7.1 Surface runoff generation processes

Surface runoff is a key player in the water cycle, shaping how rain and snowmelt move across land. It happens when water can't soak into the ground fast enough, either because the soil's full or the rain's coming down too hard.

Understanding surface runoff is crucial for managing water resources and preventing floods. It's influenced by factors like soil type, land use, and how wet the ground was before it rained. These processes determine how much water ends up in rivers and streams.

Surface Runoff Generation Mechanisms

Types of Surface Runoff

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• Surface runoff, also known as overland flow, occurs when precipitation or snowmelt exceeds the infiltration capacity of the soil or when the soil becomes saturated
• ###infiltration-excess_overland_flow_0### (Hortonian overland flow) occurs when the rainfall intensity exceeds the infiltration capacity of the soil, resulting in surface runoff
• Saturation-excess overland flow (Dunne overland flow) occurs when the soil becomes saturated, and any additional water input leads to surface runoff
• Direct precipitation onto saturated areas, such as wetlands or riparian zones, can also generate surface runoff

Subsurface Stormflow

• Subsurface stormflow (interflow) is the lateral movement of water through the soil profile, which can contribute to surface runoff when it emerges as return flow or seepage
  • Subsurface stormflow occurs within the unsaturated zone of the soil profile
  • It is driven by hydraulic gradients and the soil's hydraulic conductivity
  • Subsurface stormflow can be influenced by soil layering, preferential flow paths (macropores), and bedrock topography
  • The contribution of subsurface stormflow to surface runoff depends on factors such as soil depth, permeability, and hillslope characteristics

Infiltration and Saturation in Runoff

Infiltration Process

• Infiltration is the process by which water enters the soil surface and moves downward through the soil profile
• The infiltration capacity of the soil determines the maximum rate at which water can enter the soil under given conditions
  • Infiltration capacity is typically highest at the beginning of a rainfall event and decreases over time as the soil becomes saturated
  • The decrease in infiltration capacity is known as the infiltration curve or infiltration rate decay
• Factors influencing infiltration capacity include soil texture, structure, organic matter content, and initial soil moisture conditions
  • Coarse-textured soils (sandy soils) generally have higher infiltration rates compared to fine-textured soils (clay soils)
  • Well-structured soils with stable aggregates and high organic matter content promote infiltration by creating a network of pores and channels

Saturation and Runoff Generation

• As the soil becomes saturated, its ability to absorb additional water decreases, leading to increased surface runoff
• The soil's hydraulic conductivity, which is a measure of its ability to transmit water, affects the rate of infiltration and the development of saturated conditions
  • Hydraulic conductivity varies with soil texture, structure, and moisture content
  • Soils with high hydraulic conductivity allow for faster infiltration and drainage, while soils with low hydraulic conductivity may become saturated more quickly
• The presence of macropores, such as root channels or animal burrows, can facilitate preferential flow and rapid infiltration, even when the soil matrix is saturated
  • Macropores act as conduits for water movement, bypassing the soil matrix
  • Preferential flow through macropores can contribute to subsurface stormflow and rapid response to rainfall events

Factors Influencing Surface Runoff

Soil Properties

• Soil properties, such as texture, structure, and organic matter content, affect infiltration capacity and, consequently, surface runoff generation
  • Sandy soils generally have higher infiltration rates compared to clay soils due to larger pore spaces and better drainage
  • Well-structured soils with stable aggregates and high organic matter content tend to have higher infiltration rates and less surface runoff
• Soil crusting or sealing can reduce infiltration and increase surface runoff
  • Soil crusts form due to raindrop impact, compaction, or chemical dispersion of soil particles
  • Crusts create a barrier at the soil surface, limiting water entry and promoting runoff

Land Use and Land Cover

• Land use and land cover significantly influence surface runoff generation
  • Urbanization and impervious surfaces, such as roads and buildings, reduce infiltration and increase surface runoff
  • Vegetation cover, particularly dense canopy and ground cover, intercepts precipitation, reduces raindrop impact, and promotes infiltration, thus reducing surface runoff
• Agricultural practices, such as tillage and crop management, can affect soil structure, infiltration, and surface runoff
  • Conventional tillage practices (plowing) can disrupt soil structure and create compacted layers, reducing infiltration
  • Conservation tillage practices (no-till or reduced tillage) maintain crop residue on the soil surface, protecting against erosion and enhancing infiltration

Topography

• Topography, including slope gradient and length, affects the velocity and concentration of surface runoff
  • Steeper slopes generally result in faster and more concentrated surface runoff, while gentler slopes allow for more infiltration and slower runoff
  • Longer slope lengths provide more opportunity for runoff to accumulate and gain velocity, increasing erosion potential
• The presence of surface depressions, such as puddles or microrelief, can store water temporarily and reduce surface runoff
  • Surface depressions act as temporary storage sites for water, allowing for increased infiltration and evaporation
  • The spatial distribution and connectivity of surface depressions influence the overall runoff response of a landscape

Antecedent Moisture and Runoff Generation

Antecedent Moisture Conditions

• Antecedent moisture conditions refer to the soil moisture status prior to a rainfall event
• Higher antecedent moisture conditions reduce the soil's capacity to store additional water, leading to increased surface runoff
  • When the soil is already wet, its infiltration capacity is lower, and the likelihood of saturation-excess runoff increases
  • In dry antecedent conditions, the soil has a greater capacity to absorb water, resulting in less surface runoff
• The soil moisture content affects the soil's infiltration capacity and the development of saturated areas
  • The relationship between soil moisture and infiltration capacity is often described by the soil-water characteristic curve or moisture retention curve
  • As soil moisture increases, the infiltration capacity decreases, and the potential for runoff generation increases

Factors Influencing Antecedent Moisture

• Antecedent moisture conditions can be influenced by factors such as previous rainfall events, evapotranspiration, and groundwater levels
  • Previous rainfall events contribute to soil moisture storage and can affect the runoff response to subsequent events
  • Evapotranspiration, driven by factors such as temperature, humidity, and vegetation, removes water from the soil and influences antecedent moisture conditions
• Seasonal variations in antecedent moisture conditions can lead to different runoff responses for similar rainfall events
  • In temperate regions, antecedent moisture conditions are typically higher during the wet season (spring and fall) compared to the dry season (summer)
  • Seasonal changes in vegetation cover and evapotranspiration rates also influence antecedent moisture conditions

Variable Source Area Concept

• The concept of variable source areas suggests that the spatial extent of saturated areas contributing to surface runoff varies depending on antecedent moisture conditions and rainfall characteristics
  • Variable source areas expand and contract based on the soil moisture status and the rainfall event
  • Areas near streams, wetlands, or with shallow water tables are more likely to become saturated and contribute to surface runoff
• The connectivity of variable source areas influences the overall runoff response of a watershed
  • When variable source areas are highly connected, they can efficiently convey water to streams and generate rapid runoff
  • Disconnected variable source areas may store water temporarily and have a delayed contribution to runoff