Water in the zone of aeration
(unsaturated or vadose zone)

Infiltration

• movement of water from the surface into the unsaturated zone,
• 76% of the world's precipitation on land infiltrates
• critical stage in the hydrological cycle, because
  
  
  • it determines how much water is stored between the ground surface and the water table and is available for plant growth
  • soil water seepage is the major process of groundwater recharge
  • it greatly effects response of streams to water input events by determining proportions of water that move by overland flow versus under the ground
  
  

infiltration rate

• rate at which water enters the soil from the surface as a function of water-input rate (snow melt and rain fall) and infiltration capacity, the maximum rate that soil will accept water
• there are three infiltration scenarios
  
  
  • no ponding: infiltration rate = water input rate and is < infiltration capacity
  • saturation from above: ponding because water input rate is > infitration rate which = infiltration capacity
  • saturation from below: water table has risen to or above the surface, ponding, no infiltration
  
  
• during a water input event, infiltration declines exponentially and asymptotically to a near constant value corresponding to the K_sat of the soil
• infiltrating water is marked by a wetting front that is diffuse in clay (due to capillarity) and sharp in sand
• infiltration rates and their variation over time and space are determined by:
  
  
  • water input rate/depth of ponding
  • hydraulic conductivity of the surface: vegetation cover, frost, swelling-shrinking, washing in of fine sediment, human activities
  • water content of surface pores (antecedent water)
  • surface slope and roughness
  • soil chemistry
  • water temperature and quality
  
  

Hydrologic horizons in the zone of aeration

ground-water zone

• saturated or phreatic (Gr. well) zone
• +ve water pressure
• hydrostatic with no flow and increasing with depth

tension saturated zone (capillary fringe)

• lowest part of the vadose (L. shallow) zone adjacent to the water table and saturated with capillary water
• neagitve but relatively high pressure head from zero at the water table to the air-entry tension at the top of this zone
• the height of this zone is equal to the air entry tension (cm) and varies from virtually nothing in coarse sand to several metres in clay

intermediate zone

• zone through which water seeps from the root zone to the water table
• may be largest horizon in arid lands with low water tables, or thin or seasonally absent where water tables are near the surface and roots extend to the capillary fringe

root zone

• top layer from which plants extract water
• water is input by infiltration and output by evapotranspiration and seepage
• soil water redistribution

Soil water redistribution

• capillary forces (diffusion)
  
  
  • much larger than gravitational forces with low rates of infiltration and fine soils
  • capillary water is diffused along the vapour pressure gradient, i.e., away from the capillary fringe and from belts of gravity water seeping through the vadose zone towards drier soil
  • in heavy soils, the wetting front is diffuse, given the high capillary forces
  
  
• gravitational drainage
  
  
  • infiltrated water seeps through the zone of aeration as belts of gravity water corresponding to precipitation or snow melt events
  • the gravity water will seep to the groundwater zone, unless interflow is favoured by low vertical hydraulic conductivity and relatively higher lateral hydraulic conductivity
  • with high infiltration rates and coarse soils, there is a sharp wetting front above which soil is above field capacity or transitional to it
  
  
• infiltration and redistribution of soil water involve unsaturated flow in porous media, i.e., a matrix of individual soil grains and intervening pore spaces containing varying proportions of air and water
• thus the single most important soil property in terms of water movement is soil texture, the weight of soil in various size classes

Hydrologic parameters

porosity

• proportion of pore space in a volume of soil
• tends to decrease with depth
• derived from the relationship porosity = 1 - bulk density/particle density, where bulk density is the mass of soil per unit volume and particle density is the mass of soil particles per unit volume (e.g., 2.65 g/cc for quartz)

volumetric water content

ratio of water volume to soil volume

degree of saturation

proportion of pore space that contains water, not measured by calculated from ratio of volumetric water content and porosity

pressure head and hydraulic conductivity

• govern the rate of unsaturated flow in a porous media, according to Darcy's law
• dependent on water content

• force per unit area applied by the water in all directions, expressed relative to atmospheric pressure
• when water is unconfined, it will rise to a level according to the air pressure applied to it
• thus water pressure at an unconfined water table or piezometric surface is zero, and in the zone of saturation water pressure is positive
• in the unsaturated zone, soil pressure is negative since the water has risen above the water table against atmospheric pressure
  
  
  • it is held between soil grains by surface tension (the hydrogen bonding between water molecules)
  • the water hangs suspended in tension below the menisci
  • thus the negative soil pressure in the unsaturated zone is often called tension or suction
  • it is increasing negative as the radius of curvature of the menisci decrease, i.e., with greater surface tension, corresponding to decreasing soil water content (the surface tension is greatest in thin films of water)
  
  
• soil water pressure generally is expressed as pressure head, gravitational potential energy (pressure potential) per unit weight of water
• although water density is affected by temperature and salinity, for most hydrologic problems these can be considered constant
• so the main affect of converting soil water pressure to pressure head is to reduce the units to depth (cm)

• the negative exponential relationship between pressure head and water content
• there is zero tension head when the soil is saturated (i.e., water content = porosity)
• tension head decreases to air-entry tension, where significant volumes of air appear in the soil pores, i.e., at the top of the capillary fringe or tension-saturated zone
• then there is exponentially declining tension to low water contents where tension decreases rapidly with strong capillary and electrostatic forces in thin water films
• the moisture-characteristic curve depends on soil texture (higher tension in finer soils) and on whether the water content change represents wetting or drying, i.e., hysteresis whereby the change in tension head with water content depends on whether the water content is going up or down (analogous to Q vs. stage, or sediment concentration vs. Q, or strain vs. stress in an elastic body)

hydraulic conductivity

• rate at which water moves through a porous media under a unit potential-energy gradient
• is a function of particle size for saturated conditions and particle size and degree of saturation for unsaturated conditions
• it increases exponentially from very low values at low to moderate water contents (because the high surface tension prevents conductivity) especially in fine-textured soils
• the variation is several orders of magnitude between soil textures and up to Ksat
• the hysteresis effect is negligible and thus usually ignored

field capacity

• maximum content of capillary water (water held in tension), the water retained after saturated soil has drained and is not subject to evaporation, plant uptake of capillary rise (i.e., no outputs or inputs of capillary water)
• varies from about 10% for sand and 30% or more for clay, but in all soil the corresponding pressure head is about -340 cm

permanent wilting point

• water content when the pressure head is less than -15,000 cm, when plants cannot exert enough suctions to derive water from the soil
• from the moisture-characteristic curve this corresponds to water contents of about 5% for sand and 25% for clay
• available water content = field capacity - wilting point

hygroscopic water

• thin films held with tension head of less than -31,000
• maximum aridity of soil since at this pressure head water is absorbed directly from the air