D8 Algorithm Research Articles

A reverse tracing of the water flow path algorithm for slope length extraction based on triangulated irregular network

Terrain factor is an important factor affecting soil erosion, in which slope length is an important indicator of terrain factor. In this paper, we model the regional topographic relief with triangulated irregular network TIN, use the slope aspect of the TIN triangular surface to determine the flow direction, and propose an algorithm (RT-WFP) for extracting the slope length by tracing the water flow trajectory in reverse direction. The slope cutoff point rule is set in the algorithm to improve the accuracy of the slope length extraction results. We calculated the slope length of the experimental area of the small watershed of Golden Hook-shaped collapsing hill in Bailu Township, Ganxian District, and the rationality of the algorithm proposed in this paper is verified through the comparison and analysis with the traditional slope length extraction algorithm. The experimental results show that, compared with the traditional D8 algorithm, the water flow path extracted by this algorithm in the experimental area more closely matches the water flow path based on contour mapping, and the slope length extracted by this algorithm has a lower sensitivity to the resolution of the data, and the percentage of cells with a slope length value of no more than 300 m (the limit standard of the RUSLE model) at a resolution of 30 m reaches 94.19%.

Co-governance of open pit mines across China based on exposure risk assessment of particulate matter

Particulate matters (PMs) generated by open-pit mines (OPMs) pose a serious threat to human health and has attracted significant attention from government agencies. Previous exposure risk assessments of OPM-generated PMs focused only on environmental factors associated with individual mines, lacking large-scale assessments that integrate environmental factors with human behavior. Thus, to effectively implement governance policies for OPMs, we used the D8 algorithm to assess the risk of exposure to OPM-generated PM2.5 and PM10 across 315 Chinese cities and employed spatial Durbin modeling to explore how to co-govern the exposure risk from the perspectives of governments, companies, and the public. The results showed that 99.3% of the cities were affected by PMs originating from OPMs, and 56.5% of these cities experience a high exposure risk, approximately two-thirds of which are nonresource-based cities. The government's focus on digitalization, extractive companies' access to investment, and public participation in environmental management are key factors in reducing exposure. Cross-regional cooperation with regards to public participation and company investment is essential for reducing the effect of exposure risks on the surrounding area. This study can provide reference for the green mining construction and urban habitat improvement.

A Graph Theory Approach for Spatial Data-Based Surface Water Flow Modeling

This research proposes an innovative approach that combines graph theory with spatial data to model surface water flow with the Single Flow Direction (SFD) concept, also known as the D8 algorithm. The objective is to show the water flow from the ground surface to a lower place. The research methodology involves collecting spatial data from the Digital Elevation Model (DEMNAS) in raster data type format. Test results show that the effectiveness of the graph approach in modeling water flow can produce clear flow output. This happens because each pixel traversed by water is connected by a line that forms a well-defined water flow path. This study significantly stimulates the development of more sophisticated modeling tools and practical applications in the future. This can help in more efficient management of water resources or produce more accurate flow modeling, contributing to improved understanding and better management of the environment.

Methodology for the study of the traceability of runoff water feeding reservoirs

Abstract Water reservoirs are essential to ensure water supply to both the population and agriculture, especially in the Mediterranean basin. In some cases, analyses of water intended for human consumption have detected high levels of agrochemicals. Knowing the possible origin of these products is complex because there may be many agricultural plots within the reservoir basin. In this paper, we introduce a methodology to obtain the set of agricultural plots whose rainwater reaches the reservoir and in what proportion they affect the points where chemical analyses are performed. The method implements an extension of the D8 algorithm for the calculation of the drainage network, in which additional information about the land-use type of the area, as well as rainfall maps, are also considered. In order to facilitate the user's analysis of the data, a plugin has been implemented in QGIS. This allows usability and easy interaction with the visual information. The Rumblar reservoir basin, located in Andalusia (Spain) has been studied as a use case, surrounded by olive orchards. The result is a replicable methodology for any other water reservoir and for carrying out an individualized study of agricultural plots.

Study on urban flood simulation based on a novel model of SWTM coupling D8 flow direction and backflow effect

Urban flood is a serious challenge that increasingly affects the residents. Simulation of urban flood is becoming increasingly relevant in the world. However, how to improve the simulation accuracy is still a challenge in the field. This paper is developed to propose a novel model of SWTM (SWMM model and TELEMAC-2D model) coupling D8 flow direction and backflow effect, improving the simulation accuracy of inundation area, inundation depth and inundation duration. The model is organized into three parts: one-dimensional pipe network model establishment, two-dimensional surface inundation model, and coupling model SWTM construction. The D8 algorithm is introduced to subcatchment division. The linear interpolation in DEM is applied to improve the accuracy of D8 algorithm. It enables more accurate simulation of rainfall runoff processes using the path weighted average method for the calculation of feature width and feature slope in watershed. The overflow and backflow effect of inspection wells are both considered to simulate the flood inundation in the two-dimensional surface inundation model, which can obtain the maximum inundation depth and inundation area more accurately. In the coupled model, the overflow and backflow process of inspection wells are used as the input condition of the two-dimensional model to simulate the flooding process. The new model is applied to rainstorm waterlogging in Fangzhuang area, Beijing. It shows that SWTM model can simulate the changes of inundation range, inundation depth and inundation duration more accurately comparing with the traditional model.

Inundation analysis of Xiuzhen River based on one-dimensional hydrodynamic model

In order to deal with the mountain flood disaster for small and medium-sized rivers and reduce the loss of life and property caused by mountain flood to the people, it is necessary to calculate the flood inundation risk of the region along the river. In this paper, taking Xiuzhen River in Lanshan District of Rizhao City as an example, an one-dimensional hydrodynamic model was established and D8 algorithm was coupled to simulate flood inundation in the target area, and the submerged depth, flood amount and submerged area under different frequency of flood were analyzed. The results show that the method can calculate flood inundation risk and scope quickly and accurately. It provides flood and drought disaster risk information and scientific decision-making basis for the sustainable development of social economy.

Geovisualization of Hydrological Flow in Hexagonal Grid Systems

Recent research has extended conventional hydrological algorithms into a hexagonal grid and noted that hydrological modeling on a hexagonal mesh grid outperformed that on a rectangular grid. Among the hydrological products, flow routing grids are the base of many other hydrological simulations, such as flow accumulation, watershed delineation, and stream networks. However, most of the previous research adopted the D6 algorithm, which is analogous to the D8 algorithm over a rectangular grid, to produce flow routing. This paper explored another four methods regarding generating flow directions in a hexagonal grid, based on four algorithms of slope aspect computation. We also developed and visualized hexagonal-grid-based hydrological operations, including flow accumulation, watershed delineation, and hydrological indices computation. Experiments were carried out across multiple grid resolutions with various terrain roughness. The results showed that flow direction can vary among different approaches, and the impact of such variation can propagate to flow accumulation, watershed delineation, and hydrological indices production, which was reflected by the cell-wise comparison and visualization. This research is practical for hydrological analysis in hexagonal, hierarchical grids, such as Discrete Global Grid Systems, and the developed operations can be used in flood modeling in the real world.

A novel approach for high-quality drainage network extraction in flat terrains by using a priori knowledge of hydrogeomorphic features to extend DEMs: A case study in the Hoh Xil region of the Qinghai-Tibetan Plateau

Being a fundamental type of hydrogeomorphic parameter, drainage networks have been widely applied in the fields of geographic cartography, hydrologic and hydraulic modeling, water resource management, flood risk analysis, and so on. The rapid developments of modern remote sensing technologies have greatly improved the resolution and precision of spatial data providing a plenty of microtopographic information that could have significant impacts on the surface runoff, but most of the drainage network extraction methods have paid less attention to it especially for flat terrains. In order to deal with this problem, we propose a novel approach for high-quality drainage network extraction in flat terrains, in which the hydrogeomorphic features with the function of obstruction, guidance or collection to the runoff are considered as a priori knowledge used to extend digital elevation models (DEMs). Firstly, the hydrogeomorphic features that have obvious influences on surface runoff are classified, and used as a priori knowledge to extract drainage networks. After mathematically described by reorganizing their data into raster form and semanticizing their responses to runoff, this prior knowledge is then incorporated into a DEM to construct a Digital elevation-eXtended Model (DXM). Secondly, in order to solve the problem of erroneous or indeterminate flow directions in drainage network extraction based on the DEM alone, a method of drainage network extraction based on DXM is thereby developed. Finally, the proposed approach is applied and validated in a case study in Yanhu Lake area of Hoh Xil in the Qinghai–Tibet Plateau. The DEMs and digital orthophoto maps (DOMs) with sub-meter resolution were collected by employing unmanned aerial vehicle-based (UAV-based) structure-from-motion (SFM) photogrammetry. The hydrogeomorphic features with runoff responses were extracted from the DOMs and used as a priori knowledge. Various DXMs are constructed based on the extracted hydrogeomorphic features and DEMs to extract the drainage networks of the study area. The extraction results are qualitatively and quantitatively evaluated by visual comparisons and statistical analysis to validate the proposed approach. Additionally, the effects of DEM resolution and the presence of a priori knowledge on the quality of drainage network extraction were investigated. It is found that the DXM can clearly determine the flow directions by adding useful auxiliary information into the conventional D8 algorithm. This approach provides a new high-efficient way to extract the drainage networks in flat terrains.

Novel Technique to Investigate Glacio-fluvial Hypsometry in Hunza Using Local Indicator of Spatial Autocorrelation (LISA)

Hypsometric Integral (HI) displays the effect of active tectonics and sensitivity on geomorphic structures. In this study we calculated HI values for Hunza valley to investigate neotectonics, development of topographic structures and process of erosion using SRTM DEM 90m. ArcGIS and MATLAB is used to generate HI and hypsometric curve (HC). We generated HI and HC values by using D8 algorithm in MATLAB to extract drainage basins for 5 and 6 Strahler orders. HI and HC values show the stages of erosion for instance high values of HI and convex HC displays young and tectonically active stage. We used different grid sizes in ArcGIS to calculate maximum, mean and minimum elevation utilizing different statistical techniques. We used Local Indicator of Spatial Autocorrelation (LISA) instead of Global Moran Index to determine the extent of distribution of clustered, dispersed and randomized HI values. This technique indicates high positive z score for auto correlated data. Regions with high HI value indicate relative uplift, undissected and young structures while low HI values indicate sediment accumulation and shallow earthquakes.

Spatial-temporal analysis and background value determination of molybdenum concentration in basins with high molybdenum geochemical background - A case study of the upper Yi River basin

The environmental background value of the river section is important. It can be used to evaluate the effect of pollution control of the upstream of that river section, analyze the trend of environmental pollution, and assist the government to make decisions. Yi river is the main tributary of the Yellow River. In the headwaters of the Yi river, there are two very large molybdenum mines with a history of mining and smelting of many years. This area is also a region with a high molybdenum geochemical background.Using the collected regional molybdenum geochemical map, historical monitoring data, sampling data, remote sensing image, and spatial information of mineral enterprises, we found two reasons of why the molybdenum concentration is unusual in the basin. The first reason is the area is a high molybdenum region. The second reason is that the inherent solubility of molybdenum in the soil is changed due to human engineering activities. In this paper, we did a linear fitting on the soil samples and water samples collected from the natural areas and areas affected by human mining activities, and established a leaching model. By comparing the leaching capability of molybdenum in the soil of different areas, we found that the molybdenum release capability in areas affected by human mining was much higher than that in natural areas.Finally, this paper proposed a method to analyze the contribution rate of molybdenum concentration of this river section, using a combination of the leaching model and the D8 algorithm. The experimental results showed that the contribution rate of natural factors and human influence factors at the exit section of Yi River was 81.38% and 18.62% respectively. The background molybdenum concentration in this section was 0.16 mg/L.

Topographic Wetness Index calculation guidelines based on measured soil moisture and plant species composition

Soil moisture controls environmental processes and species distributions, but it is difficult to measure and interpolate across space. Topographic Wetness Index (TWI) derived from digital elevation model is therefore often used as a proxy for soil moisture. However, different algorithms can be used to calculate TWI and this potentially affects TWI relationship with soil moisture and species assemblages.To disentangle insufficiently-known effects of different algorithms on TWI relation with soil moisture and plant species composition, we measured the root-zone soil moisture throughout a growing season and recorded vascular plants and bryophytes in 45 temperate forest plots. For each plot, we calculated 26 TWI variants from a LiDAR-based digital terrain model and related these TWI variants to the measured soil moisture and moisture-controlled species assemblages of vascular plants and bryophytes.A flow accumulation algorithm determined the ability of the TWI to predict soil moisture, while the flow width and slope algorithms had only a small effects. The TWI calculated with the most often used single-flow D8 algorithm explained less than half of the variation in soil moisture and species composition explained by the TWI calculated with the multiple-flow FD8 algorithm. Flow dispersion used in the FD8 algorithm strongly affected the TWI performance, and a flow dispersion close to 1.0 resulted in the TWI best related to the soil moisture and species assemblages. Using downslope gradient instead of the local slope gradient can strongly decrease TWI performance.Our results clearly showed that the method used to calculate TWI affects study conclusion. However, TWI calculation is often not specified and thus impossible to reproduce and compare among studies. We therefore provide guidelines for TWI calculation and recommend the FD8 flow algorithm with a flow dispersion close to 1.0, flow width equal to the raster cell size and local slope gradient for TWI calculation.

Topographically and hydrologically signatures express subsurface geological structures in an arid region: a modified integrated approach using remote sensing and GIS

This study aims to investigate the ability of remote sensing sensors to map subsurface fault zones with the support of hydrological information collected from boreholes. We first extract linear features crosscut and drain the study area and its adjoining mountainous areas from the Phased Array type L-band Synthetic Aperture Radar (PALSAR), the Landsat 8 images and a set of slope and shaded relief maps derived from the PALSAR DEM using visual interpretation and D8 algorithm. To delineate displacements of subsurface faults were then, interpreted from interpolated maps of groundwater level and aquifer thickness. The subsurface faults are well evident in the form palaeochannels, abruption change in flow direction and the change in the groundwater table and aquifer thickness. The extracted surface linear features were compared against those that exist in the geological map. The result showed that the subsurface geological structures can be indicated from remote sensing and hydrological data.

A Method for Dynamical Sub-Watershed Delimitating by No-Fill Digital Elevation Model and Defined Precipitation: A Case Study of Wuhan, China

Watershed delimitation is very important in flood control management. The traditional sub-watersheds delimitated by a filling digital elevation model (DEM) may change the real sink area, such that it may not be the best choice in studies sensitive to sub-watershed storage. This paper proposes a dynamical watershed delimitation method using a no-fill DEM and precipitation. It considers a closed sink area containing cells that fully flow into a large special cell, which can flow out when its water level is “higher than outlet”. We took Wuhan City as a study area and defined the precipitation in return periods of 1, 5, 20, or 100 years to derive the sub-watersheds. It is found that, in the four delimitations, the ratio of isolated basic units which could not flow outside were 27%, 9%, 5%, and 1%, respectively, as the precipitation increased. The results show that the provided method satisfies the assumption that the sink area might overflow with increased precipitation. The sub-watershed delimitated by the proposed method has higher correlation with the distribution of waterlogging points than those delimitated according to the D8 algorithm. These findings indicate that the proposed method can derive reasonable sub-watershed delimitation and that it may be helpful in the practice of urban flood control management.

The isotropic organization of DEM structure and extraction of valley lines using hexagonal grid

AbstractThe automatic extraction of valley lines (VLs) from digital elevation models (DEMs) has had a long history in the GIS and hydrology fields. The quality of the extracted results relies on the geometrical shape, spatial tessellation, and placement of the grids in the DEM structure. The traditional DEM structure consists of square grids with an eight‐neighborhood relationship, where there is an inconsistent distance measurement between orthogonal neighborhoods and diagonal neighborhoods. The directional difference results in the extracted VLs by the D8 algorithm not guaranteeing isotropy characteristics. Alternatively, hexagonal grids have been proved to be advantageous over square grids due to their consistent connectivity, isotropy of local neighborhoods, higher symmetry, increased compactness, and more. Considering the merits above, this study develops an approach to VL extraction from DEMs based on hexagonal grids. First, the pre‐process phase contains the depression filling, flow direction calculation, and flow accumulation calculation based on the six‐neighborhood relationship. Then, the flow arcs are connected, followed by estimating the flow direction. Finally, the connected paths are organized into a tree structure. To explore the effectiveness of hexagonal grids, comparative experiments are implemented against traditional DEMs with square grids using three sample regions. By analyzing the results between these two grid structures via visual and quantitative comparison, we conclude that the hexagonal grid structure has an outstanding ability in maintaining the location accuracy and bending characteristics of extracted valley networks. That is to say, the DEM‐derived VLs based on hexagonal grids have better spatial agreement with mapped river systems and lower shape diversion under the same resolution representation. Therefore, the DEMs with hexagonal grids can extract finer valley networks with the same data volume relative to traditional DEM.

Debris Run-Out Modeling Without Site-Specific Data

Recent population growth and actions near hilly areas increase the vulnerability of occurring landslides. The effects of climate change further increase the likelihood of landslide danger. Therefore, accurate analysis of unstable slope behavior is crucial to prevent loss of life and destruction to property. Predicting landslide flow path is essential in identifying the route of debris, and it is essential necessary component in hazard mapping. However, current methodologies of determining the flow direction of landslides require costly site-specific data such as surface soil type, categories of underground soil layers, and other related field characteristics. This paper demonstrates an approach to predict the flow direction without site-specific data, taking a large landslide incident in Sri Lanka at Araranyaka region in the district of Kegalle as a case study. Spreading area assessment was based on deterministic eight-node (D8) and Multiple Direction Flow (MDF) flow directional algorithms. Results acquired by the model were compared with the real Aranayaka landslide data set and the landslide hazard map of the area. Debris paths generated from the proof of concept software tool using the D8 algorithm showed greater than 76% agreement, and MDF showed greater than 87% agreement with the actual flow paths and other related statistics such as maximum width of the slide, run-out distance, and slip surface area.

Study on Potential Influence of Runoff on Observatory-Based Watershed in Japan

Land use shows the interaction between human and nature, and its impacts especially on runoff have received global attention and required more research studies. However, watershed database is generally available only for very large scales and not yet adapted to small scales. This paper presents a methodology for watershed delineation and potential influence assessment of runoff on the small watersheds in Japan based on the locations of the official observatory point data. Flow direction, flow accumulation, snap pour point and watershed recognition were analyzed by using the D8 algorithm and 10 m × 10 m DEM data of Japan. Totally 3831 watersheds were delineated by defining snapping pour point distance of 50 m as the input value, and the nation-wide watershed database was established. According to the suggestion of Notification No. 521 from the Ministry of Land, Infrastructure and Transport of Japan and land use/land cover data from JAXA satellite Alos-2, comprehensive runoff coefficient (CRC) was calculated by the weighted average method and divided into 10 levels by the equal interval method using ArcGIS. The obtained watershed boundary lines were almost identical to the ridge lines in the mountain area which accounts for more than half of Japan. The CRC values in the biggest cities such as Tokyo and Osaka were the highest, indicating these cities would receive the highest impacts of runoff. These results can provide the technical support for the decision-making on watershed management. The creation of small-scale watershed database would provide basic materials for the subsequent research such as flood prevention, water shortage and biogeochemical cycle of the sustainable regional development.

Catchment Drainage Network Scaling Laws Found Experimentally in Overland Flow Morphologies

AbstractThe scaling relation between the drainage area and stream length (Hack's law), along with exceedance probabilities of drainage area, discharge, and upstream flow network length, is well known for channelized fluvial regions. We report here on a laboratory experiment on an eroding unconsolidated sediment for which no channeling occurred. Laser scanning was used to capture the morphological evolution of the sediment. High‐intensity, spatially nonuniform rainfall ensured that the morphology changed substantially over the 16‐hr experiment. Based on the surface scans and precipitation distribution, overland flow was estimated with the D8 algorithm, which outputs a flow network that was analyzed statistically. The above‐mentioned scaling and exceedance probability relationships for this overland flow network are the same as those found for large‐scale catchments and for laboratory experiments with observable channels. In addition, the scaling laws were temporally invariant, even though the network dynamically changed over the course of experiment.

THE COMPARISON OF DRAINAGE NETWORK EXTRACTION BETWEEN SQUARE AND HEXAGONAL GRID-BASED DEM

Abstract. The automatic extraction of valleys or ridges from DEM is a long term topic in the GIS and hydrology fields and a number of algorithms have been developed. The quality of drainage networks extraction depends on many impacts such as data source, DEM resolution and extraction algorithms. However, little consideration has been paid to the influence of different tessellation of grid-based DEM construction in terrain surface representation. Actually, hexagonal grid has been proved to be advantageous over square grid due to its consistent connectivity, isotropy of local neighbourhoods, higher symmetry, visual advantage, and so on. This study tries to explore the impact of different tessellation scheme for grid-based DEM on the accuracy of terrain representation reflected by the results of drainage networks extraction. The contour line data model is applied to grid-based DEM generation. Then, by analogy with traditional D8 algorithm, the D6 algorithm is introduced to extract drainage networks by calculating flow direction of each gird with the steepest slope neighbour criteria. From the comparison between D8 algorithm and D6 algorithm, we conclude that hexagonal grid-based DEM has a superior capability in maintaining the detailed shape and the characteristics of extracted drainage networks in coarser resolution.

THE STUDY OF WATERSHED TOPOGRAPHY CHARACTERISTICS IN VAKHSH RIVER BASED ON ZY3-DSM

Abstract. The Vakhsh River is one of the major rivers in Tajikistan. The quantitative analysis of watershed topography and developmental characteristics in Vakhsh River catchment can reflect the morphological characteristics of the region, which is of great significance for revealing the quantitative relationship between the hydrological and the geomorphological process. In this paper, the D8 algorithm and the spatial analysis method were used to extract the river networks, the catchment boundary profile lines and the longitudinal valley profile lines of the four major tributaries in the Vakhsh River from the ZY3-DSM of 10 meters resolution. On this basis, five quantitative indices including the frequency of wave, amplitude of wave, gully density, the longitudinal slope and roundness rate were used to analyze the watershed landform and its development degree. According to the experimental results, the catchment have a high surface complexity and a mature landform. Yovonsu river catchment which is in the downstream of Vakhsh River is oval and has low terrain complexity with large frequency and small amplitude. Among the midstream and upstream, the Mukson River has developed into geriatric terrain that is the most mature and has the highest surface complex, while the Obikhingon River and the Kizilsu River have developed into a stable maternal terrain. In terms of topography, the boundary elevation of the Obikhingon is basically in accordance with the normal distribution, while the Kizilsu and the Muksu show a peak state with elevations of 4,000–5,000 m and 5,000–5,500 m, respectively.

Impacts of DEM resolution and area threshold value uncertainty on the drainage network derived using SWAT

Many hydrological algorithms have been developed to automatically extract drainage networks from DEM, and the D8 algorithm is widely used worldwide to delineate drainage networks and catchments. The simulation accuracy of the SWAT model depends on characteristics of the watershed, and previous studies of DEM resolution and its impacts on drainage network extraction have not generally considered the effects of resolution and threshold value on uncertainty. In order to assess the influence of different DEM resolutions and drainage threshold values on drainage network extraction using the SWAT model, 10 basic watershed regions in China were chosen as case studies to analyse the relationship between extracted watershed parameters and the threshold value. SRTM DEM data at 3 different resolutions were used in this study, and regression analysis for DEM resolution, threshold value and extraction effects was done. The results show that DEM resolution influences the selected flow accumulation threshold value; the suitable flow accumulation threshold value increases as the DEM resolution increases, and shows greater variability for basins with lower drainage densities. The link between drainage area threshold value and stream network extraction results was also examined, and showed a variation trend of power function y = ax b between the sub-basin counts and threshold value, i.e., the maximum reach length increases while the threshold value increases, and the minimum reach length shows no relation with the threshold value. The stream network extraction resulting from a 250 m DEM resolution and a 50 000 ha threshold value was similar to the real stream network. The drainage network density and the threshold value also shows a trend of power function y = ax b ; the value of b is usually 0.5. Keywords: SWAT, digital elevation model (DEM), watershed delineation, threshold value