🌊Hydrology Unit 4 – Infiltration and Soil Water

Key Concepts

• Infiltration: the process by which water enters the soil surface and moves downward through the soil profile
• Soil water content: the amount of water held in the soil, typically expressed as a percentage or ratio of the soil volume
• Soil water potential: the energy state of water in the soil, which determines its tendency to move and be retained
• Hydraulic conductivity: a measure of the soil's ability to transmit water, dependent on soil properties such as texture and structure
• Wetting front: the boundary between the wet and dry portions of the soil profile during infiltration
  • Characterized by a sharp increase in soil water content and a decrease in soil water potential
• Preferential flow: the rapid movement of water through macropores, cracks, or other structural features in the soil, bypassing the soil matrix
• Infiltration capacity: the maximum rate at which water can enter the soil surface under given conditions
• Ponding: the accumulation of water on the soil surface when the infiltration rate is lower than the water supply rate

Water Movement in Soil

• Driven by hydraulic gradients: water moves from regions of high water potential (wet areas) to regions of low water potential (dry areas)
• Governed by Darcy's law: the flow rate is proportional to the hydraulic gradient and the soil's hydraulic conductivity
• Occurs in both saturated and unsaturated conditions
  • Saturated flow: all pores are filled with water, and flow is driven by pressure gradients
  • Unsaturated flow: pores contain both water and air, and flow is driven by both pressure and matric potential gradients
• Influenced by soil properties: texture, structure, organic matter content, and bulk density affect water retention and transmission
• Macropore flow: rapid movement through large pores, cracks, or channels, which can significantly influence infiltration and solute transport
• Hysteresis: the dependence of soil water content-potential relationships on the wetting and drying history of the soil
• Redistribution: the movement of water within the soil profile after infiltration, driven by matric potential gradients

Factors Affecting Infiltration

• Soil texture: coarser soils (sandy) generally have higher infiltration rates than finer soils (clay) due to larger pore sizes
• Soil structure: well-aggregated soils with stable structure have higher infiltration rates than compacted or structureless soils
• Initial soil moisture content: drier soils typically have higher initial infiltration rates due to greater matric potential gradients
• Surface conditions: vegetation cover, mulch, and surface roughness can increase infiltration by reducing runoff and evaporation
• Crusting and sealing: the formation of a low-permeability layer at the soil surface can reduce infiltration rates
  • Caused by raindrop impact, fine particle deposition, or chemical dispersion
• Hydrophobicity: the presence of water-repellent compounds (organic matter, fungi) can inhibit infiltration, especially in dry soils
• Frozen soils: the presence of ice in soil pores can significantly reduce infiltration rates and increase runoff

Infiltration Models and Equations

• Green-Ampt model: a simplified physically-based model that assumes a sharp wetting front and a constant hydraulic conductivity behind the front
  • Infiltration rate: $f(t) = K_s \left(1 + \frac{\psi_f \Delta\theta}{F(t)}\right)$
  • $K_s$: saturated hydraulic conductivity, $\psi_f$: wetting front suction, $\Delta\theta$: change in water content, $F(t)$: cumulative infiltration
• Philip's equation: an infinite series solution to the Richards equation for vertical infiltration into a homogeneous soil
  • Infiltration rate: $f(t) = \frac{1}{2} S t^{-1/2} + A$
  • $S$: sorptivity, $A$: steady-state infiltration rate
• Horton's equation: an empirical model that describes the exponential decay of infiltration rate over time
  • Infiltration rate: $f(t) = f_c + (f_0 - f_c) e^{-kt}$
  • $f_0$: initial infiltration rate, $f_c$: final (steady-state) infiltration rate, $k$: decay constant
• Kostiakov equation: another empirical model that relates infiltration rate to time using power functions
  • Infiltration rate: $f(t) = \alpha t^{-\beta}$
  • $\alpha$, $\beta$: empirical constants

Measurement Techniques

• Infiltrometers: devices used to measure infiltration rates in the field
  • Single-ring infiltrometers: a simple method that involves driving a metal ring into the soil and measuring the rate of water level decline
  • Double-ring infiltrometers: a more accurate method that uses two concentric rings to minimize lateral flow and better represent one-dimensional infiltration
• Rainfall simulators: equipment that applies artificial rainfall to a plot or soil sample to study infiltration under controlled conditions
• Tracer studies: the use of chemical or isotopic tracers to track water movement and quantify infiltration rates
  • Examples: bromide, chloride, deuterium, or tritium
• Soil moisture sensors: instruments that measure soil water content or potential, which can be used to monitor infiltration and redistribution
  • Time-domain reflectometry (TDR), capacitance sensors, tensiometers, and neutron probes
• Remote sensing: the use of satellite or airborne sensors to estimate soil moisture and infer infiltration patterns at larger scales
  • Passive and active microwave sensors (radiometers, radars), thermal infrared sensors, and multi-spectral imagery

Soil Water Content and Potential

• Soil water content: the amount of water stored in the soil, expressed as a volume or mass fraction
  • Volumetric water content ($\theta_v$): the ratio of water volume to total soil volume
  • Gravimetric water content ($\theta_g$): the ratio of water mass to dry soil mass
• Soil water potential: the energy state of water in the soil, which determines its tendency to move and be retained
  • Components: matric potential, osmotic potential, pressure potential, and gravitational potential
  • Matric potential: the attraction of water to soil particles due to capillary and adsorptive forces
• Soil water characteristic curve: the relationship between soil water content and matric potential, which is unique for each soil type
  • Typically measured using tensiometers, pressure plates, or vapor equilibration methods
• Available water capacity: the amount of water held between field capacity (upper limit) and permanent wilting point (lower limit)
  • Represents the water that is accessible to plants and microorganisms
• Soil water storage: the total amount of water held in a given soil depth or profile
  • Calculated by integrating soil water content over depth: $S = \int_{z_1}^{z_2} \theta(z) dz$

Applications in Hydrology

• Rainfall-runoff modeling: infiltration is a key component in determining the partitioning of rainfall into surface runoff and groundwater recharge
• Flood forecasting: accurate estimation of infiltration rates is essential for predicting the timing and magnitude of flood events
• Irrigation management: understanding soil water dynamics helps optimize irrigation scheduling and efficiency
  • Minimizing deep percolation losses and maximizing crop water use efficiency
• Contaminant transport: infiltration and preferential flow can greatly influence the movement of pollutants through the soil and into groundwater
• Soil erosion: the balance between infiltration and runoff affects the susceptibility of soils to erosion by water
  • Practices that enhance infiltration (cover crops, mulching, conservation tillage) can reduce erosion rates
• Ecosystem services: infiltration plays a crucial role in regulating water supply, purifying water, and supporting plant growth and biodiversity
• Land-atmosphere interactions: soil moisture and infiltration patterns influence evapotranspiration, heat fluxes, and atmospheric processes

Challenges and Future Directions

• Spatial variability: infiltration rates can vary greatly across landscapes due to heterogeneity in soil properties, vegetation, and topography
  • Requires high-resolution measurements and modeling approaches to capture this variability
• Temporal dynamics: infiltration processes are highly dynamic and can change rapidly in response to rainfall events, land use changes, and management practices
• Scaling issues: reconciling infiltration measurements and models across different spatial and temporal scales remains a challenge
  • From point measurements to field, watershed, and regional scales
• Climate change impacts: altered precipitation patterns, temperature regimes, and extreme events may significantly affect infiltration and soil water dynamics
  • Requires adaptive management strategies and resilient design of hydrologic systems
• Coupled processes: infiltration interacts with other hydrologic processes (evapotranspiration, groundwater flow) and biogeochemical cycles (carbon, nutrients)
  • Calls for integrated, multi-disciplinary approaches to understand and predict these interactions
• Advances in monitoring: the development of new sensors, remote sensing techniques, and data assimilation methods can improve our ability to quantify infiltration across scales
• Process-based modeling: the incorporation of infiltration processes into physically-based, distributed hydrologic models can enhance their predictive capabilities and support decision-making