Statistical Analysis of Discharge Fluctuations in a Semiarid Basin Using Effective Atmospheric Teleconnections: Dez River Basin in Iran

This paper investigated the variability of freshwater discharge ($Q$) in the Dez River Basin in Iran and its possible links to the North Atlantic Oscillation (NAO) and El Niño Southern Oscillation (ENSO). Nonparametric statistical tests were applied to analyze fluctuations of discharge and teleconnection indexes. The results indicated statistically significant monotonic decreases of the discharge and peak flow ($p$-value $<0.05$) across most of the subbasins. Furthermore, a downward step change was detected in discharge time series around the mid-1990s from 14.7% to 43.3% with respect to long-term values. Similarly, almost concurrent shifts detected in the Oceanic Niño Index (ONI) (from $+0.254$ to $-0.416$ in 1998) and the NAO (from $+0.196$ to $-0.155$ in 1995) that suggest changing conditions toward more cold phases of ENSO and negative phases of NAO coincided with discharge and monthly peak flow depletion. Comparing the amount of discharges during different phases showed that $Q_{(\mathrm{El}\mathrm{Ni}\tilde{\mathrm{n}}\mathrm{o})}>Q_{(\mathrm{neutral})}>Q_{(\mathrm{La}\mathrm{Ni}\tilde{\mathrm{n}}\mathrm{a})}$ and $Q_{(\mathrm{NAO}+)}>Q_{(\mathrm{NAO}-)}$. Furthermore, the Mann-Whitney U test verified statistically significant differences ($p$-value $<0.05$) between flows pertaining to different phases of both ENSO and NAO. Correlations between discharge and teleconnection indexes indicated that discharge is apparently affected by teleconnections considering the lag time.