Trends in precipitation and stream flow in the semi-arid region of Atrak River Basin, North Khorasan, Iran

In this paper trends of precipitation and stream flow are analyzed in the Atrak River basin in the North Khorasan province. Normal score linear regression, Mann-Kendall, and Seasonal Kendall trend tests were adopted. Three precipitation variables and two stream flow variables including total precipitation, maximum daily precipitation, and number of rainy days, mean discharge and peak discharge were studied. Annual, seasonal, and monthly trends of these variables were examined during a 35-year period starting from 1971 in six hydroclimatological stations. The results with all tests showed that despite no evidence of significant trend for the precipitation variables, trends for the hydrologic variables found to be significant. Whereas for most of the stations significant downward trend was observed for mean discharge, and for one third of the stations significant upward trend was observed for peak discharge. The results of seasonal trend analyses indicated that downward trends in the mean discharge are almost completely homogeneous across the seasons and significant for autumn, winter and spring flows in most of stations. Overall, although the precipitation pattern is considered as the major driving variable for the river discharge regime, but it is not the only possible cause and other causes such as landscape, land use changes and increased evapotranspiration due to higher temperature may play a role