Investigation of the relationship between base flow index with temperature and rainfall using wavelet coherence (Case study: Gorganroud watershed)

Climate change has changed meteorological parameters in different parts of the world. Drought is a problem that may lead to crisis in many countries. Therefore, the study of meteorological parameters changes and their effect on river flow is important which an important factor in supplying water supply is. The purpose of this study was to investigate the relationship between temperature and rainfall parameters with base flow index using continuous wavelet transform and wavelet coherence in Gorganroud river. At first, the base flow was determined using Eckhart's two-parameter method and then the base flow index was calculated. The base flow index indicates the portion of groundwater in river surface flow, with lowest annual index in Taghiabad station with a value of 0.30 and the highest of its in Tamer station with a value of 0.66. Then, the annual trend of temperature and rainfall was investigated with base flow index of six stations (Tamer, Lazoreh, Nodeh, Arzakkuseh, Sedegorgan and Taghi Abad) in Gorganroud watershed during a period of 33 years (1981 to 2013) using continuous wavelet transform and wavelet Coherence. The analysis of annual temperature and precipitation trend with base flow index showed that two parameters of temperature and precipitation affect the base flow index. Investigation of Coherence intensity between rainfall and base flow index in stations showed that there is the highest correlation between 1-4 years, which indirect correlation was observed in the middle years of Sadegorgan and Taghiabad stations and direct in other stations and the early and the end years of Taqiabad station. There are also correlations between the rainfall and the base flow index in the 4-8 year period, which are direct in Lazooreh and Nodeh and indirect in Tamer stations. Also, in the period of 8 to 10 years, the most correlation was found in Arzokesh and Nodeh stations with less intensity in the Sadgegran station, which arrows direction indicated direct relationship. The analysis of the intensity of coherence between temperature and base flow index showed that direct and high intensity correlation in Sadgegorgan stations between 2005 to 2009 in 3-4 year period, Lazorah between 2002 to 2006 with 4 year period and Taghiabad between 1984 to 1989 with period of 1 to 5 years. Indirect correlations was found in Nodeh and Tamer stations between 1987 to 1990 and 2011 to 2013, respectively, with a short 2-year and 3-year period and Taqi-Abad stations between 1993 to 2000 and between 1992 to 2013 with a long-term period of 8 to 10 years. The rainfall relationship with the base flow index can be direct or indirect due to the condition of the stations. Also, in some stations there is a direct relationship in the periods and indirect relationship in the rest of the period. The relationship between the base flow index and temperature is also indirect in some of the stations.