Spatial and Temporal Variations of Base Flow Index (BFI) for the Ardabil Province river, Iran

River flow is one of the most important water resources for uses in agriculture, industry and sanitary sectors. In this regard, analysis of temporal trend in river flow is essential in management of water resource. Better understanding of base flow can be used in analysis of river flow pattern and discharge regime and management of water resources in the low flow periods. Water withdrawals alter the natural patterns of stream flow regime and flow conditions. The main aim of the present research is to investigate the spatio-temporal variations of surface flow and base flow over Ardabil Province through 22 river gauge stations considering rainfall variability. Therefore, the daily discharge and rainfall records were analyzed over the period 1989-2012. A common approach to quantify the base flow components of stream flow is to use a graphical hydrograph separation approach, as the discharge data is the only requirement. The stream discharge records are widely available, and therefore, the graphical approach can be employed in different spatial scales. The base flow component of the hydrograph was estimated at a daily time step for the period of record at each station. The amount of surface run off was calculated and the local minima technique was used to separate the base flow contribution and then the base flow index (BFI) values were calculated in each station and were interpolated to detect the spatial variation pattern. The recorded data length were divided into 5-year periods and the variations of base flow and BFI were analyzed and its relationship with total runoff and rainfall amounts was investigated to detect the temporal changes in the base flow amount and base flow index. The results showed that the decreasing changes was dominant through study area. Also relationships of monthly precipitation and base flow amount indicated that the precipitation variations had low impact on the base flow decreasing pattern. Based on the results, the spatial pattern of total runoff, base flow and base flow index follows a similar regime and the highest values in three variables is related to West and South-west parts of the Ardabil Province and the South-East parts of the study area had the lowest values of interpolated amounts. The highest values of base flow index varies from 0.05 to 0.88 in the study area and highest values are concentrated in the West part of the study area. It was determined that the highest values of base flow is related to the amount of 30 cubic meter per second in the Doost Beigloo station. Also, it can be concluded that the maximum of base flow was observed in Gilandeh station. It can be noted that the West parts of the study area is snowmelt-dominated rivers where there is a persistent flow discharge during snowmelt in late spring or early summer and low-flow periods. The impacts of rainfall variations and water uses on the river flow is clearly reflected in the timing and quantity of river flow. It should be noted that the base flow and base flow amount have decreased and the rate of decreasing intensity was more intense in three months of autumn season.