# Study Notes: Hillslope Hydrology and Storm Runoff Mechanisms ## Key Concepts in Hillslope Hydrology Hillslope hydrology examines how water moves through slopes and soils within a drainage basin. This movement is critical for understanding flooding, wetland ecosystems, pollutant transport, soil formation, erosion, and slope stability. Predicting water flow is challenging due to: - **Temporal variability**: Water stores and flows change over time with rainfall, evapotranspiration, and streamflow. - **Spatial variability**: Factors like slope gradient, soil depth, and moisture content vary across short distances. - **Subsurface complexity**: Most water flows underground, making it difficult to measure. Hillslope hydrology is essential for managing water resources, mitigating flood risks, and understanding landscape evolution. --- ## Hydrological Pathways on Hillslopes Water moves through hillslopes via several pathways, each with unique characteristics and implications for water flow and storage. ### 1. **Surface Runoff (Overland Flow)** Occurs when precipitation cannot infiltrate the soil, leading to water flowing over the surface. This process is rare but significant for rapid water movement and soil erosion. - **Key factors**: Surface depressions, soil compaction, and impermeable surfaces. - **Speed**: Fastest hillslope flow process. During a heavy rainstorm on a steep, clay-covered slope, water quickly accumulates and flows downhill as surface runoff. --- ### 2. **Infiltration** The process by which water soaks into the soil. The maximum rate at which soil can absorb water is called **infiltration capacity** (measured in mm/hour). - **Influencing factors**: - Vegetation cover (slows water flow, increases infiltration). - Soil compaction (reduces infiltration). - Soil pore size (larger pores = higher infiltration). To increase infiltration in urban areas, use permeable pavements or plant vegetation to reduce surface runoff. --- ### 3. **Throughflow** Water moves downslope within the soil, typically through small pores or along preferential pathways like soil pipes and macropores. - **Characteristics**: - Slow due to high friction in small pores. - Faster in macropores (e.g., root channels, animal burrows). Throughflow is a key contributor to water movement in unsaturated soils but is slower than surface runoff. --- ### 4. **Groundwater Flow** Water flows slowly through saturated rocks and sediments beneath the water table. If the water table intersects the surface, groundwater may emerge as **return flow**. - **Speed**: Very slow compared to other pathways. - **Importance**: Sustains baseflow in rivers during dry periods. In a valley, groundwater flow may emerge as springs or seepage areas, contributing to streamflow. --- ## Soil Properties and Water Movement Soil characteristics significantly influence water storage and movement. Key properties include: ### 1. **Porosity** The proportion of soil volume made up of pore spaces (air or water). Higher porosity means more water storage capacity. - **Example**: Clay has a porosity of 0.45–0.60, meaning it can store up to 600 liters of water per cubic meter. ### 2. **Permeability** The rate at which water flows through soil. Coarse-textured soils (e.g., sand, gravel) are more permeable than fine-grained soils (e.g., clay). | **Soil Type** | **Porosity** | **Permeability (m/day)** | |---------------|--------------|--------------------------| | Clay | 0.45–0.60 | $10^{-6}$ – $10^{-4}$ | | Silt | 0.2–0.5 | $10^{-3}$ – $10$ | | Sand | 0.3–0.4 | $10$ – $10^4$ | | Gravel | 0.25–0.4 | $10^2$ – $10^6$ | Confusing porosity with permeability. High porosity does not always mean high permeability (e.g., clay has high porosity but low permeability). --- ## Storm Runoff Mechanisms Storm runoff reaches rivers through various pathways, influencing the **flashiness** of the river hydrograph (speed and magnitude of response). Key mechanisms include: ### 1. **Infiltration-Excess (Hortonian) Overland Flow** Occurs when rainfall intensity exceeds the soil's infiltration capacity, causing water to flow over the surface. - **Common in**: Areas with thin soils, steep slopes, or impermeable surfaces (e.g., urban areas). - **Hydrograph impact**: Rapid rise in discharge (steep rising limb). During a torrential downpour on a paved parking lot, water cannot infiltrate and flows directly into storm drains. --- ### 2. **Saturation-Excess Overland Flow** Occurs when soil becomes fully saturated, and additional rainfall cannot infiltrate. - **Common in**: Valley floors, floodplains, and areas with prolonged rainfall. - **Hydrograph impact**: Slower response compared to Hortonian flow. In temperate regions like lowland Britain, most surface runoff is due to saturation-excess rather than infiltration-excess. --- ### 3. **Subsurface Storm Flow** Water moves rapidly through soil macropores or fractures, displacing pre-existing water (piston flow). - **Key insight**: Much of the water in a storm hydrograph is **pre-event water** (stored in the catchment before the storm). - **Hydrograph impact**: Short lag times and steep rising limbs. Geochemical signatures (e.g., solute concentration, isotopes) can distinguish between new rainfall and pre-event water. --- ## Field Techniques in Hillslope Hydrology Studying hillslope hydrology requires careful fieldwork. Common techniques include: ### 1. **Infiltration Measurements** - Use a falling-head permeameter (e.g., a pipe filled with water) to measure infiltration rates. ### 2. **Soil Moisture Measurements** - Use soil moisture meters or laboratory drying methods to estimate soil water content. ### 3. **Throughflow Troughs** - Install gutters in a soil pit to measure throughflow rates at different depths. To study infiltration, hammer a pipe into the soil, fill it with water, and record how quickly the water level drops. --- ## Computer Modeling in Hydrology Computer models help predict how drainage basins respond to rainfall. Two main types are: ### 1. **Lumped Models** - Treat the entire catchment as a single unit. - Use average parameters (e.g., lag time) to predict runoff. - Example: IHACRES. ### 2. **Distributed Models** - Simulate processes in detail using a grid of cells. - Require extensive data (e.g., soil properties, topography). - Example: TOPMODEL. Distributed models are ideal for urban planning, as they provide detailed predictions of water movement across a basin. --- ## Key Takeaways - Hillslope hydrology is crucial for understanding water movement, flooding, and landscape processes. - Water flows through various pathways: surface runoff, infiltration, throughflow, and groundwater flow. - Soil properties like porosity and permeability control water storage and movement. - Storm runoff mechanisms (e.g., Hortonian flow, saturation-excess flow) influence river hydrographs. - Field techniques and computer models are essential tools for studying and predicting hydrological processes. Assuming all storm runoff is surface runoff. Subsurface flows and pre-event water often dominate in many catchments. --- ## Practice Questions 1. Explain the difference between infiltration-excess and saturation-excess overland flow. Provide examples of where each might occur. 2. How do soil properties like porosity and permeability affect water movement in a drainage basin? 3. Why is pre-event water significant in storm hydrographs, and how can it be identified? 4. Describe a field technique you could use to measure throughflow on a hillslope. What challenges might you face? Understanding hillslope hydrology is not just academic—it has real-world applications in flood management, agriculture, and urban planning.