Investigation and estimation of flow duration curve indices, based on lithological and hydro-climatological factors, case study: Khazar region

The flow duration curve (FDC) is one of the most important and widely used signatures of catchment runoff response. It has been used in numerous hydrological applications as a part of planning activities related to agriculture, industry, urban water supply, navigation, flood control, reservoir and sedimentation studies, in-stream flow assessment, water quality management, and impacts of land use changes. The FDC is a graph of river discharge plotted against exceedance frequency and is normally derived from the complete time series of recorded river flows. Representation of the relationship between the frequency and magnitude of stream flows, making it a compact signature of a catchment’s functioning; it can be used to diagnose the rainfall-runoff responses in gauged catchments at a holistic functional level, as well as to regionalize them to ungauged catchments. For these reasons, in the past few decades, considerable effort has been expended towards detailed studies of FDCs, especially in the context of predictions in ungauged basins (Smakhtin, 1998, 2001, Castellarina, 2004, Niadas, 2005, Santhi, 2008. Li, 2010). However, most recent studies on the FDCs have been empirically based, which generally fall into two categories: graphical and statistical. The graphical approach focuses on exploring the controls of catchment climatic and physiographic characteristics on the shape of the FDC (Smakhtin et al), while the statistical approach employs statistical distributions to fit the FDC and then relates the parameters of the distribution to the catchment’s physical characteristics (Castellarin et al., 2004; Li et al., 2010). Several studies have been conducted on the impact of geometric and hydrologic parameters on the FDC. (Booker and Snelder, 2012. Brown, 2013). In unregulated streams, the shape of FDC is controlled chiefly by the geology of the basin and research on the effects of lithology on FDC in a certain area, leading to obtaining information for use in water resources management (Bloomfield, 2009).
Although both graphical and statistical methods demonstrated that different climatic and landscape characteristics impact the shape of the FDCs in different regions of the world (Li et al., 2010), it has been difficult to generalize the results from one region to another.
The aim of this research was to determine the factors affecting the flow duration curve with emphasis on the role of lithology in the estimation of flow duration indices.
The study area comprises 18 basins with areas from 69.1 to 3941 km2 which are located in the North part of Iran and is underlain mostly by rocks belonging to the hard formations. Carbonate formations in this area are appropriate distribution and cover about 28% of the studied area. Mean annual rainfall varies from 427 to 1131 mm/ year. Median slopes vary from 2.85 to 15.9%.
In this research, using data from the daily flow of rivers, flow duration curve indices, in eighteen hydrometric stations of the Caspian Basin were determined. Physiographic parameters extracted from the GIS environment and the various factors of climate, geology and land use were also calculated. Lithological units using digital geological map, with a scale of 1: 250,000 and based on expert opinions were divided into 4 classes and the area covered by each unit was calculated. Land use map based on satellite images with the division of forest, pasture, agriculture, surface water resources, and rock outcrops were prepared. Factor analysis and cluster analysis were conducted and the regression equations were derived. The accuracy of models using the coefficient of determination and standard errors were investigated. Finally, the most appropriate models in 1% and 5%, significant of levels extracted.
Factor analysis of 21 variables showed, six factors were chosen as the most important independent factors. In the multivariate regressions between agents of geologic formations, particularly carbonate hard formation and alluvial and between morphometric parameters, slope, and main river length and from land use classes, agricultural land use was determined as the best predictor of duration curve indices. Also, according to the power and exponential models, annual precipitation as the most important factors between climatologically factors was diagnosed. The validation results showed that the distributions of errors in all models were normalized and Durbin-Watson index was varied between 1.5 to 2.5.According to regional relations obtained, the contribution of hard carbonate formations along with Quaternary deposits, indicating their role in ensuring the continuity of the region's rivers.