Document Type : Research Paper
Authors
Associate Professor, Soil Conservation and Watershed Management Research Institute, Agricultural Research, Education and Extension Organization, Tehran, Iran
Abstract
Introduction
Baseflow, as the delayed portion of river flow, mainly supplied from subsurface water and groundwater resources, plays a key role in river flow sustainability, water resources management, aquatic ecosystem protection, and drought adaptation planning. The base flow index (BFI), which is the ratio of base flow volume to the total streamflow volume, is a dimensionless measure for assessing the contribution of subsurface water to stream flow. Global studies show that, on average, more than half of river flow is provided by base flow, and BFI depends more on the inherent characteristics of the basin, such as geology, soil, topography, and morphology, than on precipitation. Despite numerous studies on baseflow estimation in Iran, limited attention has been paid to investigating its temporal changes and long-term trends at different time scales. Therefore, the aim of this study is to investigate changes in BFI on monthly, seasonal, and annual time scales, as well as analyze the long-term trend and the effect of human interventions in Barun Chay river basin.
Materials and methods
The study area is Barun Chay river basin with an area of 1025 km2, located in the Aras Basin in West Azerbaijan Province (Iran). This basin has a significant diversity in terms of permeability of geological formations, such that a major part of its surface is formed by geological formations with low and very low permeability. In this study, daily stream flow data from the Qale-Jogh hydrometric station of Barun Chay river basin were used for the period of 1976 to 2017. After extracting the physiographic characteristics of the basin using a geographic information system, the base flow was separated from the total flow hydrograph using the One Parameter Recursive Digital Filter technique and the BFI extension in the Hydro Office,2015, software. Then, time series of base flow and BFI were prepared on monthly, seasonal and annual scales. Descriptive statistical analysis and trend analysis were performed using the non-parametric Mann-Kendall method. Also, the effect of dam construction upstream of the hydrometric station on BFI was evaluated by comparing the periods before and after dam construction.
Results and discussion
The results showed that the long-term average of the annual BFI over a 41-year period was 0.552, indicating a more than 50 percent contribution of subsurface flow and groundwater to the river flow. On a seasonal scale, the highest value of BFI belongs to the winter and summer seasons and the lowest to the spring season. On a monthly scale, the maximum long-term BFI was observed in January and its minimum in May. Analysis of monthly changes in BFI showed that it had an increasing trend from the October to January and a decreasing trend from January to May, which coincided with the change in the precipitation regime and the delay in the participation of subsurface flows to stream flow. From May to September, with the decrease in precipitation, contribution of delayed flows increased and BFI showed an increasing trend again. Trend analysis of the long-term BFI using the Mann-Kendall test indicates that in most months, seasons and annual scales, the trend were negative and only in June and October an increasing trend was observed. Comparison of the annual average BFI in the periods before and after dam construction showed that the long-term average of BFI increased from 0.542 to 0.562, and the effect of human interventions resulting from dam construction on the annual BFI was estimated to be about 0.02.
Conclusion
The Overall results indicate the dominant role of subsurface flows in providing Barun Chay river flow and the greater dependence of BFI on the inherent characteristics of the basin than on precipitation changes. Understanding the temporal pattern and long-term trend of base flow can provide an effective basis for sustainable water resources management, dam operation planning, and developing drought adaptation strategies in similar watersheds. After determining BFI at different times and locations, and preparing a map of sustainable flows at regional and national scales, the initial prerequisite for developing plans appropriate to the hydrological capacity of each region will be achieved.
Keywords
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