Investigation of relationship between base flow index and flow duration curve indices at national scale in Iran

Investigation:

 of relationship between base flow index and flow duration curve indices at national scale in Iran Rahim Kazemi1 *Jahangir porhemmat2, Forood Sharifi3 1Assistant Prof., Soil Conservation and Watershed Management Research Institute, Agricultural Research, Education and Extension Organization, Tehran, Iran 2,3 Prof., Soil Conservation and Watershed Management Research Institute, Agricultural Research, Education and Extension Organization, Tehran, Iran Abstract:

Recognizing and understanding the relationship between the different components of the catchments can help improvement and development of predictions in ungagged catchments. The true value of the base flow is unclear, and since the flow duration curve (FDC) is generated using observational data, therefore recognizing and analyzing the relationships between FDC and the base flow index (BFI) leads to achieving information for optimal use of FDC indices as the estimating parameter of BFI. The purpose of this study is to investigate and identify the relationship between BFI and FDC indices in catchments of different climates of Iran.

First, by preparing the climate map of the country and its intersection with the border of the 4th order of watersheds, the catchments located in each climatic zone were separated. Then, at least 30 stations with appropriate statistics and the common period of 1976-2011 were selected in each climatic zone. The FDC was prepared using long-term daily stream flow data, and indices of, Q2, Q5, Q10, Q15, Q20, Q50, Q75, Q90, extracted. The index of the last inflection point of FDC at the zero slope of FDC was extracted using coding in MATLAB. Then BFI was calculated using one parameter recursive digital filtering algorithm and using long-term daily stream flow data. Finally, regression relationships between the average annual FDC indices and BFI in different climatic zones were computed and analyzed.

The results showed that the highest correlation between FDC and BFI in catchments of very humid region with a coefficient of determination of 0.84 was related to first part of FDC, but in both humid and semi-humid regions, the highest correlation were related to the last part of FDC with 0.63 and 0.69 coefficient of determination. The highest coefficient of determination (0.85) between FDC index at the zero slope point (QFinal) and the BFI were related to the catchments of very humid region. Correlation between end-of of FDC indices with BFI in catchments of humid, semi-humid, Mediterranean and semi-arid climate zones were strong relation and could be recommended for regional analysis, forecasting and estimation purposes in ungauged catchment. However, in the catchments of the arid region, this relationship was exceptional, and indices of first part of FDC play this role. In very humid region, the coefficient of determination of FDC indices with BFI was reliable and usable.

In overall conclusion, it is noteworthy that the correlation between the endpoint FDC indices with the average annual BFI in the humid, semi-humid, Mediterranean and semi-arid climate zones were strong, reliable and recommended for regional analysis, forecasting and estimation in ungagged catchments. However, in the catchments of the arid region, this relationship was exceptional, and first part of FDC indices play this role, and have the highest correlation. In catchments located in very humid region, the coefficient of determination of all indices with the BFI has the ability to be trusted and used. The overall results in the catchments of all climatic zones show FDC indices as a reliable and predictive parameter of BFI.