The trend of river discharge changes in arid and semi-arid regions is mostly decreasing and this trend can be influenced by climatic factors and rising temperatures in recent decades in these areas. This reduction in discharge could have a significant impact on the water storage of the Nahrin Dam and agriculture, industry and drinking water in Tabas City, which should be considered by experts. The aim of this study was to determine the trend of discharge, precipitation and temperature in the watershed of Nahrin Tabas Dam using Mann-Kendall method, Sen's slope and linear regression. After data investigation and validation the data, the statistical period of 1987 - 2017 was selected. The results of all methods showed that the discharge decreased in all scales of annually, seasonally, and monthly. Precipitation has also decreased and in this 31-year period, it has annualy decreased by 0.8 mm, in average. The temperature has been increasing and on average, it has warmed up to about 1.5 °C during this period. The declining trend of river discharge has been due to climatic factors such as drought and rising temperatures. So that, according to the standardized precipitation index (SPI), the region suffers from severe drought in the last years. There is no water extraction for agriculture and industry above the hydrometric station, and the land use of the region has not changed. Therefore, declining trend of river discharge reduces  the agriculture and other exploitations at the downstream of the Nahrin Tabas Dam, which already exists.