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Practical Laboratory and Mini-Assignment

This session combines a guided step-by-step laboratory tutorial (Part A) with an independent mini-assignment (Part B). You will apply the terrain processing and hydrological routing skills learned throughout Module 4.

Part A: Guided Laboratory Workflow

This tutorial guides you through setting up a project workspace, conditioning a raw Digital Elevation Model, routing water, extracting stream channels, and delineating a watershed boundary.

1. Workspace Configuration

Before starting calculations, configure your QGIS project:

  1. Create a standard directory tree in your local workspace:

    • data/raw/ (for raw inputs).
    • data/processed/ (for intermediate and final outputs).

  2. Open QGIS. Save a new project as Day4_Hydrology_Analysis.qgz.

  3. Set the project CRS to WGS 84 / UTM Zone 45N (EPSG:32645).

  4. Copy the local DEM output_hh.tif from the docs/data/Natural_Earth_quick_start/DEM/ folder into your local data/raw/ folder and load it into QGIS.

2. DEM Reprojection & Conditioning

Ensure the DEM coordinates are in meters and remove sinks to establish continuous flow:

  1. Reproject DEM: Go to Raster > Projections > Warp (Reproject).... Set Input to output_hh.tif, Target CRS to EPSG:32645, Resampling to Bilinear, and save as data/processed/output_hh_utm.tif.

  2. Fill Sinks: Go to Processing Toolbox > SAGA > Terrain Analysis - Hydrology > Fill Sinks (Wang & Liu).

    • DEM: data/processed/output_hh_utm.tif.
    • Minimum Slope (Degree): 0.01.
    • Filled DEM: Save as data/processed/filled_dem.tif.
    • Click Run.

3. Flow Direction and Accumulation

Compute flow direction paths and cumulative upstream drainage areas:

  1. Navigate to Processing Toolbox > SAGA > Terrain Analysis - Hydrology > Flow Accumulation (Top-Down).

  2. Configure parameters:

    • Elevation: data/processed/filled_dem.tif.
    • Method: Select [0] Deterministic 8 (D8).
    • Flow Accumulation: Save as data/processed/flow_accumulation.tif.
    • Flow Directions: Save as data/processed/flow_direction.tif.

  3. Click Run. Style flow_accumulation.tif using a logarithmic color scale to visualize stream paths.

4. Stream Network Extraction

Isolate cells representing major streams and convert them into vector lines:

  1. Open Raster > Raster Calculator....

  2. Apply an extraction threshold of 1000 cells (delineating pixels with at least \(1000\) pixels contributing drainage): "flow_accumulation@1" >= 1000

  3. Save output as data/processed/stream_network_binary.tif. Click OK.

  4. Vectorize the stream grid: Navigate to Processing Toolbox > SAGA > Vector <-> Raster > Vectorising Grid Classes.

    • Grid: stream_network_binary.tif.
    • Class Selection: Set to each class or filter for class 1.
    • Save output as a vector layer data/processed/vector_streams.gpkg.

5. Catchment Delineation

Delineate the watershed boundary draining to a selected outlet coordinate:

  1. Navigate to Processing Toolbox > SAGA > Terrain Analysis - Hydrology > Upslope Area.

  2. Configure parameters:

    • Elevation: data/processed/filled_dem.tif.
    • Flow Method: Select Deterministic 8 (D8).
    • Target X / Target Y: Click the map tool button (...) and click a point on your stream network representing the basin outlet.
    • Upslope Area: Save output as a raster data/processed/basin_boundary_raster.tif.

  3. Click Run. Convert this to a vector layer: Go to Raster > Conversion > Polygonize (Raster to Vector).... Select basin_boundary_raster.tif and save as a vector polygon data/processed/basin_boundary_polygon.gpkg.

  4. Style the polygon with a transparent fill and a thick black outline.

6. Soil Erosion Susceptibility (RUSLE)

Calculate the topographic LS factor and compile the annual soil loss grid:

  1. Calculate SAGA LS Factor: Navigate to Processing Toolbox > SAGA > Terrain Analysis - Hydrology > LS Factor.

    • Elevation: data/processed/filled_dem.tif.
    • Flow Accumulation: data/processed/flow_accumulation.tif.
    • Method: Select [0] Moore et al. (1991).
    • LS Factor: Save output as data/processed/ls_factor.tif.
    • Click Run.

  2. Compile Final Soil Loss: Open Raster > Raster Calculator....

    • Multiply the local factor rasters (\(R, K, C, P\), assuming default constants if local grids are unavailable, e.g. \(R = 250\), \(K = 0.25\), \(C = 0.1\), \(P = 1.0\)): 250 * 0.25 * "ls_factor@1" * 0.1 * 1.0
    • Save output as data/processed/annual_soil_loss.tif. Click OK.
    • Style annual_soil_loss.tif using Singleband pseudocolor classified into 5 categories (\(0-5\) Slight, \(5-10\) Moderate, \(10-20\) High, \(20-40\) Very High, \(>40\) Severe).

7. Rainfall Interpolation and Catchment Zonal Statistics

Interpolate point weather station records and compute volumetric catchment rainfall:

  1. IDW Interpolation: Navigate to Processing Toolbox > QGIS > Raster Analysis > IDW Interpolation.

    • Vector Layer: Select a point gauge layer (e.g. rain_gauges.shp).
    • Interpolation Attribute: Select the rainfall column (e.g. precip_mm).
    • Extent: Select Use Extent from > data/processed/basin_boundary_polygon.gpkg.
    • Pixel Size: Set to 30 meters.
    • Interpolated: Save output as data/processed/rainfall_idw.tif. Click Run.

  2. Calculate Zonal Statistics: Go to Processing Toolbox > Raster Analysis > Zonal Statistics.

    • Input Raster: data/processed/rainfall_idw.tif.
    • Vector Layer Containing Zones: data/processed/basin_boundary_polygon.gpkg.
    • Output Column Prefix: Type rain_.
    • Statistics to Calculate: Check Mean and Sum.
    • Click Run.

  3. Compute Volume in Field Calculator: Open the attribute table of your catchment polygon. Open the Field Calculator, create a new decimal field precip_m3, and enter the volumetric equation: ("rain_mean" / 1000) * $area Click OK. This calculates the total cubic meters (\(m^3\)) of rainfall entering the basin.

8. Height Above Nearest Drainage (HAND) Mapping

Delineate relative topography above stream channels to locate low-lying flood inundation hazard zones:

  1. Navigate to Processing Toolbox > SAGA > Terrain Analysis - Hydrology > Relative Heights.

    • Elevation: data/processed/filled_dem.tif.
    • Streams: data/processed/stream_network_binary.tif.
    • Relative Heights: Save output as data/processed/hand_model.tif.
    • Click Run.

  2. Style hand_model.tif in the Layer Styling Panel using a classified color ramp. Highlight cells between \(0\text{ m}\) and \(2\text{ m}\) in red to map regions susceptible to local flood inundation.

9. Reservoir Stage-Volume Capacity Curve Analysis

Calculate reservoir storage capacities and surface areas at varying water levels:

  1. Clip DEM to Reservoir Catchment: Delineate the upslope catchment draining through a planned dam axis. Use the Raster Calculator to clip the DEM to this catchment boundary. Save as data/processed/reservoir_dem.tif.

  2. Calculate Volume at Target Stage: Go to Processing Toolbox > QGIS Raster Analysis > Raster Surface Volume.

    • Input Layer: data/processed/reservoir_dem.tif.
    • Base Threshold: Enter the target pool height elevation (e.g. 480 meters).
    • Method: Select Count Only Below.
    • Volume / Area Output: Save as a table data/processed/stage_volume_480.html.
    • Click Run. Open the HTML report to extract pool area (\(m^2\)) and storage volume (\(m^3\)).

Part B: Mini-Assignment: Watershed Terrain Characterization

1. Objective

You will independently delineate a specific sub-watershed, calculate its topographic statistics, and compile a publication-quality map layout.

2. Required Tasks

Using the layers created in Part A, execute the following steps:

  1. Delineate a Sub-basin:

    • Identify a downstream confluence point on vector_streams.gpkg.
    • Run SAGA Upslope Area using that coordinate to delineate the upstream catchment boundary.
    • Convert the output raster to a vector polygon layer named catchment_boundary.gpkg.

  2. Calculate Topographic Statistics (Zonal Statistics):

    • Run Zonal Statistics using catchment_boundary.gpkg as the zone and data/processed/output_hh_utm.tif as the input raster. Compute the Mean, Min, and Max elevations.
    • Generate a Slope grid (slope_degrees.tif) via GDAL Slope. Run Zonal Statistics using the slope grid to find the catchment's Mean Slope in degrees (slope_mean).
    • Open the attribute table of catchment_boundary.gpkg and calculate the total catchment surface area in square kilometers: $area / 1000000.

  3. Generate Stream Ordering:

    • Open SAGA Stream Order. Select the filled DEM and your stream network. Set Method to Strahler and output strahler_order.tif.
    • Run the SAGA Vectorising Grid Classes tool to convert the Strahler raster to vector lines named ordered_streams.gpkg.

  4. Compose Print Layout Map:

    • Create an A4 landscape Print Layout in QGIS.
    • Render the styled layers in the map frame:
      • ordered_streams.gpkg styled graduated by the ORDER field (tributaries at \(0.2\text{ mm}\), main channels at \(1.5\text{ mm}\)).
      • catchment_boundary.gpkg styled with an empty fill and a solid \(0.8\text{ mm}\) red border.
      • Slope raster styled semi-transparent pseudocolor overlaying a Hillshade layer.
    • Add a text block table detailing:
      • Total Drainage Area (\(km^2\)).
      • Elevation Range (\(meters\)).
      • Mean Slope (\(degrees\)).
    • Include essential cartographic elements: Title, North Arrow, scale bar, legend, and UTM coordinate grid.

3. Submission Files

Export your print layout to PDF and compile a ZIP file containing:

  1. Basin_Terrain_Characterization_Map.pdf.

  2. Watershed_Terrain_Data.gpkg (containing catchment_boundary and ordered_streams).

  3. Your QGIS project file saved with Relative Paths.