TXT Soil Water and Its Management Flashcards

Soil Water Management: A General Overview

• Plant Water Requirements: Water is essential for photosynthesis, tissue rigidity (turgidity), and carbohydrate production. Water is drawn from the soil via roots through the plant's xylem.

• Transpiration and Gas Exchange:

  • Transpiration is the loss of water from microscopic leaf openings called stomata.

  • Stomatal opening facilitates the exchange of $CO_2$ (needed for growth) and $O_2$ (expelled during photosynthesis or taken up during respiration).

  • Transpiration is an "undesirable trade-off" of canopy gas exchange, often losing more water than necessary for turgidity.

• Water Availability Outcomes:

  • Demand > Availability: Leads to reduced growth or plant mortality.

  • Natural Vegetation: Drought periods define plant community adaptations.

  • Agriculture: Requires irrigation or water harvesting to meet demand, especially in arid climates where deep subsurface stores are used.

• Excess Water Complications:

  • Excess water hampers gas exchange between roots and soil, leading to hypoxia (oxygen starvation).

  • Gases diffuse much slower through water than air ($≈ 10,000$ times slower).

  • Toxic effects include the formation of nitric oxide during root tissue hypoxia.

• Soil Water Management Definition: The collective practices of irrigation, improving natural drainage, and soil water conservation (suppressing evaporative/drainage losses, runoff control, tillage, or soil amendments).

• Integrated Water Resources Management (IWRM): A systematic process for the sustainable development, allocation, and monitoring of water resource use in the context of social, economic, and environmental objectives.

The Soil Water Balance

• The Root Zone Water Balance Equation:

  • $\Delta\Theta = P + I + C - E - T - D - R$

  • $\Delta\Theta$: Change in soil moisture content.

  • $P$: Precipitation (Gain).

  • $I$: Irrigation (Gain).

  • $C$: Capillary rise (Gain).

  • $E$: Soil evaporation (Loss).

  • $T$: Transpiration (Loss).

  • $D$: Deep drainage (Loss).

  • $R$: Runoff (positive value = loss) or Runon (negative value = gain).

• Units: Typically expressed as depth per unit time, e.g., $mm\,H_2O\,day^{-1}$.

• Hydrological Context: The soil water store is small relative to ice or groundwater but critical for human sustenance via crop growth.

• Interception Loss: Precipitation or irrigation intercepted by vegetation that evaporates before reaching the ground. Significant in forests, less so in crops.

Infiltration and Redistribution

• Infiltration Rate: The amount of water per unit area per unit time entering the soil. It is determined by supply rate (P/I), initial moisture, and water permeability (hydraulic conductivity).

• Infiltration Capacity (Infiltrability):

  • Initially high in dry soils due to a large suction gradient between the surface and drier underlying soil.

  • Decreases as the profile wets up and the suction gradient declines.

  • Steady-State Infiltrability: Reached when topsoil is saturated; infiltration rate then equals the saturated hydraulic conductivity, $K_s$.

• Surface Storage Capacity: The total volume of surface depressions where excess water collects before runoff occurs.

• Runoff Factors: Intensity of supply vs. infiltration rate, slope, and soil roughness. Runoff is undesirable as it causes erosion and loss of fertile topsoil.

• Water Movement Categorization:

  • Redistribution: Downward movement in the absence of a shallow groundwater table.

  • Internal Drainage: Movement toward an existing groundwater table.

  • Interflow: Below-ground lateral flow within topsoil layers on sloping land.

  • Deep Drainage (Seepage): Water flowing out of the root zone into deeper substrate layers or aquifers.

• Groundwater Table (GWT): The upper surface of the completely saturated zone where all pores are water-filled.

• Perched Water Table: Occurs when an impermeable layer (e.g., hardpan) prevents internal drainage, causing saturation even if deeper layers are unsaturated.

Evapotranspiration Dynamics

• Physics of Evaporation: Requires energy (latent heat of vaporisation, $l \approx 2.45 \times 10^6\,J\,kg^{-1}$ at $20^\circ C$) to break molecular bonds.

• Atmospheric Evaporative Demand (Evaporativity): Depends on available energy, relative humidity, wind speed, and temperature.

• Potential vs. Actual Evaporation:

  • Potential ($E_p$): Maximum rate when water supply is unlimited.

  • Actual ($E_a$): Rate occurring when supply is limited ($E_a \le E_p$).

  • Advection: Can cause E_a > E_p when dry air from adjacent fallow land provides extra energy to irrigated cropland.

• Soil Evaporation Stages:

  • Stage 1 (Energy-limited): Soil surface is saturated; $E_a = E_p$.

  • Stage 2 (Water-limited): Transient process where E_a < E_p, eventually approaching zero. Persists longer in fine-textured soils than coarse-textured ones.

Water Retention in Soils

• Retention Mechanisms:

  1. Adhesion: Attraction of water to solid soil particles (London-van der Waals forces).

  2. Cohesion: Intermolecular attraction between water molecules.

  3. Surface Tension ($\sigma$): Enhancement of intermolecular forces at the water surface ($\sigma \approx 0.0728\,N\,m^{-1}$ at $20^\circ C$).

  4. Osmotic Binding: Binding in diffuse electric double layers (dominant in fine-textured soils).

• Capillary Binding: The primary mechanism in coarse to medium-textured soils. Soil pores act as non-cylindrical capillaries.

• Capillary Rise Formula:

  • $h_w = \frac{2\sigma}{\rho_w gr}$

  • $h_w$: Height of rise (m).

  • $\rho_w$: Density of water.

  • $g$: Gravity.

  • $r$: Pore radius.

• Capillary Fringe: Saturated soil layer above the GWT ($\approx 0.4\,m$ in clay; much shallower in sand).

• Soil Water Suction ($S$): Related to pore radius via $S = \frac{2\sigma}{r}$. Narrower pores require larger suction to empty.

Key Soil Moisture Concepts

• Saturated Water Content ($q_s$): Maximum moisture when all pores are filled.

• Permanent Wilting Point (PWP): Soil moisture content when roots can no longer extract water ($S \approx 1500\,kPa$ or $150\,m$).

• Field Capacity (FC): Water content after internal drainage has slowed significantly (2–3 days post-wetting); typically $S = 33\,kPa$ ($3.3\,m$) or $10\,kPa$.

• Available Soil Water (Available Water Capacity): The difference between FC and PWP ($q_{FC} - q_{PWP}$).

  • Storage pores: Diameter $2–50\,\mu m$.

  • Transmission pores: Diameter > 50\,\mu m (drain after saturation).

  • Residual pores: Diameter < 2\,\mu m (hold water at PWP).

• Total Available Water (TAW): AWC multiplied by root zone thickness.

• Readily Available Water (RAW): The fraction of TAW ($p$) a crop can extract before suffering stress.

• Water Release Curve (Retention Curve): The unique relationship between water content ($q$) and matric potential ($\psi_m$) for a soil. It exhibits hysteresis, meaning desorption (drying) curves differ from absorption (wetting) curves.

Water Flow and Darcy’s Law

• Hydraulic Potential ($\psi_h$): The sum of matric potential ($\psi_m$, negative) and gravitational potential ($\psi_g$, $z$). Water flows from high to low $\psi_h$.

• Darcy's Law:

  • $F_w = -K(\theta) \frac{\Delta\psi_h}{\Delta z}$

  • $F_w$: Water flow rate ($m\,s^{-1}$).

  • $K(\theta)$: Hydraulic conductivity as a function of moisture content.

• Hydraulic Conductivity ($K$):

  • Decreases sharply as soil dries because cross-sectional water area decreases and tortuosity (path length) increases.

  • Poiseuille’s Law: Flow is proportional to $r^4$, meaning large pores dominate conductivity.

• Preferential Flow: Water bypassing portions of the soil matrix via macropores, cracks, or "fingering" at textural interfaces.

Plant Indicators of Water Stress

• Stomatal Regulation: Controlled by leaf water status and hormones like abscisic acid (ABA).

• Isohydric vs. Anisohydric:

  • Isohydric (e.g., Maize, Poplar): Maintain stable leaf water status via strict stomatal control.

  • Anisohydric (e.g., Sunflower, Barley): Less effective control; leaf water status fluctuates with environment.

• Direct Stress Measurements:

  • Pressure Chamber (Scholander bomb): Measures leaf or stem water potential.

  • Psychrometers: Infer water potential from vapour phase equilibrium; requires high technical skill.

• Indirect Stress Measurements:

  • Stem Diameter Variations (SDV): Uses Linear Variable Differential Transformers (LVDT) to record Maximum Daily Shrinkage (MDS) and Stem Growth Rate (SGR).

  • Sap Flow (SF): Uses heat as a tracer to estimate the transpiration ratio.

  • Thermal Sensing (CWSI): Stomatal closure increases leaf temperature due to reduced transpirational cooling.

• Crop Water Stress Index (CWSI):

  • $CWSI = \frac{\Delta T - \Delta T_{nws}}{\Delta T_{dry} - \Delta T_{nws}}$

  • $\Delta T$: Canopy-air temperature difference ($T_c - T_a$).

  • $nws$: Non-water stressed baseline; $dry$: Null transpiration baseline.

Determination of Soil Moisture and ET

• Soil Water Content Measurement:

  • Gravimetric: Change in mass after drying at $105^\circ C$. Accurate but destructive and labor-intensive.

  • Neutron Moderation: Fast neutrons from a radioactive source slow down upon hitting hydrogen nuclei. Large sensing volume; requires certified personnel.

  • Electromagnetic (TDR/FDR): Measures bulk dielectric permittivity ($\varepsilon$). Water has $\varepsilon \approx 80$; soil solids $2–9$; air $1$.

• Soil Water Potential Measurement:

  • Tensiometers: Water-filled tube with a ceramic cup; limited to suctions above $-80\,kPa$.

  • Electrical Resistance/Gypsum Blocks: Porous medium in equilibrium with soil; gypsum acts as a buffer against salinity but degrades over time.

• Evapotranspiration Estimation:

  • Pan Evaporimeter: Measures potential evaporation from an open water surface.

  • Weighing Lysimetry: Continuous monitoring of weight changes in a soil column.

  • Eddy Covariance (EC): Direct research-grade method measuring the covariance of vertical wind speed ($w$) and specific humidity ($q$).

  • Bowen Ratio Energy Balance (BREB): Infers evaporation from air temperature and vapour pressure gradients at two heights.

  • Penman-Monteith (Big-Leaf) Model:

    • $\lambda ET = \frac{\Delta(R_n - G) + \rho_a c_p (e_s - e) / r_a}{\Delta + \gamma(1 + r_s / r_a)}$

Irrigation Systems and Strategies

• Irrigation Types:

  1. Surface (Furrow/Basin): Worldwide dominant ($≈ 84.5\%$); global efficiency only $≈ 37\%$.

  2. Sprinkler: Better performance via pressurized systems.

  3. Micro/Localized (Drip/Subsurface): Frequent, small applications. Subsurface drip minimizes soil evaporation.

• Deficit Irrigation (DI): Applying water below crop ET requirements to maximize WUE under scarcity.

  • Sustained (SDI): Uniform shortage throughout the season.

  • Regulated (RDI): Stress during non-critical growth stages, full water during critical periods.

  • Partial Root Zone Drying (PRD): Alternating wet/dry cycles on different parts of the root system to induce ABA signalling.

  • Supplemental Irrigation (SI): Tactical measure to complement rainfall.

• Modeling Tools: FAO’s AquaCrop (simulates yield response to water) and CROPWAT 8.0 (calculates requirements).

Salinity and Soil Management

• Salinity Mechanics:

  • Phase 1 (Osmotic): Salts reduce the plant's ability to take up water.

  • Phase 2 (Toxic): Salts accumulate to toxic levels in older leaves, causing premature senescence.

• Species Sensitivity:

  • Salt-tolerant: Wheat, sunflower, potato, maize, sugar beet.

  • Salt-sensitive: Tomato, lentil, broad bean, chickpea.

• Agronomic WUE Improvements:

  • Tillage: Deep ripping to alleviate physical constraints or minimum/conservation tillage (stubble retention) to reduce evaporation.

  • Organic Matter (SOM): Increases storage pores thus raising field capacity and improving structure.

  • Mulching: Applying materials (plastic, organic straw, gravel) to block soil pores or create hydraulic discontinuity, preventing Stage 1 drying evaporation.

  • Agroforestry: Shading decreases soil evaporation, though tree canopies may increase interception losses.

Drainage and Alleviating Excess Water

• Natural/Anthropogenic Causes: High-intensity rain, over-irrigation, native vegetation clearing, impermeable plough pans, or rising GWT.

• Remediation:

  • Preventing crusting via crop residue or SOM.

  • Deep tillage to destroy compacted subsoil pans.

  • Establishment of tree belts to lower groundwater tables.

• Agricultural Drainage Systems (ADS):

  • Surface Drainage: For soil surface ponding.

  • Subsurface Drainage: For waterlogging in the root zone (ditches, buried pipes, mole drains).

• Effects of Drainage:

  • Positive: Increased soil aeration, deeper rooting, higher nitrogen availability (nitrification), better land workability, earlier planting due to warmer spring soil.

  • Negative: Increased SOM decomposition, soil subsidence, acidification of potential acid sulphate soils, risk of drought via reduced capillary rise.

Plant Water Requirements:

Water is essential for photosynthesis, tissue rigidity (turgidity), and carbohydrate production. Water is drawn from the soil via roots through the plant's xylem.

Transpiration and Gas Exchange:

• Transpiration is the loss of water from microscopic leaf openings called stomata.

• Stomatal opening facilitates the exchange of $CO_2$ (needed for growth) and $O_2$ (expelled during photosynthesis or taken up during respiration).

• Transpiration is an "undesirable trade-off" of canopy gas exchange, often losing more water than necessary for turgidity.

Water Availability Outcomes:

• Demand > Availability: Leads to reduced growth or plant mortality.

• Natural Vegetation: Drought periods define plant community adaptations.

• Agriculture: Requires irrigation or water harvesting to meet demand, especially in arid climates where deep subsurface stores are used.

Excess Water Complications:

• Excess water hampers gas exchange between roots and soil, leading to hypoxia (oxygen starvation).

• Gases diffuse much slower through water than air ($≈ 10,000$ times slower).

• Toxic effects include the formation of nitric oxide during root tissue hypoxia.

Soil Water Management Definition:

The collective practices of irrigation, improving natural drainage, and soil water conservation (suppressing evaporative/drainage losses, runoff control, tillage, or soil amendments).

Integrated Water Resources Management (IWRM):

A systematic process for the sustainable development, allocation, and monitoring of water resource use in the context of social, economic, and environmental objectives.

The Soil Water Balance

• The Root Zone Water Balance Equation:
  $</p></li></ul><p>elta heta = P + I + C - E - T - D - R<br>$

  • $<br>elta heta$: Change in soil moisture content.

  • $P$: Precipitation (Gain).

  • $I$: Irrigation (Gain).

  • $C$: Capillary rise (Gain).

  • $E$: Soil evaporation (Loss).

  • $T$: Transpiration (Loss).

  • $D$: Deep drainage (Loss).

  • $R$: Runoff (positive value = loss) or Runon (negative value = gain).

  Units:

  Typically expressed as depth per unit time, e.g., $mm\,H_2O\,day^{-1}$.

  Hydrological Context:

  The soil water store is small relative to ice or groundwater but critical for human sustenance via crop growth.

  Interception Loss:

  Precipitation or irrigation intercepted by vegetation that evaporates before reaching the ground. Significant in forests, less so in crops.

  Infiltration and Redistribution

  • Infiltration Rate: The amount of water per unit area per unit time entering the soil. It is determined by supply rate (P/I), initial moisture, and water permeability (hydraulic conductivity).

  • Infiltration Capacity (Infiltrability): Initially high in dry soils due to a large suction gradient between the surface and drier underlying soil. Decreases as the profile wets up and the suction gradient declines.

  • Steady-State Infiltrability: Reached when topsoil is saturated; infiltration rate then equals the saturated hydraulic conductivity, $K_s$.

  • Surface Storage Capacity: The total volume of surface depressions where excess water collects before runoff occurs.

  • Runoff Factors: Intensity of supply vs. infiltration rate, slope, and soil roughness. Runoff is undesirable as it causes erosion and loss of fertile topsoil.

  • Water Movement Categorization:

    • Redistribution: Downward movement in the absence of a shallow groundwater table.

    • Internal Drainage: Movement toward an existing groundwater table.

    • Interflow: Below-ground lateral flow within topsoil layers on sloping land.

    • Deep Drainage (Seepage): Water flowing out of the root zone into deeper substrate layers or aquifers.

    • Groundwater Table (GWT): The upper surface of the completely saturated zone where all pores are water-filled.

    • Perched Water Table: Occurs when an impermeable layer (e.g., hardpan) prevents internal drainage, causing saturation even if deeper layers are unsaturated.

  Evapotranspiration Dynamics

  • Physics of Evaporation: Requires energy (latent heat of vaporisation, $l \approx 2.45 \times 10^6\,J\,kg^{-1}$ at $20^\circ C$) to break molecular bonds.

  • Atmospheric Evaporative Demand (Evaporativity): Depends on available energy, relative humidity, wind speed, and temperature.

  • Potential vs. Actual Evaporation:

    • Potential ($E_p$): Maximum rate when water supply is unlimited.

    • Actual ($E_a$): Rate occurring when supply is limited ($E_a \le E_p$).

    • Advection: Can cause E_a > E_p when dry air from adjacent fallow land provides extra energy to irrigated cropland.

  • Soil Evaporation Stages:

    • Stage 1 (Energy-limited): Soil surface is saturated; $E_a = E_p$.

    • Stage 2 (Water-limited): Transient process where $$E_a