Determining Hydrological Variability Using a Multi-Catchment Model Approach for the Western Cape, South Africa

Understanding the impacts of climate change requires the development of hydrological modelling tools. However, data scarcity hinders model application, performance, process simulation and uncertainty, especially for Sub-Saharan Africa. In this study, a multi-catchment approach was used to assess hydrological process variability in the Western Cape (WC) of South Africa using the JAMS/J2000 rainfall–runoff model and a Monte Carlo analysis (MCA). Due to much steeper slopes and lower evapotranspiration, the models suggest that WC is dominated by surface runoff from mountainous regions and regional groundwater flow. The results highlight the impact of the catchment size, availability and position of hydroclimatic and anthropogenic factors and the frequency of the signal-to-noise ratio (water balance). For large catchments (>5000 km2), the calibration was able to achieve a Nash–Sutcliffe efficiency (NSE) of 0.61 to 0.88. For small catchments (<2000 km2), NSE was between 0.23 to 0.39. The large catchments had an overall surface runoff, interflow and baseflow contribution of 44, 19 and 37%, respectively, and lower overall uncertainty. The simulated flow components for the small catchments were variable and these results are less certain. The use of a multi-catchment approach allows for identifying the specific factors impacting parameter sensitivities and in turn provides a means to improve hydrological process simulation.

Review on Co-factors Triggering Flash Flood Occurrences in Indonesian Small Catchments

Flash flood is defined as “a flood of short duration with a relatively high peak discharge,” which leaves little time to take action to reduce property damage and the risk to life. Flash floods occur not only because of heavy rainfall but some co-factors that can trigger it. This study aims to determine the co-factors that trigger the flash flood. Observations are carried out using a descriptive-qualitative approach of five small catchments in Indonesia, namely Bahorok Catchment (Langkat, North Sumatra), Kalijompo, and Kalipakis Catchment (Jember, East Java), Nasiri Catchment (Western Seram, Maluku), Wasior Catchment (Wondama Bay, West Papua). The dominant co-factors are related to rainfall IDF, morphological characteristics (slope, channel properties, flow pattern), geological conditions (rock, soil, structure, geohydrology), catchment conditions (vegetation, land use). Flash floods generally occur due to landslides in the upstream part of the river. Debris consisting of water, rock, and tree trunks can stem the river’s flow and form natural dams. In five flash flood cases under investigation, the causes of a flash flood triggered by heavy rainfall and the morphological characteristics are 60% and 40%, respectively. The quantitative measure of each co-factor that triggers flash floods is essential for further research to identify flash flood symptoms.

Contemporary Sediment Delivery Ratios for Small Catchments Subject to Shallow Rainfall Triggered Earthflows in the Waipaoa Catchment, North Island, New Zealand

<p>The Waipaoa catchment is generally considered to have high hill slope channel coupling due to the large volumes of sediment output at the river mouth. Yet the percentage of sediment that is transported within the fluvial system is low when considered in terms of the total volume of sediment mobilised during episodic failure events. Clearly, there is a discrepancy between generation of sediment and its delivery to the fluvial network. Previous research has suggested there is a strong decrease in catchment connectivity as catchment size increases. However, little research has been undertaken to understand the changes in hillslope-channel coupling over time. This study focuses on the connectivity of shallow rainfall triggered earthflows located in small catchments located within three different land systems in the Waipaoa Catchment. A multiple regression model was developed to predict the sediment delivery ratio for individual earthflows based on an empirical dataset of earthflows which occurred during a storm event in 2002. The results from this modelling were applied to five larger sub-catchments where sequential aerial photograph analysis (1940s to 2004) was used to determine connectivity. From this, spatial and temporal patterns in the catchment sediment delivery ratios were identified. The expected decrease in sediment delivery ratios was observed as catchment size increased. However, the temporal pattern to sediment delivery is not so clear. It appears that catchment evolution, referring specially to the Terrain Event Resistance Model developed by Crozier and Preston (1999), does not have a significant influence on sediment delivery ratios within the six decades examined in this thesis. Furthermore, while earthflows are considered the ultimate source of sediment during storm events, they are not always the mechanism by which this sediment enters the fluvial network. It is also vital to consider rates of gullying, sheet erosion and riparian erosion.</p>

Contemporary Sediment Delivery Ratios for Small Catchments Subject to Shallow Rainfall Triggered Earthflows in the Waipaoa Catchment, North Island, New Zealand

<p>The Waipaoa catchment is generally considered to have high hill slope channel coupling due to the large volumes of sediment output at the river mouth. Yet the percentage of sediment that is transported within the fluvial system is low when considered in terms of the total volume of sediment mobilised during episodic failure events. Clearly, there is a discrepancy between generation of sediment and its delivery to the fluvial network. Previous research has suggested there is a strong decrease in catchment connectivity as catchment size increases. However, little research has been undertaken to understand the changes in hillslope-channel coupling over time. This study focuses on the connectivity of shallow rainfall triggered earthflows located in small catchments located within three different land systems in the Waipaoa Catchment. A multiple regression model was developed to predict the sediment delivery ratio for individual earthflows based on an empirical dataset of earthflows which occurred during a storm event in 2002. The results from this modelling were applied to five larger sub-catchments where sequential aerial photograph analysis (1940s to 2004) was used to determine connectivity. From this, spatial and temporal patterns in the catchment sediment delivery ratios were identified. The expected decrease in sediment delivery ratios was observed as catchment size increased. However, the temporal pattern to sediment delivery is not so clear. It appears that catchment evolution, referring specially to the Terrain Event Resistance Model developed by Crozier and Preston (1999), does not have a significant influence on sediment delivery ratios within the six decades examined in this thesis. Furthermore, while earthflows are considered the ultimate source of sediment during storm events, they are not always the mechanism by which this sediment enters the fluvial network. It is also vital to consider rates of gullying, sheet erosion and riparian erosion.</p>

Carbon, Nutrients and Methylmercury in Water from Small Catchments Affected by Various Forest Management Operations

Forest management activities in boreal and hemiboreal environments have been found to increase the concentration of carbon, nutrients, and methylmercury (MeHg) in runoff water, thus contributing to environmental quality issues. We evaluated carbon, nutrient, and MeHg concentrations in water at eight small, forested catchments on organic soils in Latvia, subject to ditch cleaning and beaver dam removal. These management-induced disturbances were classified into a major, minor, or no disturbance classes. The concentrations of dissolved organic carbon and total nitrogen were elevated in disturbed catchments (both major and minor) compared to the catchments with no disturbance. The concentrations of MeHg in the water displayed a clear seasonal variation with higher concentrations in spring and summer, but there were no significant differences in MeHg concentrations between catchments with major, minor, and no disturbances. However, the higher concentrations of SO42− in the disturbed catchments compared to those undisturbed may promote MeHg formation if the conditions become more reduced further downstream. While most former studies of forest management effects on water quality have focused on forest harvest, our research contributes to the currently rather scarce pool of data on the impact of less-studied management operations, such as ditch cleaning and beaver dam removal, on carbon, nutrient, and MeHg concentrations in runoff water.

Assessment of the impact of climatic characteristics and landscape changes on the maximum flow of small watersheds

Introduction. The article dedicated to safety of hydraulic structures in small catchments. The research consists in the development of a methodology for assessment of influence of climatic and landscape changes on the maximum flow which determins sizes for the construction elements. The practical significance is due to the assessment of changes in the reliability of hydraulic structures, as well as the degree of potential flooding of territories and soil erosion during rain floods. In scientific terms, the practical significance lies in the justification of changes in the parameters of the formula for the maximum flow calculating. The main goal of the study was to test hypotheses about an increase in the intensity of storm rainfalls and of changes in landscape characteristics during recent decades. Accordingly the next problems were desided: assessment the increase in the daily maximums of precipitation and landscape changes over the past decades; as well as assessing the degree of influence of the above factors on the increase in the maximum water. Materials and methods. The data of the network weather station and the landscape characteristics of the catchment area served as the research materials. Methods of statistical analysis and geoinformation technologies were used as research methods. Results. The main results of the research were the characteristics of the increase in maximum daily precipitation and landscape changes over the past decades and the characteristics of their influence on the increase in maximum water flow, obtained on the base of probability theory and of the surface flow theory. Conclusions. A confirmation of the effectiveness of the proposed methodology for assessing changes in the climatic and landscape characteristics of catchments and their impact on the flood maximums. The hydraulic structures built both in the second half of the twentieth century on small catchments, and in subsequent years when using existing building regulations that do not have proper updating have lost their original reliability.

ASSESSMENT OF THE GEOECOLOGICAL SITUATION WITHIN THE SMALL DRAINS IN THE YASELDA RIVER BASIN

The main topic of research is the drainage basins of small rivers, which in recent years have become objects of close attention due to the high degree of their transformation associated with significant anthropogenic loads. The article presents a methodology for geoecological analysis of anthropogenic loads on the catchments of small rivers, an assessment of the degree of natural protection of the territory and a comprehensive description of the ecological state of the Yaselda river basin using GIS technologies. The paper considers the results of the study of the geoecological state of small catchments by assessing various indicators of the natural protection of the territory and factors of anthropogenic load. The scientific novelty of the research consists in the development of a methodology for assessing the geoecological situation of small catchments of the rivers of the Belarusian Polesye on the basis of a matrix of attributive indicators, which allows, taking into account generalized point estimates, to identify categories of lands with different geoecological situations. As a result of the research, a hydrographic zoning of the river basin was carried out. Yaselda, eight small private catchments (sub-basins) were identified, cartographic material was digitized, statistical data were collected, a database was created on various physical-geographical, socio-economic and ecological-hydrographic indicators. In this work, for the first time, the natural resource potential of small catchments of the Yaselda river basin is studied and presented, the natural environment-forming factors are assessed, and the main anthropogenic factors are identified, their qualitative and quantitative characteristics are carried out.