with moderate variability. But the limited year-to-year carryover storage in the reservoir makes the basin vulnerable to prolonged periods of drought. Water releases from the Chadegan reservoir also show a predictable pattern, with the only deviations occurring during flood events. There is a high reliability of meeting the water requirements during periods of peak demand. But releases during the winter months, at the end of the irrigation season, are lower and more variable. This results in low discharges in the Zayandeh Rud and reduced water quality, especially in the lower reaches of the river. A simple water-balance approach was used to estimate the proportion of return flows in the basin. An average annual value of 30% was obtained, with the magnitude of return flows being particularly important in the lower reaches of the basin. But more investigation, especially including groundwater and water quality aspects, needs to be carried out before a definitive value can be advanced. Given the limited supply of fresh water in the Zayandeh Rud basin, further water resources development and water management improvements can only be envisaged if there is scope for real water savings in the basin. This can be assessed if a basin-wide approach, leading to a good understanding of water use (and reuse) at the farm, system and basin levels, is adopted.