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Terrain Modelling & Hydrological Spatial Analysis

Welcome to Module 4. Today we dive into the core of Spatial Hydrology. We will focus on how terrain elevation models (DEMs) are processed to extract flow lines, stream directions, catchment divisions, and flood accumulation paths. This forms the analytical heart of water resource planning in WECS.

Learning Objectives

By the end of today's sessions, you will be able to:

  • Understand DEM surface concepts, horizontal-vertical grid resolutions, and topographic representation models (DSM, DTM).

  • Execute DEM conditioning to resolve sinks, depressions, and canopy bridges using sink filling (Wang & Liu, Planchon & Darboux), DEM breaching, and vector stream burning.

  • Generate primary and secondary terrain derivatives (slope, aspect, curvatures, hillshade, TWI, SPI, TRI, TPI) and interpret their geological and hydrological significance.

  • Calculate flow direction grids (D8, MFD, D-Infinity) and unweighted/weighted flow accumulation paths to estimate water yield discharges.

  • Delineate regional watershed basins automatically, extract stream networks based on threshold density parameters, and classify channels using Strahler and Shreve stream ordering.

  • Model peak surface runoff discharge (Rational Method), runoff volume depths (SCS Curve Number), and annual soil erosion loss susceptibility (RUSLE).

  • Interpolate point rain gauge records into continuous grids (IDW, Thin Plate Splines, Kriging), modeling orographic precipitation using elevation covariates (Regression Kriging).

  • Execute flood inundation mapping using GIS-hydraulic coupling (HEC-RAS), the HAND relative height model, and calculate reservoir storage capacities via stage-volume curves.

Learning Roadmap

The day is structured into 9 comprehensive topics:

flowchart TD
    subgraph Row1 [" "]
        direction LR
        M1["1. Intro to Terrain Analysis<br/>(DSM vs. DTM & Resolution)"] --> M2["2. DEM Preprocessing<br/>(Sink Filling & Stream Burning)"] --> M3["3. Terrain Derivatives<br/>(Slope, Aspect, TWI & Ruggedness)"]
    end
    subgraph Row2 [" "]
        direction RL
        M4["4. Flow Routing & Catchment<br/>(D8/MFD & Watershed bounds)"] --> M5["5. Basin Partitioning<br/>(r.watershed & Stream Orders)"] --> M6["6. Raster-Based Models<br/>(Rational Method, SCS CN & RUSLE)"]
    end
    subgraph Row3 [" "]
        direction LR
        M7["7. Spatial Interpolation<br/>(IDW, Kriging & Orographic Rain)"] --> M8["8. Flood & Water Planning<br/>(HEC-RAS, HAND & Reservoir Stage)"] --> M9["9. Practical Lab & Assignment<br/>(Watershed characterization)"]
    end
    M3 --> M4
    M6 --> M7

    %% Premium styled flowchart nodes
    style M1 fill:#e3f2fd,stroke:#1565c0,stroke-width:2px
    style M2 fill:#e3f2fd,stroke:#1565c0,stroke-width:2px
    style M3 fill:#e8f5e9,stroke:#2e7d32,stroke-width:2px
    style M4 fill:#f3e5f5,stroke:#6a1b9a,stroke-width:2px
    style M5 fill:#fff3e0,stroke:#d84315,stroke-width:2px
    style M6 fill:#fff3e0,stroke:#d84315,stroke-width:2px
    style M7 fill:#efebe9,stroke:#4e342e,stroke-width:2px
    style M8 fill:#fce4ec,stroke:#c2185b,stroke-width:2px
    style M9 fill:#d2f8d2,stroke:#2b8a2b,stroke-width:3px
    style Row1 fill:none,stroke:none,stroke-width:0px
    style Row2 fill:none,stroke:none,stroke-width:0px
    style Row3 fill:none,stroke:none,stroke-width:0px

Day 4 Course Materials:

  1. 01: Introduction to Terrain Analysis: DEM concepts, grid cell size and precision, differences between DSM/DTM, data sources (SRTM, ALOS, Copernicus), and horizontal-vertical datums.

  2. 02: DEM Processing and Conditioning: Sinks and depressions, mathematical flow routing problems, SAGA Wang & Liu sink filling, GRASS r.fill.dir Planchon & Darboux, DEM breaching, and AGREE stream burning.

  3. 03: Primary and Secondary Terrain Derivatives: Moving neighborhood windows (Horn's vs. Zevenbergen & Thorne), slope, aspect, standard/multi-directional hillshading, curvatures, TRI ruggedness indices, TPI landforms, TWI wetness index, and SPI stream power calculations.

  4. 04: Hydrological Analysis and Flow Routing: Flow direction models (D8, MFD, D-Infinity), flow accumulation (unweighted area vs. weighted runoff discharge), threshold-based stream network extraction, and pour point snapping single-basin upslope delineations.

  5. 05: River Basins and Stream Ordering: Automated regional sub-basin delineation using SAGA Channel Network and GRASS r.watershed. Delineating Strahler stream orders (Horton's laws, bifurcation ratios) and Shreve stream magnitude ordering.

  6. 06: Raster-Based Hydrological and Soil Erosion Modelling: Runoff peak discharge modeling (Rational Method), runoff volume depth estimation (SCS Curve Number), and annual soil loss susceptibility mapping (RUSLE).

  7. 07: Rainfall Gridding and Spatial Interpolation: Proximity-based IDW, Thin Plate Splines, Ordinary Kriging (nuggets, sills, ranges), elevation-based Regression Kriging, and Zonal statistics volumetric basin inputs.

  8. 08: Flood Mapping and Water Resource Applications: Pre- and post-processing coupling with HEC-RAS, Height Above Nearest Drainage (HAND) inundation models, and reservoir capacity stage-volume analysis curves.

  9. 09: Practical Laboratory and Mini-Assignment: Guided step-by-step workspace configuration, DEM conditioning, flow accumulation routing, and watershed delineations, followed by an independent catchment characterization, zonal stats, and QGIS print layout mapping assignment.