Unit 6.1 Runoff Generation Mechanisms

Summary

In this lecture, Jochen Wenninger, a hydrologist and senior lecturer at IHE Delft, explores the mechanisms behind runoff generation in hydrological systems. He delves into the processes that transform rainfall into runoff and streamflow, covering concepts like surface runoff, baseflow, and the various factors influencing them, such as topography, soil types, climate, and land use. The lecture emphasizes the complexity of runoff generation, with different mechanisms such as overland flow, interflow, and groundwater flow playing crucial roles. The interaction between climate, vegetation, and hydrological processes is also highlighted.

Takeaways

• 😀 The runoff generation process involves transforming rainfall into streamflow, with key components including surface runoff and baseflow.
• 😀 A catchment or watershed is the area where water flows towards a river and eventually exits via a gauging station, forming the basis for water balance calculations.
• 😀 Runoff is the portion of precipitation that flows into streams, either on the surface or within the ground, and is expressed as a unit of length per time (mm/year).
• 😀 Hydrographs represent discharge over time, showing both long-term (baseflow) and short-term (event-based) streamflow responses.
• 😀 The hydrograph's shape is influenced by several factors, including topography, soil types, storm characteristics, land use, and vegetation.
• 😀 The hydrograph can be divided into four phases: pre-event baseflow, initial abstraction, effective precipitation, and recession.
• 😀 Runoff mechanisms include channel precipitation, overland flow, subsurface flow (interflow), and groundwater flow, all of which contribute to streamflow during rainfall events.
• 😀 Overland flow can be classified into two types: Hortonian overland flow (caused by high rainfall intensity exceeding soil infiltration capacity) and saturation excess overland flow (occurring on saturated areas).
• 😀 Subsurface flow, including interflow and groundwater flow, plays a crucial role in runoff generation, especially in humid and temperate climates.
• 😀 Climate, topography, soil types, and vegetation all interact to influence runoff processes, with different regions (arid vs. humid) exhibiting different dominant runoff mechanisms.

Q & A

What is runoff and how is it related to precipitation?

Runoff is the portion of precipitation that flows towards streams either on the surface (surface runoff) or within the ground (baseflow). It is a key component in the water balance of a catchment area and can be quantified as the volume of water flowing through a river cross-section over a unit time.

What is the difference between surface runoff and baseflow?

Surface runoff is water that travels overland to the stream channel, whereas baseflow is the streamflow that results from precipitation infiltrating the soil and participating in subsurface processes. Baseflow is often considered groundwater flow or dry weather flow.

How does a hydrograph represent the streamflow response to rainfall?

A hydrograph is a graphical representation of streamflow (or discharge) versus time. It shows how the discharge at a catchment outlet changes in response to a rainfall event, reflecting both high and low flow conditions.

What factors influence the shape of a hydrograph?

The shape of a hydrograph is influenced by topography, soil types, geology, antecedent wetness conditions, storm characteristics (like rainfall intensity, amount, and duration), land use, vegetation, and the degree of urbanization in the catchment.

What is effective precipitation and how does it contribute to runoff?

Effective precipitation refers to the portion of rainfall that contributes directly to runoff. This is the rainfall that exceeds initial abstractions like surface interception and infiltration, and is translated into runoff via overland flow or subsurface processes.

What are the key runoff mechanisms in a catchment?

Runoff generation is influenced by several mechanisms: channel precipitation, overland flow, subsurface throughflow (or interflow), and groundwater flow. These processes work together to produce surface runoff and baseflow.

What are the differences between Hortonian overland flow and saturation excess overland flow?

Hortonian overland flow occurs when rainfall intensity exceeds the soil's infiltration capacity, causing water to flow over the surface. In contrast, saturation excess overland flow happens when precipitation falls on saturated areas, causing those areas to contribute to runoff.

What role do macropores play in subsurface runoff?

Macropores, formed by earthworms and bioactivity in the soil, provide rapid pathways for water to travel through the subsurface, accelerating the movement of water from the soil to the stream channel faster than matrix flow.

How do groundwater flow and interflow contribute to streamflow during a rainfall event?

During a rainfall event, groundwater flow (or baseflow) contributes to streamflow by exfiltrating water towards the stream, while interflow, which moves laterally through the unsaturated zone, also contributes to the runoff, often before the water reaches deeper groundwater levels.

How do the runoff mechanisms differ between arid and humid climates?

In arid and semi-arid regions, the runoff is mainly driven by surface processes like Hortonian overland flow due to high rainfall intensities and low soil infiltration capacities. In humid climates, subsurface processes such as interflow and groundwater flow are more dominant in generating runoff.