Soil water relationship

Soil Water Relationship
• Explain the relationships between soil
properties and water retention

Types of Groundwater
• Gravitational – also called “free water.” –
• This is the water that drains out of the soil after it
has been wetted. - This water moves downward
through the soil because of the pull of gravity. -
This water also feeds wells and springs.

• Capillary – water that moves into and is held
in the soil by capillary forces (or pertaining to
the attraction or repulsion between a solid
and a liquid). - Plant roots can absorb or take
up this moisture. - The size of the soil pore will
influence the amount of water held by
capillary forces. - Provides most of the
moisture for plant growth.

• Hygroscopic - very thin water films around the
soil particles. - These films are held by
extremely strong forces that cause the water
molecules to be arranged in a semi-solid form.
- This water is unavailable to plants.

How is Soil Water Classified
• Hygroscopic Water is held so strongly by the soil
particles (adhesion), that it is not available to the
plants.
• Capillary Water is held by cohesive forces greater
than gravity and is available to plants.
• Gravitational Water is that water which cannot
be held against gravity.
• –as water is pulled down through the soil,
nutrients are "leached" out of the soil (nitrogen)

Levels of Water in Soil
• Saturation Point – the moisture point at which
all of the pore spaces are filled with water. -
Occurs when an area receives a lot of rain on a
daily basis and the water does not get
absorbed by plants, evaporation is at a low do
to the lack of sunlight, and runoff areas
(ditches, drains) are to capacity.

Levels of Water in Soil
• Field Capacity – the maximum amount of
water left in the soil after losses of water to
the forces of gravity have ceased and before
surface evaporation begins. - Occurs when the
soil contains the maximum amount of
capillary water.

• Wilting Point – the point at which the plant
can no longer obtain sufficient water from the
soil to meet its transpiration needs. - At this
point the plant enters permanent wilt and
dies.

• Available Soil Water – that amount present in
a soil which can be moved by plants. - It is
designated as the difference between the field
capacity and the wilting point.

The Hydrological (Water) Cycle
• Water is constantly moving through the
atmosphere and into and out of the soil.
• Soil moisture is one portion of the cycle which
can be controlled to the greatest extent by
affecting the soil.

How Does Water Enter the Soil?
• through pores in the soil
• sandy soils have the largest pores, but are
often filled with other material
• medium textured soils (loamy) have good
water entry properties
• clays, pores swell shut when they get wet

Objective 2
• Explain the relationships between soil water
and plant growth.

• Water Holding Capacity – is a soil property
which represents the amount of water a soil
can retain after it has been saturated by rain
and downward movement has ceased.
• Transpiration – the process by which water, as
a vapour, is lost by living plants

• Translocation – the process by which water
moves through a plant from the roots to the
leaves.
• Wilting Point – the moisture content of a soil
in which growing plants wilt and will not
recover after water is added.

• Evapotranspiration - the combination of water
that is lost from the soil through evaporation
and through transpiration from plants as a
part of their metabolic processes
• Adhesion – the attraction of two different
molecules (water to soil)
• Cohesion – the attraction of two similar
molecules (water to water)

• Water Holding Capacities of Soils
• The amount of water a soil can retain is
influenced by:
• –soil texture
• –soil structure
• –organic matter

• Soil Texture
• –The smaller the soil particles, the greater the
soil’s water holding capacity. Clay has more water
holding capacity than sand.
• –Small soil particles (clay) have more small pores
or capillary spaces, so they have a higher water
holding capacity. Large soil particles (sand) have
fewer capillary spaces, therefore less ability to
hold water

• Soil Structure
• -A soil structure has a direct correlation to the amount
of water it can retain.
• Organic Matter
• – Organic matter aids in cementing particles of clay,
silt, and sand together into aggregates which increases
the water holding capacity.
• – Decomposition of organic matter also adds vital
nutrients to the soil.

• Soil moisture content
• The soil moisture content indicates the amount of
water present in the soil.
• It is commonly expressed as the amount of water
(in mm of water depth) present in a depth of one
metre of soil.
• For example: when an amount of water (in mm of
water depth) of 150 mm is present in a depth of
one metre of soil, the soil moisture content is 150
mm/m

• Saturation capacity: this is the total water
content of the soil when all the pores of the
soil are filled with water.
• It is also termed as the maximum water
holding capacity of the soil.
• At saturation capacity, the soil moisture
tension is almost equal to zero.

• Field capacity: this is the water retained by an
initially saturated soil against the force of gravity.
• Hence, as the gravitational water gets drained off
from the soil, it is said to reach the field capacity.
• At field capacity, the macro-pores of the soil are
drained off, but water is retained in the
microspores.

• Permanent wilting point: plant roots are able
to extract water from a soil matrix, which is
saturated up to field capacity.
• However, as the water extraction proceeds,
the moisture content diminishes and the
negative pressure increases.
• At one point, the plant cannot extract any
further water and thus wilts.

• Two stages of wilting points are recognized and
they are:
• Temporary wilting point: this denotes the soil
water content at which the plant wilts at day
time, but recovers during right or when water is
added to the soil.
• Ultimate wilting point: at such a soil water
content, the plant wilts and fails to regain life
even after addition of water to soil.

• It must be noted that the above water
contents are expressed as percentage of water
held in the soil pores, compared to a fully
saturated soil

• Depth of Penetration
• Can be viewed as sequentially filling the soil
profile in layers
• Deep percolation: water penetrating deeper
than the bottom of the root zone
• Leaching: transport of chemicals from the root
zone due to deep percolation

Water requirements of crops
• Content… Depth of water applied during
irrigation- Duty of water and delta
improvement of duty- command area and
intensity of irrigation consumptive use of
water and Evapotranspiration- irrigation
efficiencies- assessment of irrigation water.

• Introduction…
• The term water requirements of a crop means
the total quantity of all water and the way in
which a crop requires water, from the time it is
sown to the time it is harvested.
• The water requirement of crop varies with the
crop as well as with the place.
• The same crop may have different water
requirements at different places of the same
country.

• Factors affecting water requirement
• Water table: high water table less requirement,
vice versa.
• Climate: In hot climate evaporation loss is more,
hence requirement more, vice versa.
• Ground slope: ground is steep, the water flows
down very quickly and soil gets little time to
absorb, so requirement more. If ground is flat less
requirement

• Intensity of irrigation: if intensity of irrigation for a
particular crop is high, then more area comes under
the irrigation system and requirement is more, vice
versa.
• Type of soil: sandy soil water percolates very quickly, so
requirement is more. Clay soil retention capacity is
more, so less requirement.
• Method of application of water: surface method more
water is required to meet up evaporation. In sub
surface and sprinkler method less water required.

• Method of ploughing: In deep ploughing less
water required, because soil can retain
moisture for longer period. In shallow
ploughing more water required.

• Base
• Base is defined as the period from the first to
the last watering of the crop just before its
maturity.
• Also known as base period.
• Denoted as “B” and expressed in no of days.

• Kharif crop Delta in cm
• Rice 125
• Maize 45
• Ground nut 30
• Millet 30
• Rabi crop Delta in cm
• Wheat 40
• Mustard 45
• Gram 30
• Potato 75

• Methods of improving duty
• Proper ploughing: Ploughing should be done properly and
deeply, so that moisture retaining capacity of soil is
increased.
• Methods of supplying water: this should be decided
according to the field and soil conditions.
• Furrow method – crops shown in row
• Contour method – hilly area
• Basin method – for orchards
• Flooding method – plain lands

• Canal lining: to reduce percolation loss the canals
should be lined according to site condition.
• Transmission loss: to reduce this canals should be
taken close to the irrigable land as far as possible.
• Crop rotation: crop rotation should be adopted to
increase the moisture retaining capacity and
fertility of the soil.
• Implementation of tax: the water tax should be
imposed on the basis of volume of water
consumption.

• Types of soil
• Alluvial soil: formed by deposition of silt
carried by river water during flood.
• Silt is formed due to weathering action of
rocks by heavy current of river water in the
hilly regions.
• Found in Indo –Gangetic plains, Brahmaputra
plains.

• Black soil: originated by weathering action on
rocks like granite, basalt etc.
• Mainly found in AP, MP, TN, Gujarat.
• They are sticky when wet and very hard when
dry.
• Suitable for cultivation of cotton

• Red soil: formed by weathering action of rocks
of igneous and metamorphic group.
• Water absorbing capacity very low.
• Found in Karnataka, TN, Orissa, WB,
Maharashtra etc.

• Laterite soil: formed by weathering action of
laterite rocks.
• Yellowish red in color and having good
drainage property.
• Found in Kerala, Karnataka, Orissa, Assam etc.

• Consumptive use of water
• It is defined as total quantity of water used for
the growth of plants by transpiration and the
amount of lost by evaporation.
• It is also known as evapo-transpiration.
• Expressed in hectare-meter or as depth of water
in m.
• The value of consumptive use of water is vary
from crop to crop, time to time, place to place

Evapotranspiration
• a) Evaporation: The process by which water is changed from the
liquid or solid state into the gaseous state through the transfer of
heat energy.
• b) Transpiration: The evaporation of water absorbed by the crop
which is used directly in the building of plant tissue in a specified
time. It does not include soil evaporation.
• c) Evapotranspiration, ET: It is the sum of the amount of water
transpired by plants during the growth process and that amount
that is evaporated from soil and vegetation in the domain occupied
by the growing crop. ET is normally expressed in mm/day.

Factors that affect Evapotranspiration
• Weather parameters
• Crop Characteristics
• Management and Environmental aspects are
factors affecting ET

• Weather Parameters:
• The principal weather conditions affecting
Evapotranspiration are:
• Radiation
• Air temperature
• Humidity and
• Wind speed.

• Crop characteristics that affect ET :
• Crop Type
Variety of Crop
• Development Stage
• Crop Height
• Crop Roughness
• Ground Cover
• Crop Rooting Depth

Management and Environmental
Factors :
• Factors such as soil salinity,
• Poor land fertility,
• Limited application of fertilizers,
• Absence of control of diseases and
• Pests and poor soil management
• May limit the crop development and reduce
soil Evapotranspiration.

• Other factors that affect ET are ground cover,
plant density and soil water content.
• The effect of soil water content on ET is
conditioned primarily by the magnitude of the
water deficit and the type of soil.
• Too much water will result in water logging which
might damage the root and limit root water
uptake by inhibiting respiration.

Determination of ET
• Evapotranspiration is not easy to measure.
• Specific devices and accurate measurements of
various physical parameters or the soil water
balance in lysimeters are required to determine
ET.
• The methods are expensive, demanding and used
for research purposes.
• They remain important for evaluating ET
estimates obtained by more indirect methods

Water Balance Method
• The Water Balance or Budget Method is a
measurement of continuity of flow of water.
• This method consists of drawing up a balance sheet of
all the water entering and leaving a particular
catchment or drainage basin.
• The water balance equation can be written as:
• ET = I + P – RO – DP + CR + SF + SW
• Where: I is Irrigation, P is rainfall, RO is surface runoff,
DP is deep percolation, CR is capillary rise, SF and SW
are change in sub-surface flow and change in soil water
content respectively

Lysimeter Method
• A water tight tank of cylindrical shape having dia about 2 m
and depth about 3 m is placed vertically in ground.
• The tank is filled with sample soil.
• Bottom of the tank consists of sand layer and a pan for
collecting surplus water.
• The consumptive use of water is measured by the amount
of water required for the satisfactory growth of plants with
in tank.
• Cu = Wa - Wd (Cu = consumptive use, Wa = water applied,
Wd = Water drained off)

• Field experimental method
• Some fields are selected for expt.
• The quantity of water is applied in such a way
that it is sufficient for satisfactory growth of
crops.
• There should be no percolation or deep runoff.
• If there is any runoff it should be measured and
deducted from the total quantity of water
applied.

Soil moisture study
• Several plots of land are selected where irrigation water is
to be supplied.
• The soil samples are taken from diff depths at the root zone
of the plants before and after irrigation.
• Then water contents of the soil samples are determined by
laboratory tests.
• The depth of water removed from soil determined by Dr =
pwd/ 100
• (Dr= depth of water removed in m, p = % of water content,
w = sp. Gr. Of soil, d= depth of soil in m)

• The total quantity of water removed in 30
days period is calculated.
• Then a curve of water consumption versus
time is prepared.
• From this curve the water consumption for
any period can be calculated.

Irrigation efficiencies
• Efficiency is the ratio of the water output to the water
input, and is usually expressed as percentage.
• Input minus output is nothing but losses, and hence, if
• losses are more, output is less and, therefore,
efficiency is less.
• Hence, efficiency is inversely proportional to the losses.
• Water is lost in irrigation during various processes and,
therefore, there are different kinds of irrigation
efficiencies

• Efficiency of Water-conveyance (ηc)
• It is the ratio of amount of water applied to the land to
the amount of water supplied from the reservoir.
• It may be represented by ηc.
• ηc = (Wl/Wr) × 100
• where, ηc = Water conveyance efficiency,
• Wl = amount of water applied to land, and
• Wr = Water supplied from reservoir.

• Water application efficiency (ηa)
• Ratio of water stored in root zone of plants to the
water applied to the land.
• Denoted by ηa= (Wz/Wl) × 100
• ηa = water application efficiency
• Wz = amount of water stored in root zone
• Wl = amount of water applied to land

Water use efficiency (ηu )
• Ratio of the amount of water used to the amount
of water applied.
• Denoted by ηu .
• ηu = (Wu/Wl) × 100
• ηu = water efficiency use.
• Wu = water used
• Wl = water applied

Consumptive use efficiency (ηcu )
• Ratio of the consumptive use of water to the
amount of water depleted from the root zone.
• ηcu= (Cu/Wp) × 100
• ηcu = consumptive use efficiency
• Cu = consumptive use of water
• Wp = amount of water depleted from root zone

Soil water relationship

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