Trend Analysis of Streamflow across the Hamadan Province

Drought and water deficit is a challenge in arid and semi-arid regions which have recently intensified because of climate change. In recent years, the combined effect of climate change and socioeconomic factors exacerbate desertification processes, especially due to lack of water resources in land, wetlands and lakes, which appears in most parts of the country. Climate change is one of the challenges which lead to many environmental consequences. Trend analysis of river flows is an important issue in water resources planning and management and can provide valuable information. Heretofore, a numerous studies used parametric and non-parametric methods to examine the existence of significant trends in hydro-climatic time series. Most of the studies used non-parametric methods for trend analysis and a few studies used linear regression test. The non-parametric methods were used in this study because the non-parametric methods are distribution-free, robust against outliers, and have a higher power for non-normally distributed data. The Mann-Kendall (MK) method (Mann, 1945; Kendall, 1975) is the most commonly used non-parametric method that has recommended for identification of monotonic trends in different hydrologic and climatologic time series by World Meteorological Organization (WMO). The serial dependence between observations should not exist when the original classic MK test used for trend detection. However, in most of the hydro-meteorological time series, significant autocorrelation with different time lags, in addition to lag-1, may exist among observations. In such a situation, application of the classic version of the MK test for trend analysis could yield unreliable results. As some of previous studies showed that the presence of positive auto-correlation overestimates the significance of both positive and negative trends, whereas negative auto-correlation underestimates the significance of both positive and negative trends. The existence of more than one significant auto-correlation among data called as long-term persistence (LTP). To incorporate the LTP behavior in MK test, Hamed (2008) suggested to remove the effect of all significant serial correlation before applying the classic MK test. The surface water is one of the main resource for providing irrigation demand in Hamadan province. However, in recent decays, because of increasing the farm land area, the available surface water resources cannot provide the agricultural demand for water completely, so the farmers have drilled a lot of wells to extract groundwater for irrigation uses. The overexploitation of groundwater led to severe decline of the water table in most parts of the Hamadan province. In this study, the trend of river flows of the Hamadan province was investigated in monthly, seasonal and annual time scales by using Mann-Kendall non-parametric test, after removing the effect of all significant serial correlation. For this purpose, monthly stream flow data of 17 hydrometric stations during 1985 to 2013 were used. The Sen’s slope estimator was used to estimate trend line slope. Also, the abrupt change points in the stream flow time series were detected using the Pettitt test. The results showed that in annual time scale all stations had negative trends, as about half of them were significant at the 10 % level or less. The most severe significant negative trend in 1% level belonged to Bujin station with a Z value of -3.28. At seasonal time scale, the discharges of most rivers were experienced decreasing trend which the summer ranked first. In monthly time scale, among 204 considered series (12*17), only 15 stream flow series showed a significant positive trend (at 10% significance level) and 102 stream flow series have experienced a significant decreasing trend (at 10% significance level) and 87 series had no significant trend. The most significant negative trend of monthly stream flow series belonged to Kooshkabad stations in June with Z value of -4.45. The maximum number of stations with significant negative trend at monthly time scale at the level of 10% or less belonged to April. The highest slope of the trend line for annual time scale belonged to the Aran station, which was equal to 0.36 m3/s/yr. In general, trends of river flows in Hamadan province were statistically negative at 10% level. The results of applying the Pettitt test showed that in most stations, the significant change point in annual stream flow time series were occurred between 1995 and 1999. The results of investigating the trend of precipitation across the Hamadan province reveal that there is no negative trend in precipitation, and it seems the main reason of decreasing stream flow in this province is due to water extraction at the upstream of rivers in recent years. The results of the present study may be used by water resources planners to alter surface water allocations based on the trend of river flows.