6.4 Root zone water balance

Root zone water balance is crucial for understanding soil water dynamics. It tracks water inputs like precipitation and irrigation, outputs like evapotranspiration and runoff, and changes in soil water storage within the plant root zone.

Measuring these components helps quantify water availability for plants. Vegetation impacts soil water through root uptake and by altering water fluxes. Models of root zone water balance aid in optimizing irrigation and assessing water management strategies for crops.

Root zone water balance components

Water fluxes and storage in the root zone

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• The root zone water balance quantitatively describes water fluxes and storage changes within the soil layer where plant roots are present
• Main components include precipitation, irrigation, evapotranspiration, surface runoff, deep percolation, and changes in soil water storage
• Precipitation and irrigation are the primary water inputs to the root zone (rainfall, sprinkler systems)
• Evapotranspiration, surface runoff, and deep percolation are the main water outputs (plant water use, overland flow, leaching)
• Soil water storage represents the amount of water held in the soil pores within the root zone and can change over time due to the balance between water inputs and outputs (field capacity, wilting point)

Interactions between water balance components

• The interactions between water balance components determine water availability for plant uptake and overall water dynamics within the root zone
• Precipitation and irrigation add water to the root zone, increasing soil water storage and potentially generating surface runoff or deep percolation if the infiltration capacity is exceeded
• Evapotranspiration removes water from the root zone, reducing soil water storage and potentially causing plant water stress if the soil water supply cannot meet the atmospheric demand
• Surface runoff and deep percolation occur when water inputs exceed the soil's infiltration capacity or when the soil water content reaches saturation, resulting in water losses from the root zone
• The balance between water inputs, outputs, and storage changes determines the temporal and spatial patterns of soil water availability for plant growth and other ecosystem processes

Quantifying water in the root zone

Measuring water inputs and outputs

• Precipitation can be measured using rain gauges or weather station data to quantify water inputs from rainfall or snowmelt
• Irrigation inputs can be quantified using flow meters or by estimating application rates based on the irrigation system characteristics (sprinkler output, drip emitter flow rates)
• Evapotranspiration, including both soil evaporation and plant transpiration, can be estimated using methods such as the Penman-Monteith equation, crop coefficients, or direct measurements using lysimeters
• Surface runoff can be quantified using infiltration equations (Green-Ampt, Philip's equation) or by measuring flow rates at the field or watershed scale (runoff plots, stream gauges)
• Deep percolation, the downward movement of water below the root zone, can be estimated using soil water balance calculations or by measuring soil water content changes at the bottom of the root zone

Determining changes in soil water storage

• Changes in soil water storage can be determined by measuring soil water content using techniques such as time-domain reflectometry (TDR), capacitance probes, or neutron probes
• Soil water content measurements are typically taken at multiple depths within the root zone to capture the vertical distribution of water
• The change in soil water storage over time is calculated by comparing soil water content measurements at different time points (daily, weekly, seasonally)
• Accurate quantification of soil water storage changes requires consideration of soil properties (texture, bulk density) and the spatial variability of soil moisture within the field or landscape
• Remote sensing techniques, such as satellite imagery or ground-penetrating radar, can provide estimates of soil water content at larger spatial scales

Vegetation impact on soil water

Root water uptake and plant water stress

• Plant roots extract water from the soil to meet the transpiration demands of the vegetation, playing a crucial role in soil water dynamics
• The distribution and density of plant roots influence the spatial pattern of water uptake within the root zone (tap roots, fibrous roots)
• Root water uptake is driven by the water potential gradient between the soil and the plant leaves, which is influenced by factors such as soil moisture, plant hydraulic conductivity, and atmospheric demand
• Plant water stress occurs when the soil water supply cannot meet the transpiration demand, leading to reduced plant growth and water use efficiency (wilting, stomatal closure)

Vegetation effects on water balance components

• The presence of vegetation modifies the soil water dynamics by altering the partitioning of water fluxes between evaporation and transpiration
• Transpiration typically dominates over evaporation in well-developed canopies, while evaporation may be more significant in sparse or newly planted vegetation
• Vegetation characteristics, such as leaf area index (LAI), rooting depth, and canopy structure, influence the magnitude and timing of evapotranspiration fluxes
• Plant water uptake can reduce soil water storage, particularly during periods of high atmospheric demand or limited water supply (drought conditions)
• Vegetation can also affect surface runoff and infiltration processes by intercepting rainfall, modifying soil structure, and creating preferential flow paths through root channels

Root zone water balance models

Model development and components

• Root zone water balance models are mathematical representations of the water fluxes and storage changes within the root zone, based on the conservation of mass principle
• These models typically include components such as precipitation, irrigation, evapotranspiration, surface runoff, deep percolation, and changes in soil water storage
• The models simulate soil water dynamics over time, considering the specific soil properties, crop characteristics, and environmental conditions of the site
• Accurate input data, including soil properties, crop parameters, weather data, and irrigation information, are required for model development and application
• Proper calibration and validation against field measurements are essential to ensure the reliability and robustness of the model predictions

Model applications in water management

• Root zone water balance models can be used to optimize irrigation scheduling by determining the timing and amount of irrigation required to maintain soil moisture within the desired range for optimal crop growth
• The models can assess the impact of different irrigation strategies, such as deficit irrigation or partial root zone drying, on crop water use efficiency and yield (regulated deficit irrigation, alternate furrow irrigation)
• Coupling root zone water balance models with crop growth models allows for more comprehensive assessments of water management strategies, simulating the interactions between soil water availability and crop development
• The models can also be used to evaluate the effects of soil management practices (tillage, mulching) or climate variability (drought, heat waves) on root zone water dynamics and crop performance
• At larger scales, root zone water balance models can inform regional water resources planning and management by estimating irrigation water requirements, assessing the impacts of land use changes, and supporting the development of sustainable water allocation strategies