Soil moisture is the
water content of the
soil
Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...
. It can be expressed in terms of volume or weight.
Soil moisture measurement
Water content or moisture content is the quantity of water contained in a material, such as soil (called soil moisture), rock, ceramics, crops, or wood. Water content is used in a wide range of scientific and technical areas, and is expressed as ...
can be based on ''in situ'' probes (e.g.,
capacitance probe Capacitance sensors (or Dielectric sensors) use capacitance to measure the dielectric permittivity of a surrounding medium.
The configuration is like the neutron probe where an access tube made of PVC is installed in the soil; probes can also be mo ...
s,
neutron probes) or
remote sensing
Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring information about Ear ...
methods.
Water that enters a field is removed from a field by
runoff
Runoff, run-off or RUNOFF may refer to:
* RUNOFF, the first computer text-formatting program
* Runoff or run-off, another name for bleed, printing that lies beyond the edges to which a printed sheet is trimmed
* Runoff or run-off, a stock marke ...
,
drainage,
evaporation
Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when h ...
or
transpiration
Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is used for growth ...
. Runoff is the water that flows on the surface to the edge of the field; drainage is the water that flows through the soil downward or toward the edge of the field underground; evaporative water loss from a field is that part of the water that evaporates into the atmosphere directly from the field's surface; transpiration is the loss of water from the field by its evaporation from the plant itself.
Water affects
soil formation,
structure
A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such a ...
, stability and
erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is d ...
but is of primary concern with respect to
plant growth. Water is essential to plants for four reasons:
# It constitutes 80%-95% of the plant's
protoplasm
Protoplasm (; ) is the living part of a cell that is surrounded by a plasma membrane. It is a mixture of small molecules such as ions, monosaccharides, amino acid, and macromolecules such as proteins, polysaccharides, lipids, etc.
In some defi ...
.
# It is essential for
photosynthesis
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...
.
# It is the solvent in which
nutrients
A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excr ...
are carried to, into and throughout the plant.
# It provides the
turgidity by which the plant keeps itself in proper position.
In addition, water alters the soil profile by dissolving and re-depositing mineral and organic
solutes
In chemistry, a solution is a special type of homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. If the attractive forces between the solven ...
and
colloid
A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend ...
s, often at lower levels, a process called
leaching. In a
loam soil, solids constitute half the volume, gas one-quarter of the volume, and water one-quarter of the volume of which only half will be available to most plants, with a strong variation according to
matric potential.
Water moves in soil under the influence of
gravity
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
,
osmosis
Osmosis (, ) is the spontaneous net movement or diffusion of solvent molecules through a selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of low water potential (region ...
and
capillarity. When water enters the soil, it displaces air from interconnected
macropores by
buoyancy
Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the ...
, and breaks
aggregates into which air is entrapped, a process called
slaking.
The rate at which a soil can absorb water depends on the soil and its other conditions. As a plant grows, its roots remove water from the largest pores (
macropores) first. Soon the larger pores hold only air, and the remaining water is found only in the intermediate- and smallest-sized pores (
micropores
A microporous material is a material containing pores with diameters less than 2 nm. Examples of microporous materials include zeolites and metal-organic frameworks.
Porous materials are classified into several kinds by their size. The recom ...
). The water in the smallest pores is so strongly held to particle surfaces that plant roots cannot pull it away. Consequently, not all soil water is available to plants, with a strong dependence on
texture.
When saturated, the soil may lose nutrients as the water drains. Water moves in a draining field under the influence of
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...
where the soil is locally saturated and by
capillarity pull to drier parts of the soil. Most plant water needs are supplied from the suction caused by evaporation from plant leaves (
transpiration
Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is used for growth ...
) and a lower fraction is supplied by suction created by
osmotic pressure
Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of its pure solvent across a semipermeable membrane.
It is also defined as the measure of the tendency of a solution to take in a pure ...
differences between the plant interior and the soil solution. Plant roots must seek out water and grow preferentially in moister soil microsites, but some parts of the root system are also able to remoisten dry parts of the soil. Insufficient water will damage the yield of a crop. Most of the available water is used in transpiration to pull nutrients into the plant.
Soil water is also important for
climate modeling
Numerical climate models use quantitative methods to simulate the interactions of the important drivers of climate, including atmosphere, oceans, land surface and ice. They are used for a variety of purposes from study of the dynamics of the cl ...
and numerical
weather prediction. The
Global Climate Observing System specified soil water as one of the 50 Essential Climate Variables (ECVs). Soil water can be measured in situ with
soil moisture sensor
Soil moisture sensors measure the volumetric water content in soil. Since the direct gravimetric measurement of free soil moisture requires removing, drying, and weighing of a sample, soil moisture sensors measure the volumetric water content ind ...
s or can be estimated at various scales and resolution: from local or
wifi measures via
sensors in the soil to
satellite imagery
Satellite images (also Earth observation imagery, spaceborne photography, or simply satellite photo) are images of Earth collected by imaging satellites operated by governments and businesses around the world. Satellite imaging companies sell ima ...
that combines
data capture and
hydrological model A hydrologic model is a simplification of a real-world system (e.g., surface water, soil water, wetland, groundwater, estuary) that aids in understanding, predicting, and managing water resources. Both the flow and quality of water are commonly stud ...
s. Each method exhibits pros and cons, and hence, the integration of different techniques may decrease the drawbacks of a single given method.
Moisture level concepts
; Field capacity
: A flooded field will drain the gravitational water under the influence of
gravity
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
until water's
adhesive
Adhesive, also known as glue, cement, mucilage, or paste, is any non-metallic substance applied to one or both surfaces of two separate items that binds them together and resists their separation.
The use of adhesives offers certain advant ...
and
cohesive forces resist further drainage at which point it is said to have reached
field capacity. At that point, plants must apply
suction to draw water from a soil. By convention it is defined at 0.33 bar suction.
; Available water and unavailable water
: The water that plants may draw from the soil is called the
available water. Once the available water is used up the remaining moisture is called unavailable water as the plant cannot produce sufficient suction to draw that water in.
; Wilting point
: The
wilting point is the minimum amount of water plants need to not wilt and approximates the boundary between available and unavailable water. By convention it is defined as 15 bar suction. At this point, seeds will not germinate, plants begin to wilt and then die unless they are able to recover after water replenishment thanks to species-specific adaptations.
Water retention
Water is retained in a soil when the
adhesive force of attraction that water's
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
atoms have for the
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
of soil particles is stronger than the
cohesive forces that water's hydrogen feels for water oxygen atoms. When a field is flooded, the soil
pore space is completely filled by water. The field will drain under the
force of gravity until it reaches what is called
field capacity, at which point the smallest pores are filled with water and the largest with water and gases. The total amount of water held when field capacity is reached is a function of the
specific surface area of the soil particles. As a result, high clay and high organic soils have higher field capacities.
The potential energy of water per unit volume relative to pure water in reference conditions is called
water potential
Water potential is the potential energy of water per unit volume relative to pure water in reference conditions. Water potential quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure and ...
. Total water potential is a sum of
matric potential which results from
capillary action,
osmotic
Osmosis (, ) is the spontaneous net movement or diffusion of solvent molecules through a selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of low water potential (region ...
potential for
saline soil, and
gravitational
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the str ...
potential when dealing with downward water movement. Water potential in soil usually has negative values, and therefore it is also expressed in
suction, which is defined as the minus of water potential. Suction has a positive value and can be regarded as the total force required to pull or push water out of soil. Water potential or suction is expressed in units of kPa (10
3 pascal
Pascal, Pascal's or PASCAL may refer to:
People and fictional characters
* Pascal (given name), including a list of people with the name
* Pascal (surname), including a list of people and fictional characters with the name
** Blaise Pascal, Frenc ...
),
bar
Bar or BAR may refer to:
Food and drink
* Bar (establishment), selling alcoholic beverages
* Candy bar
* Chocolate bar
Science and technology
* Bar (river morphology), a deposit of sediment
* Bar (tropical cyclone), a layer of cloud
* Bar ( ...
(100 kPa), or
cm H2O (approximately 0.098 kPa).
Common logarithm
In mathematics, the common logarithm is the logarithm with base 10. It is also known as the decadic logarithm and as the decimal logarithm, named after its base, or Briggsian logarithm, after Henry Briggs, an English mathematician who pioneered ...
of suction in cm H
2O is called pF. Therefore, pF 3 = 1000 cm = 98 kPa = 0.98 bar.
The forces with which water is held in soils determine its availability to plants. Forces of
adhesion
Adhesion is the tendency of dissimilar particles or surfaces to cling to one another ( cohesion refers to the tendency of similar or identical particles/surfaces to cling to one another).
The forces that cause adhesion and cohesion can ...
hold water strongly to
mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...
and
humus
In classical soil science, humus is the dark organic matter in soil that is formed by the decomposition of plant and animal matter. It is a kind of soil organic matter. It is rich in nutrients and retains moisture in the soil. Humus is the Lati ...
surfaces and less strongly to itself by cohesive forces. A plant's root may penetrate a very small volume of water that is adhering to soil and be initially able to draw in water that is only lightly held by the cohesive forces. But as the droplet is drawn down, the forces of adhesion of the water for the soil particles produce increasingly higher
suction, finally up to 1500 kPa (pF = 4.2). At 1500 kPa suction, the soil water amount is called
wilting point. At that suction the plant cannot sustain its water needs as water is still being lost from the plant by
transpiration
Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is used for growth ...
, the plant's
turgidity is lost, and it wilts, although
stoma
In botany, a stoma (from Greek ''στόμα'', "mouth", plural "stomata"), also called a stomate (plural "stomates"), is a pore found in the epidermis of leaves, stems, and other organs, that controls the rate of gas exchange. The pore is bo ...
tal closure may decrease transpiration and thus may retard wilting below the
wilting point, in particular under
adaptation
In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the po ...
or
acclimatization to
drought
A drought is defined as drier than normal conditions.Douville, H., K. Raghavan, J. Renwick, R.P. Allan, P.A. Arias, M. Barlow, R. Cerezo-Mota, A. Cherchi, T.Y. Gan, J. Gergis, D. Jiang, A. Khan, W. Pokam Mba, D. Rosenfeld, J. Tierney, an ...
. The next level, called air-dry, occurs at 100,000 kPa suction (pF = 6). Finally the oven dry condition is reached at 1,000,000 kPa suction (pF = 7). All water below wilting point is called unavailable water.
When the soil moisture content is optimal for plant growth, the water in the large and intermediate size pores can move about in the soil and be easily used by plants.
The amount of water remaining in a soil drained to field capacity and the amount that is available are functions of the soil type. Sandy soil will retain very little water, while clay will hold the maximum amount.
The available water for the silt loam might be 20% whereas for the sand it might be only 6% by volume, as shown in this table.
The above are average values for the soil textures.
Water flow
Water moves through soil due to the force of
gravity
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
,
osmosis
Osmosis (, ) is the spontaneous net movement or diffusion of solvent molecules through a selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of low water potential (region ...
and
capillarity. At zero to 33 kPa
suction (
field capacity), water is pushed through soil from the point of its application under the force of gravity and the pressure gradient created by differences in the pressure of water; this is called saturated flow. At higher
suction, water movement is pulled by capillarity from wetter toward drier soil. This is caused by water's
adhesion
Adhesion is the tendency of dissimilar particles or surfaces to cling to one another ( cohesion refers to the tendency of similar or identical particles/surfaces to cling to one another).
The forces that cause adhesion and cohesion can ...
to soil solids, and is called unsaturated flow.
Water infiltration and movement in soil are controlled by six factors:
# Soil texture
# Soil structure. Fine-textured soils with granular structure are most favourable to infiltration of water.
# The amount of organic matter. Coarse matter is best and if on the surface helps prevent the destruction of soil structure and the creation of
soil crusts.
# Depth of soil to impervious layers such as
hardpans or
bedrock
In geology, bedrock is solid rock that lies under loose material ( regolith) within the crust of Earth or another terrestrial planet.
Definition
Bedrock is the solid rock that underlies looser surface material. An exposed portion of be ...
# The amount of water already in the soil
# Soil temperature. Warm soils take in water faster while frozen soils such as
permafrost
Permafrost is ground that continuously remains below 0 °C (32 °F) for two or more years, located on land or under the ocean. Most common in the Northern Hemisphere, around 15% of the Northern Hemisphere or 11% of the global surface ...
may not be able to absorb depending on the type of freezing.
Water infiltration rates range from 0.25 cm per hour for high clay soils to 2.5 cm per hour for sand and well stabilized and aggregated soil structures. Water flows through the ground unevenly, in the form of so-called ''gravity fingers'', because of the
surface tension
Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than water such as razor blades and insects (e.g. water striders) t ...
between water particles.
Tree roots, whether living or dead, create preferential channels for rainwater flow through soil, magnifying infiltration rates of water up to 27 times.
Flooding temporarily increases
soil permeability in
river beds, helping to
recharge aquifers.
Water applied to a soil is pushed by
pressure gradients from the point of its application where it is
saturated locally, to less saturated areas, such as the
vadose zone. Once soil is completely wetted, any more water will move downward, or
percolate
Percolation (from Latin ''percolare'', "to filter" or "trickle through"), in physics, chemistry and materials science, refers to the movement and filtering of fluids through porous materials.
It is described by Darcy's law.
Broader applicati ...
out of the range of
plant roots, carrying with it clay, humus, nutrients, primarily cations, and various
contaminants
Contamination is the presence of a constituent, impurity, or some other undesirable element that spoils, corrupts, infects, makes unfit, or makes inferior a material, physical body, natural environment, workplace, etc.
Types of contamination
...
, including
pesticides
Pesticides are substances that are meant to control pests. This includes herbicide, insecticide, nematicide, molluscicide, piscicide, avicide, rodenticide, bactericide, insect repellent, animal repellent, microbicide, fungicide, and la ...
,
pollutants,
viruses
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.
Since Dmitri Ivanovsky's ...
and
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...
, potentially causing
groundwater contamination
Groundwater pollution (also called groundwater contamination) occurs when pollutants are released to the ground and make their way into groundwater. This type of water pollution can also occur naturally due to the presence of a minor and unwante ...
. In order of decreasing solubility, the leached nutrients are:
* Calcium
* Magnesium, Sulfur, Potassium; depending upon soil composition
* Nitrogen; usually little, unless nitrate fertiliser was applied recently
* Phosphorus; very little as its forms in soil are of low solubility.
In the United States
percolation water due to rainfall ranges from almost zero centimeters just east of the
Rocky Mountains
The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in straight-line distance from the northernmost part of western Canada, to New Mexico ...
to fifty or more centimeters per day in the
Appalachian Mountains
The Appalachian Mountains, often called the Appalachians, (french: Appalaches), are a system of mountains in eastern to northeastern North America. The Appalachians first formed roughly 480 million years ago during the Ordovician Period. The ...
and the north coast of the
Gulf of Mexico
The Gulf of Mexico ( es, Golfo de México) is an ocean basin and a marginal sea of the Atlantic Ocean, largely surrounded by the North American continent. It is bounded on the northeast, north and northwest by the Gulf Coast of the United ...
.
Water is pulled by
capillary
A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the bod ...
action due to the
adhesion
Adhesion is the tendency of dissimilar particles or surfaces to cling to one another ( cohesion refers to the tendency of similar or identical particles/surfaces to cling to one another).
The forces that cause adhesion and cohesion can ...
force of water to the soil solids, producing a
suction gradient
In vector calculus, the gradient of a scalar-valued differentiable function of several variables is the vector field (or vector-valued function) \nabla f whose value at a point p is the "direction and rate of fastest increase". If the gr ...
from wet towards drier soil and from
macropores to
micropores
A microporous material is a material containing pores with diameters less than 2 nm. Examples of microporous materials include zeolites and metal-organic frameworks.
Porous materials are classified into several kinds by their size. The recom ...
. The so-called
Richards equation allows calculation of the time rate of change of moisture content in soils due to the movement of water in
unsaturated soils. Interestingly, this equation attributed to Richards was originally published by Richardson in 1922. The
soil moisture velocity equation, which can be solved using the
finite water-content vadose zone flow method The finite water-content vadose zone flux methodTalbot, C.A., and F. L. Ogden (2008), A method for computing infiltration and redistribution in a discretized moisture content domain, ''Water Resour. Res.'', 44(8), doi: 10.1029/2008WR006815.Ogden, F. ...
,
describes the velocity of flowing water through an unsaturated soil in the vertical direction. The numerical solution of the Richardson/Richards equation allows calculation of unsaturated water flow and solute transport using software such as
Hydrus, by giving soil hydraulic parameters of hydraulic functions (
water retention function and unsaturated hydraulic conductivity function) and initial and boundary conditions. Preferential flow occurs along interconnected
macropores, crevices, root and worm channels, which
drain water under
gravity
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
.
Many models based on soil physics now allow for some representation of preferential flow as a dual continuum, dual
porosity
Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measur ...
or dual
permeability options, but these have generally been "bolted on" to the Richards solution without any rigorous physical underpinning.
Water uptake by plants
Of equal importance to the storage and movement of water in soil is the means by which plants acquire it and their nutrients. Most soil water is taken up by plants as passive
absorption
Absorption may refer to:
Chemistry and biology
*Absorption (biology), digestion
**Absorption (small intestine)
*Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials
*Absorption (skin), a route by which s ...
caused by the pulling force of water evaporating (
transpiring) from the long column of water (
xylem sap flow) that leads from the plant's roots to its leaves, according to the
cohesion-tension theory. The upward movement of water and solutes (
hydraulic lift) is regulated in the roots by the
endodermis
The endodermis is the central, innermost layer of cortex in land plants. It is a cylinder of compact living cells, the radial walls of which are impregnated with hydrophobic substances (Casparian strip) to restrict apoplastic flow of water to th ...
and in the plant foliage by
stomatal conductance, and can be interrupted in root and shoot
xylem vessels by
cavitation
Cavitation is a phenomenon in which the static pressure of a liquid reduces to below the liquid's vapour pressure, leading to the formation of small vapor-filled cavities in the liquid. When subjected to higher pressure, these cavities, ca ...
, also called ''xylem embolism''. In addition, the high concentration of salts within plant roots creates an
osmotic pressure
Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of its pure solvent across a semipermeable membrane.
It is also defined as the measure of the tendency of a solution to take in a pure ...
gradient that pushes soil water into the roots. Osmotic absorption becomes more important during times of low water transpiration caused by lower temperatures (for example at night) or high humidity, and the reverse occurs under high temperature or low humidity. It is these processes that cause
guttation and
wilting, respectively.
Root extension is vital for plant survival. A study of a single winter
rye plant grown for four months in one cubic foot (0.0283 cubic meters) of loam soil showed that the plant developed 13,800,000 roots, a total of 620 km in length with 237 square meters in
surface area
The surface area of a solid object is a measure of the total area that the surface of the object occupies. The mathematical definition of surface area in the presence of curved surfaces is considerably more involved than the definition of ...
; and 14 billion
root hairs
Root hair, or absorbent hairs, are outgrowths of epidermal cells, specialized cells at the tip of a plant root. They are lateral extensions of a single cell and are only rarely branched. They are found in the region of maturation, of the root. Roo ...
of 10,620 km total length and 400 square meters total area; for a total surface area of 638 square meters. The total surface area of the loam soil was estimated to be 52,000 square meters. In other words, the roots were in contact with only 1.2% of the soil volume. However, root extension should be viewed as a dynamic process, allowing new roots to explore a new volume of soil each day, increasing dramatically the total volume of soil explored over a given growth period, and thus the volume of water taken up by the root system over this period. Root architecture, i.e. the spatial configuration of the root system, plays a prominent role in the adaptation of plants to soil water and nutrient availability, and thus in plant productivity.
Roots must seek out water as the unsaturated flow of water in soil can move only at a rate of up to 2.5 cm per day; as a result they are constantly dying and growing as they seek out high concentrations of soil moisture. Insufficient soil moisture, to the point of causing
wilting, will cause permanent damage and
crop yield
In agriculture, the yield is a measurement of the amount of a crop grown, or product such as wool, meat or milk produced, per unit area of land. The seed ratio is another way of calculating yields.
Innovations, such as the use of fertilizer, the c ...
s will suffer. When grain
sorghum was exposed to soil suction as low as 1300 kPa during the seed head emergence through bloom and seed set stages of growth, its production was reduced by 34%.
Consumptive use and water use efficiency
Only a small fraction (0.1% to 1%) of the water used by a plant is held within the plant. The majority is ultimately lost via
transpiration
Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is used for growth ...
, while
evaporation
Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when h ...
from the soil surface is also substantial, the transpiration:evaporation ratio (T/ET) varying according to vegetation type and climate, peaking in
tropical rainforests and dipping in
steppes and
deserts. Transpiration plus evaporative soil moisture loss is called
evapotranspiration
Evapotranspiration (ET) is the combined processes by which water moves from the earth’s surface into the atmosphere. It covers both water evaporation (movement of water to the air directly from soil, canopies, and water bodies) and transp ...
. Evapotranspiration plus water held in the plant totals to
consumptive use, which is nearly identical to evapotranspiration.
The total water used in an agricultural field includes
surface runoff
Surface runoff (also known as overland flow) is the flow of water occurring on the ground surface when excess rainwater, stormwater, meltwater, or other sources, can no longer sufficiently rapidly infiltrate in the soil. This can occur when t ...
,
drainage and consumptive use. The use of loose
mulches will reduce evaporative losses for a period after a field is irrigated, but in the end the total evaporative loss (plant plus soil) will approach that of an uncovered soil, while more water is immediately available for plant growth.
Water use efficiency
Water-use efficiency (WUE) refers to the ratio of water used in plant metabolism to water lost by the plant through transpiration. Two types of water-use efficiency are referred to most frequently:
*photosynthetic water-use efficiency (also cal ...
is measured by the
transpiration ratio, which is the ratio of the total water transpired by a plant to the dry weight of the harvested plant. Transpiration ratios for crops range from 300 to 700. For example, alfalfa may have a transpiration ratio of 500 and as a result 500 kilograms of water will produce one kilogram of dry alfalfa.
References
[Rakesh Chandra Joshi, Gary J. Sheridan, Dongryeol Ryu, Patrick N.J. Lane, How long is the memory of forest growth to rainfall in asynchronous climates?, Ecological Indicators, Volume 140, 2022, 109057, ISSN 1470-160X, https://doi.org/10.1016/j.ecolind.2022.109057.]
Bibliography
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