PROJECT 7 : WATERSHED DELINEATION
A watershed, also known as a drainage basin or catchment area, is a fundamental hydrological unit that plays a pivotal role in understanding water movement, distribution, and quality. At its core, a watershed is a land area that channels precipitation and surface water originating from rainfall, snowmelt, or other sources into lower elevations. From there, this water converges at a common outlet, such as a river, lake, stream, or wetland, or infiltrates the ground to recharge aquifers. Watersheds are delineated by natural topographical features, such as ridges and hills, which define the boundaries within which all surface water flows toward a shared endpoint.
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| Figure 1: Sample image of water delineation map |
Watersheds serve as critical units for environmental and water resource management, as they directly influence the availability, quality, and flow of water. They act as natural catchment systems that not only sustain aquatic ecosystems but also supply water for agricultural, industrial, and domestic needs. Understanding the behavior of a watershed is essential for managing issues like flooding, soil erosion, and pollution, as well as for promoting sustainable development. The process of watershed delineation is key to hydrological studies and typically involves the use of Geographic Information Systems (GIS) tools and techniques. A common and efficient approach employs a Digital Elevation Model (DEM), a three-dimensional representation of terrain elevations, alongside the hydrology toolset in software such as ArcGIS. DEMs provide accurate data on land surface features, enabling precise mapping of flow directions, accumulation patterns, and watershed boundaries. By leveraging GIS technology, researchers and planners can analyze the interplay between topography and hydrology, gaining valuable insights into water flow pathways and catchment characteristics.
In this project, our focus is on delineating the watershed for the Selangor River Basin, one of the most significant river systems in Selangor, Malaysia. Situated in the upper part of the state, the Selangor River Basin plays a vital role in sustaining local biodiversity, supporting agriculture, and supplying water to surrounding communities. By utilizing a DEM and GIS tools, we aim to accurately map the watershed's boundaries, analyze its hydrological characteristics, and contribute to a better understanding of its role in regional water resource management. This study not only highlights the importance of watershed analysis in addressing environmental challenges but also underscores the potential of GIS technology in supporting sustainable water management practices.
HOW ?
The process of preparing a watershed delineation begins with pre-processing the Digital Elevation Model (DEM) data to ensure its accuracy for hydrological analysis. The DEM represents the terrain's elevation values and serves as the foundation for mapping water flow across the landscape. However, raw DEM data often contains depressions or sinks artificially low areas that disrupt the natural flow of water. These depressions may result from errors in data collection or processing and can misrepresent water flow paths, leading to incorrect watershed delineation. To address this, the Fill tool is used as the first step in hydrological preprocessing.
To begin, open ArcMap and load the DEM file (DEM.tif) into the workspace, displaying terrain elevations for the study area. Next, navigate to Toolboxes > ArcToolboxes > Spatial Analyst Tools > Hydrology > Fill in ArcCatalog. The Fill tool removes depressions or sinks in the DEM that could disrupt water flow modeling. Set the Input surface raster to DEM.tif and save the corrected output as Fill.tif. This ensures a hydrologically conditioned DEM, allowing continuous and accurate water flow modeling. The primary purpose of running the Fill tool is to create a "hydrologically conditioned" DEM. This means the DEM is refined to allow water to flow uninterrupted across the surface, following realistic drainage patterns. Without this step, sinks or flat areas could trap water, resulting in inaccurate flow direction and accumulation calculations, which would compromise subsequent steps in the watershed delineation process. By filling these artifacts, the tool ensures the DEM accurately represents the natural landscape, enabling reliable hydrological analysis and watershed boundary mapping.
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| Figure 2: Selangor water basin map in DEM format |
After filling the DEM to remove sinks, the next steps in watershed delineation involve generating the flow direction and flow accumulation rasters. These are essential for analyzing how water flows across the landscape and for identifying key hydrological features like drainage networks and watersheds.
The Flow Direction tool analyzes the terrain to determine the direction water will flow from each cell based on the steepest downslope neighbor, using the filled DEM (Fill.tif) as input. This process generates a flow direction raster (Flowdir.tif), which is essential for modeling water pathways and understanding how surface water moves across the landscape. Building on this, the Flow Accumulation tool calculates the total flow accumulating at each cell by considering the number of upstream cells contributing to it, using the flow direction raster as input. The resulting accumulation raster (Flowacc.tif) highlights areas where water converges, such as streams or rivers, and provides key insights for identifying drainage networks and understanding surface water dynamics in the watershed. These outputs are critical for hydrological modeling and watershed delineation.
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| Figure 3: After generating flow direction from filled DME |
The Flow Accumulation tool analyzes the flow direction data to calculate the total accumulated flow reaching each cell. Cells with higher flow accumulation values represent areas where water from larger upstream regions converges, often corresponding to rivers or streams. This output helps identify the drainage network, assess watershed connectivity, and locate potential areas of water pooling or runoff concentration. Both steps are fundamental for watershed analysis. The flow direction raster ensures accurate mapping of water movement across the terrain, while the flow accumulation raster highlights the relative importance of each cell in the hydrological network. Together, these outputs provide a detailed understanding of surface water dynamics and form the foundation for delineating watersheds and analyzing hydrological processes.
The next step is creating an outlet involves defining the point where water exits the watershed, which is essential for accurately delineating watershed boundaries. To do this, a new shapefile is created to serve as the outlet layer, with a suitable coordinate system like Kertau RSO Malaya (Meters) to ensure spatial accuracy within the study area. Once the shapefile (Outlet.shp) is added, the specific outlet point is manually selected by editing the shapefile. This step requires careful identification of the watershed's drainage outlet, typically located at a river's downstream point or the basin's lowest elevation. Defining this outlet is crucial as it serves as the reference point for delineating the watershed, ensuring that all contributing areas are correctly identified in the subsequent analysis.
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| Figure 4 : Creating watershed's drainage outlet |
Delineating a watershed involves using the Watershed tool to identify the area contributing water flow to a specific outlet point. The tool requires input data, including the flow direction raster (Flowdir.tif) and the pour point layer (Outlet.shp) created earlier. The pour point represents the designated outlet where all surface water converges. By running the tool, the watershed area upstream of this outlet is delineated, showing the boundaries of the region that drains into the selected point. This step is crucial for understanding the spatial extent of the watershed and for analyzing hydrological processes, resource management, and potential impacts within the defined area. To delineate the watershed, the Watershed tool is used to identify the area contributing water flow to a defined outlet point. The tool requires input data, including the flow direction raster (Flowdir.tif) and the outlet point shapefile (Outlet.shp). By processing these inputs, the tool determines the boundaries of the watershed, ensuring that all areas draining toward the outlet are accurately captured.Once the watershed is delineated, the stream network within the watershed can be defined using the Raster Calculator in the Map Algebra toolset. This process helps to identify and visualize streams based on flow accumulation thresholds, highlighting the primary drainage paths. These steps are critical for hydrological studies, as they provide a comprehensive understanding of the watershed's structure and water flow dynamics, enabling effective water resource management and environmental planning.
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| Figure 5: Delineated watershed identification |
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| Figure 6 : Watershed polygon |
Extracting streams and DEM (Digital Elevation Model) for the watershed focuses on isolating data specifically within the watershed boundary to streamline analysis. The Clip tool is used to extract streams that lie within the watershed by clipping the vector stream network with the watershed boundary. This ensures that only the relevant portion of the stream network is retained for further analysis. Similarly, the Extract by Mask tool is applied to the DEM using the watershed boundary as a mask. This operation isolates the DEM to the watershed area, making it easier to focus on elevation data specific to the region of interest. These steps are essential for refining the dataset, ensuring that analyses are precise and restricted to the watershed, and eliminating irrelevant data outside the area of focus. To calculate the watershed area, the vector watershed layer’s attribute table is used. By adding a new field and calculating its geometry, the total area of the watershed is determined. For the Selangor River Basin, the watershed area is approximately 188,390 hectares, which can be compared with existing studies to validate the results. These steps are fundamental for detailed spatial and hydrological analysis, allowing for accurate characterization of the watershed and its features.
Calculating the watershed area is a key step in understanding the extent and scale of the watershed, which is essential for hydrological, environmental, and resource management studies. This process involves working with the vector representation of the watershed, typically stored as a polygon layer. By opening the attribute table of the watershed layer, a new field is added, often named "Area," to store the computed area values. The Calculate Geometry function is then applied to this field, leveraging the layer's coordinate system to compute the area in appropriate units, such as square meters or hectares. This ensures accuracy and consistency with the spatial data's projection system.
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| Figure 7: Watershed Delineation Map |
In addition to water resource management, the delineation process plays a key role in flood risk assessment. By identifying areas prone to high water accumulation, it aids in predicting flood-prone zones. Analyzing the flow accumulation map allows the identification of regions at risk of surface runoff or flooding during heavy rainfall, which supports disaster preparedness and mitigation planning. Understanding stream networks within the watershed also enables planners to model flood scenarios and implement infrastructure improvements, such as retention basins or drainage systems, to reduce flood risks. The analysis is also significant for environmental conservation and land management. It highlights the terrain and hydrology of the watershed, which can guide conservation efforts by pinpointing critical areas for maintaining ecological balance. Protecting headwaters and streambanks, for example, helps preserve biodiversity and water quality. Land-use planners can use the data to avoid overdevelopment in sensitive areas, such as steep slopes or high-flow regions, reducing soil erosion and sedimentation in water bodies.
Watershed analysis also supports water quality monitoring and pollution control. By delineating the watershed, the contributing land area can be studied to identify potential sources of pollution, such as agricultural runoff, industrial discharge, or urban stormwater. Monitoring stations can be strategically placed within the watershed to assess water quality effectively. This analysis helps decision-makers reduce pollutant loads by focusing on areas with the highest impact on water quality. For irrigation and agricultural planning, farmers and agricultural planners can use the delineated watershed to understand water availability. The analysis ensures that irrigation systems are aligned with the natural water flow, preventing waterlogging or depletion of upstream resources. Additionally, watershed-scale planning helps optimize the use of rainwater and manage water recharge areas, ensuring sustainability for agriculture in the region.
In terms of infrastructure development, watershed analysis provides critical insights into drainage patterns and elevation data, supporting the design of infrastructure such as roads, bridges, and reservoirs. Engineers can use this information to minimize disruption to natural water flow and avoid construction in flood-prone areas. The analysis also aids in siting hydropower facilities, ensuring efficient water use while preserving the surrounding environment. Furthermore, the watershed analysis provides a baseline understanding of the region's hydrology, which is crucial for assessing the impacts of climate change, such as altered rainfall patterns or rising temperatures. This enables the development of adaptive strategies to cope with changes in water availability or increased flood risks.
Finally, the watershed delineation map serves as an essential tool for community awareness and policy development. It helps raise awareness about the importance of watersheds and the need for their protection, acting as a communication tool to educate stakeholders about water flow dynamics and the interconnection between land use and water quality. Policymakers can rely on these analyses to develop regulations for land use, water management, and environmental protection, ensuring a balance between development and sustainability. In summary, the watershed delineation process provides a foundational understanding of the hydrological and geographical dynamics of a region. It equips researchers, planners, and policymakers with the necessary tools to address water resource challenges, mitigate environmental impacts, and develop sustainable management strategies for the Selangor River Basin and similar systems.
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