Trend and change-point detection for the annual stream-flow series of the Karun River at the Ahvaz hydrometric station

Abstract

Karun River watershed located in southwest of Iran experiences great changes in the last decade due to hydro climatic changes, large-scale land use and land cover changes, construction of dams and agricultural and industrial development. Stream-flow series of the annual maximum, annual minimum and annual mean in Ahvaz hydrometric station as representative of the watershed have been considered. In order to determine the change points, we use the Pettitt test while the trend in the annual stream-flow series of this station is evaluated by the Mann-Kendall test. However, the Pettitt test cannot detect any change point in these series, but the Mann-Kendall test shows that there is an increasing trend. This increasing trend has no fitness with observed data in recent decade. Therefore, the potential change points were determined and consequently, these series converted to two sub-series based on the results obtained from the Pettitt test, wide variation from human activities and variation of hydro climatic conditions. We then applied the Mann-Kendall test in these two sub-series which showed significant decreasing trend in both of them. From this method, it can be concluded that the averages of the second sub-series are larger than the first one in the three annual stream-flow series. This increasing ratio is equal to 1.34, 1.16 and 1.2 for stream-flow series of annual maximum, annual minimum and annual mean respectively. These changes are similar to changes in annual rainfall time series. We can say that the sudden increase of precipitation in recent decades makes the averages of the second sub-series larger. Results showed that inspite of wide range human induced changes, stream-flow series is more affected from the watershed rainfall time series variations. In addition, significant decreasing trend in the two sub-series can be due to the following causes: the growth of population, temperature rise, construction of three dams on the Karun River, implementing the different projects for developing agricultural, and conservation from soil and water resources. Key words: Pettit test, Mann-kendall test, annual stream-flow series, Ahvaz hydrometric station.