Csáfordi P., Kalicz P., Gribovszki Z. (2013): What did we learn from the ten-years-long sediment dataset from the Rák Brook? In: Gribovszki Z., Hlavčová K., Kalicz P., Kohnová S. (eds.): Catchment processes in regional hydrology: from experiment to modeling in Carpathian drainage basins (CD). Nyugat-magyarországi Egyetem Kiadó, Sopron: 6-1-6-16. WHAT DID WE LEARN FROM THE TEN-YEARS-LONG SEDIMENT DATASET FROM THE RÁK BROOK? by P. Csáfordi (1) , P. Kalicz (1) and Z. Gribovszki (1) (1) Institute of Geomatics and Civil Engineering, Faculty of Forestry, University of West Hungary, Sopron, Hungary (petercsafordi@gmail.com, kaliczp@emk.nyme.hu, zgribo@emk.nyme.hu) ABSTRACT Based on ten-years-long dataset, spatial and temporal variability of suspended sediment concentration and its control factors have been analysed in two small forested catchments of the Sopron Hills. Difference in the sediment fluxes of the catchments can be explained by the geomorphology. Low flow temporal fluctuation at different time-scales may refer to the impact of sediment exhaustion–replenishment since the previous flood event and the freeze-thaw effect determining the in-channel fine material availability. Three types of hysteresis loops have been identified during the flood events which indicate the intra-event variability of suspended sediment transport. Based on the observed data and the developed regression equations, total sediment load was 124.7 tons in the hydrological year 2008-2009. The wide scatter in the data and the insignificant correlation coefficients point out that other stochastic factors influence the sediment load, besides the hydrological, hydro-meteorological and climate variables. Exhaustion of a sediment deposit behind a log jam has been investigated in the Farkas Valley. Since the outwashed sediment deposit had 13% proportion (15.8 tons) to the total annual sediment yield, we can conclude that other processes contribute to the stream sediment sources as well. Keywords: forested catchment, suspended sediment concentration, temporal variability, sediment control factors 1 INTRODUCTION Nowadays sediment transport and soil erosion processes are even more important research issue due to the global climate changes. Precipitation scenarios for Hungary agree that intensity and frequency of extreme precipitation will increase, while the total precipitation amount will decrease. Namely the change of rainfall distribution leads to the increase of drought and heavy rainstorm frequency (Bartholy and Pongrácz, 2007; Gálos et al. 2007; Kis 2011). The intensification of rainfall events accompanies the increase of soil detachment and transported sediment yield as well. Sediment in streams has manifold ecological and economical impacts. Several environmental problems may relate to the sediment such as reduction of the lifetime of water structures, diminishing channel and stream bank stability, decrease of the reservoir capacity and the cross-section of stream channels. Sediment amount can determine the habitat character by the influence over the hydrodynamical conditions. High suspended sediment concentration can impair the irrigation systems, diminish the drinking water quality and directly damage the aquatic organism. Suspended particle-bond substances can lead to the contamination of aquatic ecosystems such as eutrophication caused by the nutrients and toxicity due to contaminants such as metals. Sediment load can also reflect the erosion potential of the area and gives answer whether the land use practices are convenient for soil conservation. Several authors have described that sediment dynamics, especially the suspended form, has significant spatial and temporal variability. Temporal fluctuation occurs over a wide range of time scales, and the suspended sediment yield (SSY) can vary over a number of orders of magnitude at any one discharge (Q) in the same stream. Morehead et al. (2003) listed more reasons, which can lead to the intra-annual variability of SSY. These are the seasonal changes of water sources (rain versus snowmelt), the altering channel morphology due to the changing climatic conditions, variability of the sediment supply processes and the unstable availability of the fine material in the channel. Bronsdon and Naden (2000) have observed that monthly fluctuations of SSY can also be influenced by the diatom growth and death. Duvert et al. (2010) found, the analysis of sub-daily (or inter-event) variability of sediment fluxes in small mountainous catchments is inevitably necessary for the accurate calculation of annual SSY. They reported about strong bias obtained on annual SSY estimation based on daily sampling due to the very short hydrologic response (1-3 h) of the small catchments (3-12 km 2 ). Most of the sediment can be transported during short-time periods. It is not a specific case when the 15% of total Q transfers the 50% of the total SSY in 2% of the