Assessing the effects of mountainous Bandsars on river flow regime and downstream water resources, Case study: Marandiz river basin, Razavi Khorasan province, Iran

In the arid regions of Iran, the construction of Bandsar has been one of the methods of runoff harvesting. However, despite the large extent of these structures in some mountainous catchments, so far, no study has been done in Iran on the effects of mountainous Bandsars on the river flow regime and other components of the water cycle in a catchment. In the present study, the effects of mountainous Bandsars on the Marandiz river, located in the southwest of Razavi Khorasan province, and its effects on downstream water resources have been investigated. Based on field studies and satellite images more than 150 Bandsars were identified in the study area. The area of agricultural lands behind the Bandsars was determined based on satellite images, which is equal to 4.8 km2 (2.2% of the total area of the river basin). In order to estimate the runoff from rainfall on a daily basis, the Curve Number Method (SCS) was used for the period 1998-2018. The average annual runoff volume in the Marandiz river basin was estimated about 2.6 MCM and the average annual runoff coefficient was estimated as 12.4%. The minimum and maximum storage capacity of the Bandsars (SCB)were estimated to be 0.95 to 1.43 MCM, respectively. If the daily runoff volume is more than SCB, the flood can reach the watershed-outflow point and enter the downstream plain. The results showed that a maximum and minimum numbers of floods reached the watershed-outflow point were 14 (5% of days with runoff) and five floods (2% of days with runoff) during the 15-year period. Therefore, the mountainous Bandsars in the Marandiz river basin have been reduced significantly the floods entering the downstream plain which has resulted in a decrease in the downstream alluvial aquifer recharge and increasing the groundwater salinity.