😅Hydrological Modeling Unit 6 – Infiltration and Soil Water Processes

Key Concepts and Definitions

• Infiltration: the process by which water on the ground surface enters the soil
• Infiltration rate: the rate at which water enters the soil, typically measured in mm/hr or cm/hr
• Infiltration capacity: the maximum rate at which water can infiltrate into the soil under given conditions
• Soil water content: the amount of water held in the soil, often expressed as a percentage or fraction of the soil volume
• Soil water potential: the energy state of water in the soil, which determines its tendency to move
  • Matric potential: the component of soil water potential due to the attraction between water and soil particles
  • Gravitational potential: the component of soil water potential due to the elevation of the water above a reference level
• Hydraulic conductivity: a measure of the soil's ability to transmit water, dependent on soil properties and water content
• Wetting front: the boundary between the wet and dry soil zones during infiltration

Physical Processes of Infiltration

• Infiltration begins with water entering the soil surface through pores, cracks, and other openings
• As water infiltrates, it displaces air in the soil pores and increases the soil water content
• The rate of infiltration is initially high but decreases over time as the soil becomes saturated
• Water moves through the soil profile under the influence of gravity and capillary forces
  • Gravity pulls water downward, while capillary forces can move water in any direction, depending on the soil water potential gradient
• The wetting front advances deeper into the soil as infiltration continues, with a sharp contrast in water content between the wet and dry zones
• When the infiltration rate exceeds the soil's saturated hydraulic conductivity, water may pond on the surface or run off
• Infiltration eventually reaches a steady state, where the rate equals the saturated hydraulic conductivity of the soil

Factors Affecting Infiltration Rates

• Soil texture: coarser soils (sandy) generally have higher infiltration rates than finer soils (clay) due to larger pore spaces
• Soil structure: well-aggregated soils with stable structure have higher infiltration rates than compacted or poorly structured soils
• Initial soil water content: dry soils have higher infiltration rates than wet soils, as there is more available pore space for water to enter
• Surface conditions: vegetation, litter, and surface roughness can increase infiltration by slowing runoff and providing pathways for water entry
  • Conversely, surface sealing or crusting can reduce infiltration by blocking pores
• Soil layering: the presence of layers with different hydraulic properties can affect infiltration
  • A less permeable layer below a more permeable layer can cause water to accumulate and form a perched water table
• Antecedent moisture conditions: the soil water content prior to an infiltration event influences the infiltration rate and capacity
• Land use and management practices: activities such as tillage, compaction, and vegetation removal can alter soil properties and infiltration rates

Soil Water Movement and Storage

• Once water infiltrates into the soil, it moves through the pore spaces and is stored in the soil profile
• Water movement in unsaturated soils is driven by differences in soil water potential
  • Water moves from areas of high potential (wet) to areas of low potential (dry)
• The rate and direction of water movement are determined by the hydraulic conductivity and the soil water potential gradient
• Hydraulic conductivity is a function of soil water content, with higher conductivity in wetter soils
• Water is held in the soil by capillary forces and adsorption to particle surfaces
  • The amount of water stored depends on the soil texture, structure, and organic matter content
• Field capacity is the amount of water held in the soil after excess water has drained away by gravity
• Permanent wilting point is the soil water content at which plants can no longer extract water and begin to wilt
• Available water capacity is the difference between field capacity and permanent wilting point, representing the water that plants can use
• Soil water storage is important for plant growth, groundwater recharge, and baseflow in streams

Mathematical Models of Infiltration

• Mathematical models are used to quantify and predict infiltration processes based on soil properties and boundary conditions
• The Green-Ampt model is a simplified physically-based model that assumes a sharp wetting front and a constant hydraulic conductivity behind the front
  • It calculates infiltration rate as a function of time, initial soil water content, and soil hydraulic properties
• The Philip infiltration model is a semi-analytical solution to the Richards equation, which describes unsaturated flow in porous media
  • It expresses infiltration as a series of time-dependent terms, with the first two terms representing the main components of infiltration
• The Horton equation is an empirical model that describes the exponential decay of infiltration rate over time
  • It includes parameters for initial and final infiltration rates, as well as a decay constant
• The Kostiakov equation is another empirical model that relates infiltration to time using power functions
  • It is simple to use but does not have a clear physical basis
• Numerical models, such as HYDRUS and SWMS, solve the Richards equation using finite difference or finite element methods
  • These models can handle complex boundary conditions and heterogeneous soil properties but require more input data and computational resources

Field Measurement Techniques

• Measuring infiltration rates in the field is important for understanding soil water dynamics and validating models
• Infiltrometers are devices used to measure infiltration rates by applying water to a confined area of the soil surface
  • Single-ring infiltrometers are simple to use but may overestimate infiltration rates due to lateral spreading of water
  • Double-ring infiltrometers use an outer ring to minimize lateral flow, providing a more accurate estimate of vertical infiltration
• Rainfall simulators can be used to measure infiltration under controlled rainfall conditions
  • They allow for the study of infiltration processes under different intensities and durations of rainfall
• Tension infiltrometers apply water to the soil surface under a negative pressure head, allowing for the measurement of unsaturated infiltration rates
• Tracer techniques, such as dye or isotope tracers, can be used to visualize and quantify water movement in the soil profile
• Soil moisture sensors, such as time-domain reflectometry (TDR) probes, can monitor changes in soil water content over time and depth
• Remote sensing techniques, such as satellite imagery and ground-penetrating radar, can provide estimates of soil moisture and infiltration patterns at larger scales

Applications in Hydrological Modeling

• Infiltration is a key component of the hydrological cycle and is essential for accurate modeling of watershed processes
• Infiltration models are used to estimate the partitioning of rainfall into infiltration and runoff
  • This information is critical for predicting flood events, erosion, and contaminant transport
• Infiltration parameters are important inputs for hydrological models, such as the Soil and Water Assessment Tool (SWAT) and the Variable Infiltration Capacity (VIC) model
• Coupled surface-subsurface models, such as ParFlow and HydroGeoSphere, simulate infiltration and groundwater flow processes simultaneously
  • These models can provide a more comprehensive understanding of the interactions between surface water and groundwater
• Infiltration modeling is also important for irrigation management, as it helps determine the optimal amount and timing of water application
• In urban hydrology, infiltration models are used to design and evaluate stormwater management practices, such as permeable pavements and bioretention systems
• Climate change studies rely on infiltration modeling to assess the impacts of changing precipitation patterns on water resources and ecosystem functions

Challenges and Limitations

• Measuring and modeling infiltration processes can be challenging due to the high spatial and temporal variability of soil properties and boundary conditions
• Soil heterogeneity, such as the presence of macropores or layering, can lead to preferential flow paths that are difficult to capture in models
• The scale of measurement and modeling can affect the accuracy and representativeness of infiltration estimates
  • Point measurements may not capture the larger-scale variability, while coarse-resolution models may miss important local processes
• Temporal variability in soil properties, such as changes in structure or hydrophobicity, can affect infiltration rates over time
• Surface conditions, such as vegetation or surface sealing, can be difficult to parameterize in models
• Calibrating and validating infiltration models requires extensive field data, which can be time-consuming and expensive to collect
• The choice of infiltration model depends on the available data, computational resources, and the specific application
  • Simpler models may be sufficient for some purposes, while more complex models may be needed for detailed process understanding
• Uncertainty in model parameters and structure can propagate through hydrological simulations, affecting the reliability of predictions
• Integrating infiltration processes with other hydrological and ecological processes remains a challenge, particularly at larger scales