6.3 Soil water movement and storage
5 min read•july 30, 2024
Soil water movement and storage are crucial aspects of hydrology, influencing water availability for plants and groundwater . This topic explores the forces driving water flow in soils, from saturated to unsaturated conditions, and how soil properties affect water retention and transmission.
Understanding soil water dynamics is essential for predicting , runoff, and plant water uptake. We'll examine key concepts like soil water potential, , and water retention curves, as well as the equations governing water flow in different soil conditions.
Soil water potential, conductivity, and retention
Soil water potential and its components
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- Soil water potential is the energy state of water in the soil, which determines the direction and rate of water movement
- Composed of matric potential, osmotic potential, pressure potential, and gravitational potential
- Matric potential arises from the attraction between water and soil particles (adhesion) and the attraction between water molecules (cohesion)
- Osmotic potential is caused by the presence of dissolved solutes in the soil water, which reduces the water potential
- Pressure potential refers to the hydrostatic pressure exerted by overlying water on soil water
- Gravitational potential is the potential energy of water due to its position in the gravitational field
Hydraulic conductivity and water retention
- Hydraulic conductivity is a measure of the soil's ability to transmit water, expressed as the rate of water flow per unit gradient of hydraulic head
- Depends on soil texture, structure, and water content
- Coarse-textured soils (sand) have higher hydraulic conductivity than fine-textured soils (clay) due to larger pore sizes
- Water retention refers to the soil's capacity to hold water against gravitational forces
- Determined by the soil's pore size distribution and surface area
- The relationship between soil water content and matric potential is described by the soil water retention curve
- is the amount of water retained in the soil after gravitational (typically at a matric potential of -0.03 MPa)
- is the soil water content at which plants can no longer extract water (typically at a matric potential of -1.5 MPa)
- Available water capacity is the difference between field capacity and permanent wilting point, representing the water that plants can readily use
Soil water flow: Saturated vs unsaturated
Saturated flow
- Saturated flow occurs when all soil pores are filled with water, and the matric potential is zero or positive
- Water flows in response to differences in hydraulic head, following Darcy's law
- Hydraulic head is the sum of matric potential and gravitational potential, expressed in units of length (cm or m)
- Darcy's law states that the flow rate is proportional to the hydraulic head gradient and the soil's saturated hydraulic conductivity
- Example: Groundwater flow in aquifers or water flow in saturated soil layers
Unsaturated flow
- Unsaturated flow occurs when soil pores contain both water and air, and the matric potential is negative
- Water flows in response to gradients in both matric potential and gravitational potential, following the Richards equation
- The Richards equation is a non-linear partial differential equation that describes water flow in unsaturated soils
- Infiltration is the process of water entry into the soil surface, influenced by soil surface conditions, initial water content, and rainfall intensity
- Ponding occurs when the rainfall intensity exceeds the soil's infiltration capacity
- Redistribution is the movement of water within the soil profile after infiltration, driven by differences in matric potential
- Results in the formation of a wetting front (advancing front of water) and a drying front (receding front of water)
- is the combined process of water loss from the soil surface (evaporation) and plant leaves (transpiration), driven by atmospheric demand and regulated by plant characteristics
Factors influencing soil water storage
Soil properties
- Soil texture determines the pore size distribution and surface area, which affect water retention and hydraulic conductivity
- Coarse-textured soils (sand) have lower water retention and higher hydraulic conductivity compared to fine-textured soils (clay)
- Soil structure refers to the arrangement of soil particles into aggregates, influencing the size and continuity of pores
- Well-structured soils have better water retention and transmission properties than poorly structured soils
- Organic matter content increases soil water retention by improving soil structure and increasing surface area
- Enhances soil aggregation and promotes the formation of macropores
Landscape and vegetation factors
- Soil depth and layering affect the total water storage capacity and the vertical distribution of water
- The presence of impermeable layers (hardpans) or bedrock can restrict water movement and create perched water tables
- Topography influences soil water redistribution through surface and subsurface flow processes
- Water tends to move from higher to lower elevations, following the gradient of hydraulic head
- Vegetation affects soil water storage and redistribution through root water uptake, canopy interception, and modification of soil structure
- Different plant species have varying water use strategies and rooting depths
- Example: Deep-rooted plants (trees) can access water from deeper soil layers compared to shallow-rooted plants (grasses)
Quantifying water movement in soil profiles
Darcy's law for saturated flow
- Darcy's law describes the rate of water flow in saturated soils as a function of hydraulic conductivity and hydraulic head gradient
- Expressed as Q = -K * A * (dH/dL), where Q is the flow rate, K is the hydraulic conductivity, A is the cross-sectional area, and dH/dL is the hydraulic head gradient
- The negative sign indicates that water flows from high to low hydraulic head
- Example: Calculating the flow rate of water through a saturated soil column in a laboratory experiment
Richards equation for unsaturated flow
- The Richards equation is a non-linear partial differential equation that describes water flow in unsaturated soils
- Considers the relationships between water content, matric potential, and hydraulic conductivity
- Expressed as ∂θ/∂t = ∂/∂z [K(h) * (∂h/∂z + 1)], where θ is the volumetric water content, t is time, z is depth, h is the matric potential, and K(h) is the unsaturated hydraulic conductivity function
- The van Genuchten-Mualem model is commonly used to describe the soil water retention curve and the unsaturated hydraulic conductivity function
- Relates water content to matric potential using empirical parameters that depend on soil properties
Numerical methods and field measurements
- Numerical methods, such as finite difference or finite element schemes, are often employed to solve the Richards equation
- Simulate soil water flow in complex soil profiles and boundary conditions
- Example: Using a numerical model to predict soil water dynamics in a heterogeneous soil profile under variable rainfall conditions
- Field measurements of soil water content and matric potential can be used to parameterize soil water flow models and validate their predictions
- Time-domain reflectometry (TDR) and capacitance probes are commonly used to measure soil water content
- Tensiometers and heat dissipation sensors are used to measure soil matric potential
Key Terms to Review (18)
Capillary Water: Capillary water refers to the water that occupies the spaces between soil particles and is held in the soil due to surface tension. This form of water is crucial for plant growth, as it is the moisture available for uptake by roots. It plays a significant role in soil water movement and storage, influencing how effectively plants can access water and nutrients.
Discharge: Discharge is the volume of water that flows through a given cross-section of a river or stream per unit of time, typically measured in cubic meters per second (m³/s). It is a critical measure in hydrology, as it reflects the movement of water through different environments and is influenced by factors such as precipitation, soil moisture, and groundwater flow. Understanding discharge helps in assessing water availability, flood risks, and ecosystem health.
Drainage: Drainage refers to the process of removing excess water from soil or land, which is essential for maintaining optimal conditions for plant growth and preventing waterlogging. Effective drainage helps balance water levels in the root zone, influences soil water movement and storage, and is key in calculating water balance equations. It also plays a critical role in understanding soil water retention and hydraulic conductivity, as it affects how water is stored and moved through different soil layers.
Drainage Systems: Drainage systems refer to the network of structures and processes that manage the flow and storage of water in a landscape. They play a vital role in controlling excess water, preventing flooding, and facilitating the movement of water through soil and aquifers, which directly influences soil water movement and storage.
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.
Field Capacity: Field capacity is the amount of soil moisture or water content held in the soil after excess water has drained away and the rate of downward movement has decreased. This state occurs when the soil is saturated and gravity has pulled away the excess water, leaving behind moisture that can be absorbed by plant roots. Understanding field capacity is crucial for assessing root zone water balance, soil water movement, and storage, as well as managing irrigation systems effectively.
Gravitational water: Gravitational water is the portion of water in soil that is held under the influence of gravity and drains away from the soil profile. This type of water moves through soil layers, typically downward, due to the force of gravity and is not retained by soil particles. It plays a crucial role in soil water movement and storage as it affects how much water is available for plants and how quickly it can be replenished.
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.
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.
Irrigation efficiency: Irrigation efficiency refers to the ratio of the amount of water beneficially used by crops to the amount of water applied through an irrigation system. This concept is crucial as it helps in evaluating how effectively water resources are managed for agricultural purposes, ensuring that crops receive adequate moisture while minimizing waste. Understanding irrigation efficiency can also influence soil water movement and storage, as well as impact the modeling and allocation of water resources in irrigation systems.
Percolation: Percolation is the process by which water moves downward through the soil and porous rock layers, driven primarily by gravity. This movement is crucial for understanding how water is stored and transferred in the hydrological cycle, as it influences groundwater recharge and the availability of water for plants and other organisms. The rate and extent of percolation depend on soil characteristics, such as texture, structure, and moisture content, as well as environmental factors like precipitation and land use.
Permanent Wilting Point: The permanent wilting point (PWP) is the soil moisture level at which plants can no longer extract water, leading to irreversible wilting. This critical threshold impacts plant health and growth by defining the limit of soil moisture available for uptake, ultimately influencing the overall balance of water in the root zone, the movement of water through soil layers, and the soil's ability to retain water and its hydraulic properties.
Recharge: Recharge refers to the process by which water from precipitation, surface water, or irrigation infiltrates the ground and replenishes the groundwater aquifers. This process is essential for maintaining the balance of groundwater resources and impacts soil moisture levels and overall hydrology. Understanding recharge is crucial as it directly connects to both soil water movement and the broader hydrologic cycle.
Richards' Equation: Richards' Equation is a fundamental partial differential equation that describes the movement of water in unsaturated soils, capturing the dynamics of water flow due to gravity and soil moisture content. This equation is crucial for understanding how water moves through soil layers, influences soil water storage, and impacts water availability for plants. It relates to the retention characteristics of soil and helps model preferential flow paths in soil profiles.
Saturation: Saturation refers to the condition in which the soil or a given volume of water holds as much water as it can without any air spaces being present. This state is crucial for understanding various hydrological processes, including how water moves through the soil, how plants access moisture, and how precipitation interacts with the environment. When saturation occurs, it influences factors like drainage, runoff, and the availability of water for plant uptake.
Soil Horizon: A soil horizon is a distinct layer within the soil profile that has unique physical and chemical properties, resulting from processes like weathering, organic matter accumulation, and leaching. These layers can vary in composition, color, texture, and moisture content, impacting how water moves through the soil and is stored. Understanding soil horizons is crucial for predicting water retention, drainage capacity, and overall soil health.
Water table: The water table is the upper surface of saturated soil or rock where the pore spaces are completely filled with water. It marks the boundary between the unsaturated zone, where soil and rock contain both air and water, and the saturated zone below it, where all voids are filled with water. Understanding the water table is crucial for assessing groundwater resources, as well as its interaction with soil moisture, aquifers, and groundwater flow dynamics.