TREND ANALYSIS OF SEDIMENT YIELDS IN RELATION WITH ANNUAL PRECIPITATION AND DISCHARGE IN THE MAIN TRIBUTARIES OF THE TIREH RIVER, LORESTAN PROVINCE, IRAN

activities and climate change have dramatically affected discharge and sediment regime of the rivers in the past half century. Detection of these changes in time series is usually carried out using trend analysis tests. Literature review indicated that there is little information on the sediment discharge trends across the Iran. Therefore, in this research the sediment yield trend in the main tributaries of the Tireh River in the Lorestan province and its likely relationship with variation in annual time series of precipitation and discharge have been investigated during the period of 1975 through 2004. To this end, the parametric test of simple linear regression and the non-parametric test of Mann-Kendall have been applied on hydro-climatological timeseries of the study area using the R statistical package (Version 2.6.0.). The results of both tests indicated that despite no significant decreasing trend in the main tributaries of the Tireh River basin, a significant decreasing trend has been observed in the outlet of whole basin for both discharge and sediment yield. However, no significant decreasing trend for the annual precipitation of the basin has been observed and no major dam or reservoirs have been constructed within the basin. Therefore, other factors were considered which may have influenced the discharge and sediment yield regime of the basin. It appeared that expansions of the agricultural lands, decline in the water table which in turn increase the initial abstraction and recharge rate of hydrologic components, as well as the implementation of bio-mechanical soil and water conservation practices on hillslopes and check dams across the headwater courses are the main causes of decreasing trends of discharge and sediment yield in the Tireh River basin of the Lorestan province.