SWAT modeling of best management practices for Chungju dam watershed in South Korea under future climate change scenarios

Suitable and practicable best management practices (BMPs) need to be developed due to steadily increasing agricultural land development, intensified fertilization practices, and increased soil erosion and pollutant loads from cultivated areas. The soil and water assessment tool model was used to evaluate the present and future proper BMP scenarios for Chungju dam watershed (6,642 km²) of South Korea, which includes rice paddy and upland crop areas. The present (1981–2010) and future (2040s and 2080s) BMPs of streambank stabilization, building recharge structures, conservation tillage, and terrace and contour farming were examined individually in terms of reducing nonpoint source pollution loads by applying MIROC3.2 HiRes A1B and B1 scenarios. Streambank stabilization achieved the highest reductions in sediment and T-N, and slope terracing was a highly effective BMP for sediment and T-P removal in both present and future climate conditions.     

Mediating effect of stream geometry on the relationship between urban land use and biological index

Numerous studies have stated that land use in urbanized areas can adversely affect water quality of streams. Previous studies reported that edges may play important roles in controlling flow rate of material in two systems. In this study, we examined the mediating effects of streamline geometry on the relationships between urban land use and the index of biological integrity (IBI) of the Nakdong River in Korea. Two mediation models with streamline geometry, including the partial mediation effect model and the full mediation effect model, were estimated to explore the mediating effect of streamline geometry. To measure streamline geometry, we calculated the fractal dimension of a streamline within a 1 km buffer from the sampling sites. The proportion of urban land use within a 5 km buffer of the sampling sites was also computed by GIS. We used biological assessment data from the National Aquatic Ecological Monitoring Program in Korea to delineate the IBI at the sampling sites. The results showed that urban land use significantly degraded stream morphology and IBI in the Nakdong River. It was also observed that streamline geometry mediated the influence of urban land use on the IBI. The results of structural equation modeling analysis for the two mediation models with the fractal dimension and IBI suggested that only two paths were significant in the partial mediation model, whereas all paths were significant in the estimated full mediation model.     

Movement patterns of three freshwater fish species after upstream transportation by fishway in the Jangheung Dam

We investigated the post-passage movement patterns of three freshwater fish species, Carassius auratus, Hemibarbus longirostris, and Silurus asotus, frequently captured in a fishway trap as they were released into the lentic dam reservoir. Acoustic telemetry was used to monitor fish movement. Acoustic tags were attached to 10 individuals (C. auratus, [n = 6], Ca1–6; H. longirostris, [n = 2], Hl1 and Hl2; and S. asotus, [n = 2], Sa1 and Sa2). The individual (Sa1) was particularly monitored to changes of vertical distribution between day and night a depth transmitter. The lentic species, C. auratus and S. asotus, were grouped into those that moved into upper streams (Ca1, Ca2, Ca3, and Sa1) and those that stayed in the reservoir (Ca4, Ca5, Ca6, and Sa2); all individuals utilized almost the entire area of the reservoir. However, H. longirostris, which favor lotic environments, showed different responses; one (Hl1) immediately moved into one of upper the streams, whereas the other (Hl2) utilized the entire dam reservoir area. No significant correlations were observed between size of the C. auratus individuals (total length and total weight) and monitored parameters (total movement distance, movement distance per day, and number of receivers encountered). Moreover, S. asotus used the water at a mean depth of 2.89 ± 1.52 m, and its vertical distribution changed more actively during the night than that during the day. Therefore, the efficacy of the Jangheung Dam fishway could be improved through release of the confluence of the inflow streams at the dam reservoir, which would provide both lentic and lotic environments without extended migration for fish.     

Ecosystem health evaluation of agricultural reservoirs using multi-metric lentic ecosystem health assessment (LEHA) model

The objectives of this study were to determine the trophic state of agricultural reservoirs within the four major watersheds and evaluate ecosystem health using a multi-metric fish modeling approach of the lentic ecosystem health assessment (LEHA) in South Korea. Fish survey for the LEHA model was sampled twice from 12 reservoirs (oligotrophic to hypereutrophic reservoirs) during 2008–2010 along with data analysis of water quality monitored by the Ministry of Environment, Korea. The fish sampling gears used in the field were fyke net, trammel net, casting net, and kick net. A regression analysis of N:P ratios showed a negative linear function with total phosphorous (TP) values but not with total nitrogen. The greater relationship to TP was due to low variability and high ambient concentrations of N relative to P. The plots of “TSI (CHL)–TSI (TP)” and “TSI (CHL)–TSI (SD)” suggest that other factors other than phosphorus limited algal biomass (CHL–TP < 0), and that non-algal particles dominated light attenuation (CHL–SD < 0). Nutrients input into the reservoirs during the Asian monsoon exceeded actual nutrient availability for phytoplankton growth. A regression model of the fish trophic components against log-transformed TP showed that the proportions of omnivore and insectivore fish had a positive linear function and negative relationship with TP, respectively. According to the multi-metric LEHA model for agricultural reservoirs, the model value averaged 26 ± 6.5, indicating a “Fair” condition. Overall, ecosystem health was directly influenced by organic matter pollution and high algal production. This new tool appeared particularly relevant to detect the contamination effects on fish communities in reservoirs. The new national biological monitoring methodology in lentic ecosystem would be used as a key tool for ecological restorations and species conservations in Korean lentic ecosystems.     

Effects of streamline complexity on the relationships between urban land use and ecological communities in streams

Urban land use can adversely affect the water quality of adjacent streams through interactions at the edges of the two ecosystems. From a landscape ecological perspective, edges control the flow rate of materials between two adjacent systems. Based on the rationale that the streamline complexity formed between land uses and water bodies may function in this manner, we investigated the potential role of streamline complexity in the relationship between stream ecological communities and urban land use in Korea. Various indices of biological indicators including the diatom assemblage index for organic pollution, the trophic diatom index, the Korean saprobic index, and the index of biotic integrity were applied at 80 sampling sites in the Nakdong River system. We computed the fractal dimensions of the streamlines within 1-km buffers and the proportion of urban land use in the watershed within 5-km buffers around the sampling sites. A moderation model was adopted to investigate the role of streamline complexity in the relationship between urban land use and the biological indices of ecological communities. A comparison between a simple geometry regression model and complex geometry models indicated that streamline complexity may moderate the adverse impacts of urban land use on ecological communities in streams. The moderating effect of streamline complexity was particularly significant for assemblages of macroinvertebrates and fishes. Therefore, to enhance the ecological integrity of streams, we strongly recommend considering streamline complexity when restoring channelized streams in developed areas.     

Assessing the effect of watershed slopes on recharge/baseflow and soil erosion

Aquatic ecosystems are threatened by increasing variability in the hydrologic responses. In particular, the health of river ecosystems in steeply sloping watersheds is aggravated due to soil erosion and stream depletion during dry periods. This study suggested and assessed a method to improve the adaptation ability of a river system in a steep watershed. For this, this study calibrated soil and water assessment tool (SWAT) for runoff and sediment, and quantified the changes in hydrologic responses such as groundwater recharge rate soil erosion and baseflow according to two scenarios for adjustment of the watershed slope (steep to mild). Here, one scenario was set by three measured slopes, and the other was set by fixing the entire watershed slopes with 5 %. Moreover, SWAT and web-based hydrograph analysis tool (WHAT) models were applied to estimate groundwater recharge, soil erosion, and baseflow in the Haean-myeon watershed in South Korea. The results show that the reduction of watershed slope increased groundwater recharge and baseflow, and decreased sediment. Specifically, groundwater recharge rate was increased from 257.10 to 364.60 mm, baseflow was increased from 0.86 to 1.19 m³/s, and sediment was decreased from 194.6 to 58.1 kg/km². Based on these results, the suggested method will positively contribute to aquatic ecosystems and farming environments in a steeply sloping watershed due to improvements in the quantity and quality of river water.     

Development of a water quality index model for lakes and reservoirs

Lake water quality and trophic state are evaluated using various parameters which may have different interpretations. Therefore, it is useful to adopt a proxy index that shows normalized values of parameters having different units and distribution characteristics. In this study, a model for integrated water quality index was developed for lakes and reservoirs in Korea. Water quality and phytoplankton were examined in 36 lakes, two natural lakes and 34 artificial lakes. The study lakes were selected to represent the range geographic regions and lake morphology in Korea. After investigating the interrelationships among water quality parameters, four parameters (total organic carbon, chlorophyll-a, total phosphorus, and turbidity) were selected as surrogate indicators of overall water quality. A relative evaluation system was developed by adopting a logistic function index that describes a cumulative distribution function and reflects the relative position of each parameter among the study lakes. The cumulative distribution probability ranging from 0 to 1 was multiplied by 100 and then transformed into the Korean Lake Water Quality Index (LQI) ranging from 0 to 100. A score of 50 was assigned to the median value of the dataset, 0 to the highest concentration value and 100 to the lowest concentration value. Thus, the LQI is an integrated easy-to-understand index that provides information about the relative status of each lake. The results of this study can represent a model to provide a relative evaluation system for lake and reservoir water quality, which can be useful for ecosystem management within an ecoregion or a jurisdictional district.