7.1 Surface runoff generation processes
5 min read•july 30, 2024
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
Top images from around the web for Types of Surface Runoff
HESS - Modelling the hydrological interactions between a fissured granite aquifer and a valley ... View original
Is this image relevant?
Frontiers | Characterization of Surface Runoff Pathways and Erosion Using Hydrological ... View original
Is this image relevant?
HESS - On the representation of water reservoir storage and operations in large-scale ... View original
Is this image relevant?
HESS - Modelling the hydrological interactions between a fissured granite aquifer and a valley ... View original
Is this image relevant?
Frontiers | Characterization of Surface Runoff Pathways and Erosion Using Hydrological ... View original
Is this image relevant?
1 of 3
Top images from around the web for Types of Surface Runoff
HESS - Modelling the hydrological interactions between a fissured granite aquifer and a valley ... View original
Is this image relevant?
Frontiers | Characterization of Surface Runoff Pathways and Erosion Using Hydrological ... View original
Is this image relevant?
HESS - On the representation of water reservoir storage and operations in large-scale ... View original
Is this image relevant?
HESS - Modelling the hydrological interactions between a fissured granite aquifer and a valley ... View original
Is this image relevant?
Frontiers | Characterization of Surface Runoff Pathways and Erosion Using Hydrological ... View original
Is this image relevant?
1 of 3
- Surface runoff, also known as , occurs when precipitation or snowmelt exceeds the of the soil or when the soil becomes saturated
- ###-excess_overland_flow_0### (Hortonian overland flow) occurs when the rainfall intensity exceeds the infiltration capacity of the soil, resulting in surface runoff
- (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
- () is the lateral movement of water through the soil profile, which can contribute to surface runoff when it emerges as or seepage
- Subsurface stormflow occurs within the unsaturated zone of the soil profile
- It is driven by hydraulic gradients and the soil's
- Subsurface stormflow can be influenced by soil layering, preferential flow paths (), 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 or infiltration rate decay
- Factors influencing infiltration capacity include , structure, organic matter content, and initial 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
- , 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 , 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 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
- 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
- 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, , 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
- 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
Key Terms to Review (30)
Antecedent moisture conditions: Antecedent moisture conditions refer to the level of soil moisture present before a rainfall event or storm. This term is crucial because it affects how much water can infiltrate the soil versus how much becomes surface runoff, influencing hydrological responses in various scenarios.
Capillarity: Capillarity, or capillary action, refers to the ability of water to flow in narrow spaces without the assistance of external forces, such as gravity. This phenomenon occurs due to the intermolecular forces between water molecules and surrounding materials, leading to the rise or fall of liquid in a thin tube or porous medium. Capillarity plays a crucial role in processes like infiltration, where water moves through soil and other substrates, as well as influencing surface runoff generation by affecting how water interacts with various surfaces.
Empirical Methods: Empirical methods refer to a set of research techniques that rely on observation and experimentation to gather data and derive conclusions. These methods are based on real-world evidence rather than theory or pure logic, making them essential in understanding various phenomena, including surface runoff generation processes. By employing empirical methods, researchers can quantify relationships, test hypotheses, and develop predictive models that reflect actual conditions observed in the environment.
Evapotranspiration: Evapotranspiration is the combined process of water evaporation from the soil and other surfaces, along with plant transpiration from leaves. This process is crucial for understanding water movement in the environment and plays a significant role in various hydrological processes, such as water balance, surface runoff, and the overall health of ecosystems.
Green-ampt model: The Green-Ampt model is an infiltration model used to describe the movement of water into the soil, based on the concepts of suction and hydraulic conductivity. It quantifies how water infiltrates into soil layers, particularly during rainfall, and helps to predict surface runoff generation, soil water movement, and how moisture is stored within the soil profile.
Hydraulic conductivity: Hydraulic conductivity is a property of soil or rock that describes its ability to transmit water when subjected to a hydraulic gradient. It plays a crucial role in understanding how water moves through the soil, influencing infiltration, drainage, and groundwater flow in various contexts, such as during rainfall events or in aquifer systems.
Hydrologic cycle: The hydrologic cycle, also known as the water cycle, is the continuous movement of water within the Earth and atmosphere, involving processes such as evaporation, condensation, precipitation, and runoff. This cycle is crucial for maintaining ecosystem health and influencing climate patterns. It connects various hydrological processes, including surface runoff generation, which impacts water availability and quality, and serves as a fundamental framework for hydrological modeling, providing insights into water resource management and environmental protection.
Hydrological simulation: Hydrological simulation refers to the use of mathematical models to replicate and predict the movement and distribution of water within a hydrological system. This process helps in understanding complex interactions between precipitation, soil moisture, surface runoff, and groundwater, providing valuable insights into water resource management and flood forecasting.
Infiltration: Infiltration is the process by which water on the ground surface enters the soil. It plays a crucial role in the movement of water through the hydrological cycle, impacting groundwater recharge, surface runoff, and overall watershed health.
Infiltration capacity: Infiltration capacity refers to the maximum rate at which water can enter the soil surface, influenced by various soil properties and moisture conditions. This concept is crucial as it determines how much rainfall can infiltrate into the ground versus how much will become surface runoff, impacting water availability and ecosystem health. Understanding infiltration capacity helps in predicting surface runoff generation, selecting appropriate measurement techniques, and considering the spatial and temporal variability of water movement in the hydrologic cycle.
Infiltration Curve: An infiltration curve is a graphical representation that shows the relationship between the rate of infiltration of water into soil and time. This curve is essential for understanding how much rainfall can be absorbed by the soil before excess water begins to flow over the surface, contributing to surface runoff. The shape of the curve can vary based on soil characteristics, vegetation cover, and land use, making it a vital tool in hydrological modeling and water resource management.
Infiltration-excess overland flow: Infiltration-excess overland flow occurs when the rate of rainfall exceeds the soil's ability to absorb water, leading to surface runoff. This phenomenon is significant in areas with saturated soil or impervious surfaces, causing excess water to flow over the land. Understanding this process is crucial as it impacts both hydrological modeling and the management of land use and land cover.
Interflow: Interflow refers to the lateral movement of water within the soil layers, occurring below the surface but above the groundwater table. This process is crucial in understanding how water moves through the landscape, as it contributes to both subsurface flow and surface runoff generation. Interflow can help determine how quickly and efficiently rainfall infiltrates into the soil and eventually reaches streams or rivers, significantly impacting hydrological modeling and watershed management.
Land use changes: Land use changes refer to the alteration of land cover and its functions due to human activities such as agriculture, urbanization, deforestation, and industrialization. These changes can significantly impact hydrological processes, influencing how water is absorbed into the ground, how it flows over the surface, and how it is distributed across drainage networks. The effects of these changes can also be seen in the water balance of a region, as they affect evaporation, infiltration, and runoff patterns.
Macropores: Macropores are large soil pores that can enhance the movement of water and air through the soil profile. They play a crucial role in hydrological processes, particularly by facilitating rapid infiltration and drainage, which can significantly impact surface runoff generation, soil water retention, and hydraulic conductivity.
Moisture Retention Curve: The moisture retention curve is a graphical representation that illustrates the relationship between the water content of a soil and the soil's matric potential, showing how much water the soil can hold at different tensions. This curve is critical for understanding how soils retain and release moisture, impacting infiltration rates, drainage, and the generation of surface runoff during precipitation events. It plays a vital role in hydrological modeling, as it helps predict how water moves through different soil types and informs decisions regarding land management and water conservation.
Overland flow: Overland flow refers to the movement of water across the land surface, occurring when rainfall or snowmelt exceeds the infiltration capacity of the soil. This process is crucial in the generation of surface runoff, as it contributes to the transport of water to streams, rivers, and other bodies of water. Overland flow can be influenced by factors such as land slope, soil type, vegetation cover, and precipitation intensity.
Precipitation Surplus: Precipitation surplus refers to the amount of precipitation that exceeds the total evaporation and transpiration from a given area, resulting in an excess of water available for surface runoff, groundwater recharge, and other hydrological processes. This concept is crucial for understanding how excess water can contribute to surface runoff generation, especially during periods of heavy rainfall, and is vital for water resource management and flood risk assessment.
Return flow: Return flow refers to the portion of water that re-enters the groundwater system or surface water bodies after being applied for agricultural, municipal, or industrial use. This concept is crucial for understanding how water moves through the hydrological cycle, particularly in relation to how water is utilized and then subsequently returns to the environment. Recognizing return flow is essential for managing water resources sustainably and predicting the impacts of land use and climate on hydrology.
Saturation-excess overland flow: Saturation-excess overland flow occurs when the soil becomes saturated and can no longer absorb water, leading to surface runoff. This phenomenon is driven by factors like soil moisture, precipitation, and land surface characteristics. It is crucial in understanding how excess rainfall contributes to flooding and runoff generation processes, while also being influenced by land use and cover that affect soil infiltration and saturation rates.
SCS Curve Number Method: The SCS Curve Number Method is a widely used hydrological technique developed by the Soil Conservation Service (now part of the Natural Resources Conservation Service) for estimating direct runoff from a rainfall event. This method uses a curve number (CN) that reflects the land use, hydrologic soil group, and moisture conditions to predict the amount of runoff generated from rainfall. It connects closely with various hydrological modeling approaches, surface runoff generation processes, urban hydrology particularly concerning impervious surfaces, and hydrograph analysis by providing a simplified yet effective way to estimate runoff characteristics.
Slope gradient: Slope gradient refers to the steepness or degree of incline of a surface, typically expressed as a percentage or an angle. It plays a crucial role in influencing water movement, erosion rates, and runoff generation. A higher slope gradient generally leads to faster surface runoff due to gravity, while gentler slopes allow for greater infiltration and less runoff. This relationship is significant when examining surface runoff processes, watershed characteristics, and the variability of the hydrologic cycle.
Soil moisture: Soil moisture refers to the water held in the spaces between soil particles, which is crucial for plant growth and plays a vital role in the hydrological cycle. This moisture impacts various processes including runoff generation, evapotranspiration, and is influenced by precipitation and other hydrological components. Understanding soil moisture is essential for effective land management and assessing water availability in ecosystems.
Soil Properties: Soil properties refer to the physical, chemical, and biological characteristics of soil that influence its behavior and capacity to retain and transmit water. These properties include texture, structure, permeability, porosity, and moisture content, all of which play critical roles in determining how water interacts with the soil during surface runoff generation processes.
Soil Structure: Soil structure refers to the arrangement of soil particles and the spaces between them, which affects how water, air, and roots move through the soil. This arrangement influences various soil properties such as porosity, permeability, and water retention, making it essential for understanding water movement and retention processes in soils. The organization of soil particles can significantly impact how runoff is generated, how effectively water infiltrates, and how water is retained in the soil profile.
Soil Texture: Soil texture refers to the composition and size distribution of soil particles, including sand, silt, and clay. This property significantly influences various hydrological processes, such as the movement of water through soil, its capacity to hold water and nutrients, and how it affects plant growth. Understanding soil texture is crucial for predicting infiltration rates, runoff generation, and the overall water balance in the root zone.
Subsurface Stormflow: Subsurface stormflow is the lateral movement of water through the soil layer below the surface during and after precipitation events. This process is critical in understanding how water travels through the landscape and contributes to surface runoff generation, as it can significantly influence streamflow dynamics, soil saturation levels, and overall hydrological response in a watershed. By moving laterally, subsurface stormflow acts as a vital link between subsurface water storage and surface water systems.
Surface runoff: Surface runoff is the flow of water, typically rainwater, that occurs when excess water from precipitation or melting snow cannot be absorbed by the soil and instead flows over the land surface. This phenomenon plays a crucial role in the hydrological cycle, influencing processes such as water balance in root zones, hydrological modeling, hydrograph analysis, and the use of geographic information systems for terrain analysis.
Variable Source Area Concept: The variable source area concept refers to the idea that the areas contributing to surface runoff in a watershed change over time, influenced by factors like precipitation, soil moisture, and land cover. This concept highlights that runoff does not originate from a fixed area but rather from a dynamic landscape where wet areas expand and contract depending on hydrological conditions, affecting how water flows across the surface and enters streams.
Watershed: A watershed is an area of land that drains rainwater and snowmelt into a common outlet, such as a river, lake, or ocean. It serves as a crucial component in understanding surface runoff generation, watershed management practices, and the overall functioning of the hydrologic cycle. The boundaries of a watershed are defined by its topography, making it essential for delineation techniques and effective water resource management.