DIGITALNA ARHIVA ŠUMARSKOG LISTA
prilagođeno pretraživanje po punom tekstu
| ŠUMARSKI LIST 5-6/2021 str. 48 <-- 48 --> PDF |
these factors are combined with other factors, the results obtained may vary from region to region (Wang et al., 2018). Therefore, for effective watershed management planning and sustainable water resources management, further studies are needed to determine how various factors affect the water and sediment yield of the watershed (Yan et al., 2013). Among land use types, forests are known to play a very important role in the hydrological processes of watersheds (Li et al., 2017; Shi et al., 2018). Studies on watersheds in various countries (Swank et al., 2001; Siriwardena et al., 2006; Serengil et al., 2007; Ganatsios et al., 2010; Wei and Zhang, 2010; Nadal-Romero et al., 2016; Yu et al., 2019), review articles (Bosch and Hewlett, 1982; Brown et al., 2005; Cui et al., 2012; Li et al., 2017; Zhang et al., 2017), and hydrological model simulations (Yu et al., 2010; Can et al., 2015) have shown that treatments in forests such as clear cutting, thinning, and changing the land use affect water yield positively, whereas afforestation has a negative effect. Another study investigating the contribution of different vegetation types to the watershed water yield determined that forests provided the lowest contribution to water yield (Liu et al., 2011; He et al., 2012). The hydrological responses of watersheds to increasing or decreasing of forest areas may not be the same. In fact, some researchers working on this subject in different countries have reached a common conclusion that the response of watersheds to forest change is quite variable and that watershed characteristics differ as well (Chen et al., 2010; Nippgen et al., 2011; Price, 2011; Zhang and Wei, 2014; Karlsen et al., 2016; Liu et al., 2016; Duan and Cai, 2018; Li et al., 2018). Warburton et al. (2012), who investigated the hydrological impacts of land use change, reported that the contribution of land use to watershed streamflow is not proportional to the land use area and suggested that the location of the changing land use in the watershed may affect the response on the streamflow. Therefore, it is not correct to make generalizations because hydrological process are complex and results may vary from watershed to watershed (Zhang and Wei, 2014). It has been stated in watershed scale studies that compared to forest land, agricultural land use causes an increase in water yield; in other words, the rate of water loss in forested areas is higher than that of agricultural land (Zhang et al., 2010; Lana-Renault et al., 2011; Yan et al., 2013; Dias et al., 2015; Shi et al., 2018). Contrary to these studies, Rientjes et al. (2010) found a decrease in the water yield although it had been 32 years since the forested land in the watershed had been turned into agricultural lands. They maintained that the reason for this was that the decrease in annual rainfall amount and the change in the distribution of dry and wet season rainfall. Chow et al. (2011) found that the average annual water yield of forested watersheds was higher than that of agricultural watersheds and that this may have been related to the soil and water conservation measures applied in agricultural areas. Sriwongsitanon and Taesombat (2011) found that as the percentage of forest increased, the runoff coefficient between the watersheds increased for large flood events, and decreased for small flood events, and they associated this with the antecedent moisture content of the forest soils being higher. In studies comparing the hydrological relationships in different watersheds, the effects of the watershed characteristics are striking. In a study conducted in 23 watersheds, Zhang et al. (2014) found a positive correlation between the runoff coefficient and the slope and a negative correlation between the runoff coefficent and proportion of carbonate rock. Wang et al. (2018) found that the surface flow, base flow, and streamflow of the steep sloped forested watershed were higher than of flat sloped agricultural watershed. However, in 29 watersheds, Zengin et al. (2017) found no significant relationship between the runoff coefficient and slope and some other watershed characteristics. Peng and Wang (2012) reported that the surface flow and soil loss of karstic slopes were lower than in non-karstic slopes. The factors that have a significant impact on the sediment yield of watersheds are also quite diverse and are known to vary region to region. In order to understand the relationship between these factors and sediment yield, a large number of measurements for different precipitation-streamflow events and seasons are needed over a long period of time (Nearing et al., 2007; Sadeghi et al., 2008). Therefore, as they vary greatly depending on time and site, watersheds sediment yields have a complex process (Nu-Fang et al., 2011; Bywater-Reyes et al., 2018; Ebabu et al., 2018). Agricultural and forested areas have a significant effect on the total suspended sediment (TSS) carried from the watersheds. Many studies have found a positive relationship between TSS and agricultural areas and a negative relationship between TSS and forest areas (Ahearn et al., 2005; Yan et al., 2013; Kibena et al., 2014). Chow et al. (2011) found that the annual average suspended sediment yield increased from watersheds with low agricultural intensity to those with high agricultural intensity. Erdoğan et al. (2018) determined that the average monthly suspended sediment concentration in streamwater increased after 18% forest thinning. Turkey ranks first among the few countries in the world that produce hazelnuts and the country’s largest hazelnut production areas are located in the Black Sea region. The research watersheds consist of two adjacent subwatersheds of the Big Melen in the Western Black Sea Region. The Big Melen Stream is the most important source of water for Istanbul, one of the most populous metropolitan areas in |