The investigation of stream f low changes in Chaharmahal- Bakhtiari Rivers

Introduction Climate change will affect the availability of water resources and the sustainability of their management. The impact of climate change on water resources has caused concerns and serious challenges around the world. Global climate change has decreased water resources in many regions and especially in arid and semi arid regions such as Iran. Water shortages decrease both agricultural production and food security and can also limit the economic development and ecosystem health. Also, there is growing evidence that climate change is changing the hydrological cycle.
Chaharmahal- Bakhtiari is the main supplier of water for Khuzestan and Isfahan provinces and inter-basin water transfer talk has always been for these two provinces. However, there is not a comprehensive study on river discharge changes in this province and we cannot judge about the future water transfer. In the Iran, there have been several studies about the seasonal changes in precipitation and other climatic variables but assessments of river ows are limited. Therefore, the purpose of this research is to document statistically significant trends in seasonal and annual changes in stream flow in the main rivers in Chaharmahal-Bakhtiari.
Materials&Methods The total monthly and seasonal streamflow time series for the period 1358–1389 (32 years) corresponding to 12 hydrometric stations across the Chaharmahal- Bakhtiari province are obtained from the Chaharmahal- Bakhtiari Regional Water Organization. This study is done with latest data (1389) published by Chaharmahal- Bakhtiari Regional Water Organization. At the first, the linear trends are determined on annual and seasonal timescales, using Mann–Kendall (MK) non-parametric trend tests.The non-parametric Mann-Kendall tests are based on the calculation of Kendall's tau between two samples, which is itself based on the ranks with the samples. This test have been widely used to detect trends in series of environmental data, climate data or hydrological data, because it is less sensitive to the non-normality of the distribution and less affected by extreme values or outliers in the series.Also, the magnitude of trend, if a linear trend is present in a timeseries, is estimated by using the non-parametric Sen’ estimator test. The Sen’ estimator method is commonly used for calculating the trend slope. Thus, in this study this method is also used to calculate the trend slopes to obtain the extent of trend changes.
A number of methods can be applied to determine change points of a time series. In this study, the change point in the stream flow time series in 12 studied hydrometric stations is identified by Pettitt’s test. This test is a non-parametric approach for detecting change points based on the mann-whitney test. It has been demonstrated that the Pettitt’s test is a useful technique for examining the occurrence of abrupt changes in climatic records.
Results&Discussion The results of the non-parametric Mann-Kendall tests (MK) for all studied rivers through the Chaharmahal- Bakhtiari province showed a decreasing trend in the stream flow in all 12 hydrometric stations for both seasonal and annual scale. The amount of reduction based on the Sen’ estimator method, varies at different stations and seasons, from 0.8 m3/s to 17 m3/s in Gordbysheh and Beheshtabad, respectively. On seasonal scale the results indicate a significant reduction in stream flow of all stations in summer, while in winter, except at two stations (Deh- Cheshmh and Soulegan) significant trend was not observed. The results of Pettitt's test also suggesting, the abrupt downward change (statistically significant) in most studied stations occurs in the early 1370s. On the other words, in two recent decades a significant decreasing trend in stream flow is occurred in Karon and Zayandeh Rood Basin. The results also showed that in addition to the mean of stream flow, the extreme values of stream flow in these revers also experience a very sharp decries during the studied period.
Conclusion Decreasing trend in all studied hydrometric stations through the Chaharmahal- Bakhtiari province clearly shows the reduction of water resources in this province and also the reduction of the role of the province in water supply to the country. The abrupt downward change that occurs in the early 1370s in most studied hydrometric stations is also show that the reduction of water production in the province have started from about 20 years ago. Therefore, for any water transfer plan from this province to other regions the practical principles should be considered. Another important issue that must be considers is that the water resources in Chaharmahal- Bakhtiari province is sharply decreasing and the water plants should not be based on the old information.