Comparison of the effects of land use change and precipitation on annual discharge (case study: Kiar watershed)

The development of agricultural and urban areas and overexploitation of natural resources have caused significant changes in the hydrological regime of watersheds. Furthermore, population increase, unprincipled exploitations and climate changes have intensified this process. It is therefore necessary to know and study it continuously. The trend of hydrological series may also change due to climate change, land use and watershed management. Therefore, identifying the presence or absence of changes in time in rainfall and discharges of an area and presenting it in statistical analysis seems necessary. The Mann-Kendall test is one of the most widely used nonparametric methods for analyzing the flow of such data. Studying periods of rainfall and discharge as well as land use change can be useful in expressing the role of climatic and human factors. Therefore, the present study was conducted to investigate the trend of land use change and the role of each in the discharge trend of Kiar watershed. 

The present study was conducted in Kiar watershed located in Chaharmahal and Bakhtiari province as one of the northern sub-basins of Karun River. In order to conduct the current research and achieve the desired goals, after obtaining the information and data required for the research from the Chaharmahal and Bakhtiari Regional Water Organization, a joint statistical course of study stations was determined. Finally, the mentioned period was selected from 1363-1390. Then, Mann-Kendall non-parametric test was used to investigate the trend of changes in precipitation and discharge of stations in Kiar watershed. Then, land use changes in the study area in three time periods were extracted using the object-oriented classification method in IDRISI Taiga 16.03 software environment using Landsat's TM sensor images. Using the parameters of overall accuracy and kappa coefficient, the accuracy of the classification was calculated. Finally, the trend of land use change and its relationship with the trend of discharge were evaluated.

The results of trend analysis for the Dey River time series (on seasonal and annual scales) at the Burnt Mountain Station show that discharge has had a decreasing trend on both scales. The highest discharge trend in the seasonal series belongs to summer and the lowest discharge trend belongs to fall at the significant level of 1%. The study of trends in the rainfall time series of the region shows a different behavior. While the trend in Borujen station was upward, Shahrekord and Beheshtabad exhibited a downward trend, and no trend was found in Farsan station. The decreasing trend of discharge and precipitation in Kuh-e-Sokhteh hydrometric station and rain gauge stations in Shahrekord and Beheshtabad are on the same side, which can be attributed to the decrease in discharge due to the rainfall behavior of the region. The results of the trend of land use change showed that residential and agricultural lands have an increasing trend and rangeland and barren lands have a decreasing trend. Increasing the level of agriculture and residential areas can be a reason for reducing the trend in discharge.

Based on the results of the present study, the results of trend analysis for the time series of the river at the burnt mountain station showed that the discharge trend in seasonal and annual scales is Downward. The highest discharge trend in the seasonal series belongs to the summer season and the lowest discharge trend belongs to the autumn season. Also, the results of studying the trend in the time series of rainfall in the region showed different behaviors. While, the trend in Borujen station was upward, Shahrekord and Beheshtabad exhibited a downward trend, and no trend was found in Farsan station.