Trend Analysis of Gradual and Rapid Variations of Hydro-Climatological Factors ( Case study: Lighvanchai Basin)

In this research, gradual and rapid changes of hydro-climatological variables, trends were analyzed for the Lighvanchai basin. Also, the natural (stationary) and impacted periods were identified. For this purpose, the traditional Mann-Kendall (MK1), modified (MK3) and Pettitt methods were used and for determination of the trend line slope the Senʼs estimator was used. Results showed that streamflow values of Lighvanchai River (in Hervy station) had a downward trend in the all time scales. In the spring, all precipitation related variables showed a decreasing trend. In the case of temperature variables, there was an increasing trend at the most of the studied time scales, which were significant from January to  May. Furthermore, the differences between Tmax and Tmin showed an increasing trend. On the other hand, air relative humidity time series showed a decreasing trend (specially in winter). It could be concluded that the combination of these changes led to decreasing trend in basin runoff. The result of Pettitt test showed that  the streamflow of  Lighvanchai (in Hervy) had a sudden change point, so that the average discharge in the recent period had been decreased about 35% compared to the first period. Most of studied climatic variables such as air temperature and RH, had a sudden change point that the date changes of these variables and the basin discharges (at Hervy station) were occurred in the mid-1990s. Regarding the discharges of Lighvan station (upstream of basin) the null hypothesis of abrupt change point were rejected. Therefore, along with the effects of climatic factors the effectiveness of the human factors on reducing the runoff in the basin outlet at Hervy station was confirmed.