Assessment of drought and ENSO impact on surface water changes in Zanjan and Hamedan provinces

Climate change is one of challenging subjects that affect the vast majority of human life. It can influence the quality and quantity of surface water resources in particular rivers. Furthermore, El-Niño Southern Oscillation (ENSO) is considered as a climatic phenomenon that has significant effect on weather, climatic and hydrology changes. The impact of ENSO on discharge of rivers has been assessed in several investigations. Also, drought is known as a natural phenomenon influencing hydrological parameters such as surface water flow. Due these assessments, the main purpose of this survey is evaluating the impact of ENSO and drought indices on surface water changes.

Effect of ENSO and drought on discharge of hydrometric stations in Hamedan and Zanjan provinces during a 32-year period were assessed. For this purpose the standard precipitation index (SPI) and 9 ENSO indexes including SOI, PNA, MEI, NINO3, NINO4, NINO1+2, NINO3.4, TNI and  SST from 1981 till 2012 were evaluated.
Among existing drought indices, SPI is likely the simplest drought index because it can be compared across different climatic regions. It is designed to be a spatially invariant quantity that can be computed, to provide precipitation excesses and deficits during multiple timescales. The value of SPI was determined based on the following equation in DIP software:                                                                                                 (1)
Where, : amount of precipitation in time t, : average of long term precipitation, SD: standard deviation of data. 

Comparison of different ENSO indices affecting discharge changes, indicated that maximum correlation belonged to South Oscillation Index (SOI). In the second step, the average discharge of hydrometric stations in each phase of ENSO was evaluated.
According to the obtained result, it can be inferred that the El Nino has led to an increase in the discharge of both stations. The highest increase belonged to Vasaj station, by 345 percent. This value is in line with the report of Nazemosadat et al. (2006). Assessment of impact of drought on discharge of each station indicated that the impact of SPI on discharge changes is different in each station. So that in dry years, the amount of discharge of Bahador-Beyg and Tagsimab has decreased, in comparison to an increase in other stations. One of the reasons can be that the effect of non-precipitation parameters such as plant cover and geology on hydrologic changes.

In general, effect of ENSO on discharge changes was more than drought effect in a way which the average percentage of stations with significant correlation with SOI in Hamedan and Zanjan provinces were 36.5 and 42.42 %., respectively. On the contrary, the average percentage of stations with significant correlation with SPI in Hamedan and Zanjan were 24.5 and 24.83 %, respectively. Also, El Nino phase effect led to increasing discharge of hydrometric stations.