Soil WaterNEW

8/3/2019 Soil WaterNEW

1/61

Soil water


2/61

Functions of water

Plant cells have 50 - 90% water

Keeps turgor

Seed germination

Transpiration

Photosynthesis

Moves products

Nutrients available

Lowers soil strength

Chemical reactions

Microbial activity


3/61

Forces on Soil Water

Soil-water potential


4/61


Capillarity


5/61

1- Gravitational

2- Capillary

3- Hygroscopic

Types of soil water


6/61



7/61

Gravitational

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.


8/61

Capillary

Capillary water that moves into and is

held in the soil by capillary forces .

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.


9/61

Hygroscopic

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.


10/61

Types of Soil Water


11/61

How is Soil Water Classified?

1) Hygroscopic Water is held so strongly bythe soil particles (adhesion), that it is not

available to the plants.(10.000 to 31 atm.)2) Capillary Water (31 to 0.33 atm) is held by

cohesive forces greater than gravity and issome of it is available to plants (0.33 to 15atm.) and some of it is not available ( 15 to 31atm.).

3) Gravitational Water ( 0.33 to Zero atm. ) is

that water which cannot be held against

gravity.


12/61


13/61

Levels of Water in Soil

Saturation Point the moisture point at

which all of the pore spaces are filled with

water.


14/61


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. Water forced by

0.1 to 0.55 with average 0.33 atm. Or bar occurs when the soil contains the

maximum amount of capillary water.


15/61

Field capacity : the amount of water that

remains in the soil after gravitational

drainage

F.C depends on soil type ;sands have a

lower field capacity than clays


16/61


Permanent 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. The water

forced by 15 atm.


17/61

Wilting point: the amount of water

that is so tightly held in soil that plant

can not utilize it.


18/61


19/61

Plant available water = fc- wp

Soil moisture deficit SMD = TAW =

(fc- wp)* Z r (root depth zone)

Readily available water (RAW)

RAW = P(vfc- vwp)* Z r

Where P is the allowed depletion factor

which depend on type of plants


20/61


21/61

Water Holding Capacities of Soils

The amount of water a soil can retain is

influenced by:

soil texture

soil structure

organic matter.


22/61

Soil Texture

The smaller soil particles, the greater thesoils water holding capacity. Clay has

more water holding capacity than sand.

Small soil particles (clay) have more smallpores or capillary spaces, so they have a

higher water holding capacity. Large soil

particles (sand) have fewer capillaryspaces, therefore less ability to hold water.


23/61


24/61

Some examples of field capacity and wilting

points for different soil textures

Field capacity

(m3/m3)

Wilting point

(m3/m3)

Textural class

0.400.25Clay

0.350.15Silt

0.300.10Loam

0.100.05Sand


25/61


26/61

Soil Structure

A soil structure has a direct correlation to

the amount of water it can retain. Soils

which have a good structure such as

granular structure have a high water

holding capacity.


27/61

Organic Matter

Organic matter aids in cementing

particles of clay, silt, and sand together

into aggregates which increases the

water holding capacity.


28/61

Developing a Plan for Controlling Soil Water

- Cover Crop close growing crop planted toprotect the soil and prevent erosion

Crop Rotation planting of different crops

in a given field every year or every severalyears

Mulch material placed on soil to break

the fall of rain drops (preventing erosion),

prevent weeds from growing, or reduceevaporation from the soil surface.

Strip Cropping alternating strips of row

crops with strips of close growing crops


29/61

Water Retention and Movement

Soil Texture and water movement


30/61

Soil structure and water movement


31/61

Practical Measuring Devices

Gravimetric method

Potentiometers (tensiometers)

Resistance Blocks (gypsum blocks)


32/61


33/61


34/61

Crop WaterRequirements

Crop water use, consumptive use andevapo-transpiration (ET) are the terms that

are used to describe the water consumed

by a crop. Water requirement dependmainly on the nature and stage of growth

of the crop and environmental conditions.

Different crops have different water-userequirements under the same weather

conditions.


35/61

Evapotranspiration ( ET ) the combination

of water that is lost from the soil

through evaporation and through

transpiration from plants as a part of their

metabolic processes


36/61

Factors affecting evapotranspiration

Weather parameters

Crop factors

Management and environmental

conditions


37/61


38/61

Weather parameters

The principal weather parameters affecting

evapotranspiration are radiation, air temperature,

humidity and wind speed. Several procedures have

been developed to assess the evaporation rate from

these parameters. The evaporation power of the

atmosphere is expressed by the reference crop

evapotranspiration (ETo).


39/61

The reference crop evapotranspiration ETo

represents the evapotranspiration from a

standardized vegetated surface.


40/61

Crop factors

The crop type, variety and development

stage should be considered when

assessing the evapotranspiration from

crops grown in large, well-managed fields.


41/61

Crop evapotranspiration under standard

conditions (ETc)

refers to the evaporating demand from crops that

are grown in large fields under optimum soil

water, excellent management and environmental

conditions, and achieve full production under the

given climatic conditions.


42/61

Management and environmental

conditions

Factors such as soil salinity, poor land

fertility, limited application of fertilizers,

the presence of hard or impenetrable soilhorizons, the absence of control of

diseases and pests and poor soil

management may limit the cropdevelopment and reduce the

evapotranspiration.


43/61

Reference crop evapotranspiration

(ETo)The evapotranspiration rate from a

reference surface, not short of water, is

called the reference crop

evapotranspiration or reference

evapotranspiration and is denoted as

ETo. The reference surface is ahypothetical grass reference crop with

specific characteristics.


44/61

The only factors affecting ETo are climatic

parameters. Consequently, ETo is aclimatic parameter and can be computed

from weather data. ETo expresses the

evaporating power of the atmosphere at a

specific location and time of the year and

does not consider the crop characteristics

and soil factors. The FAOPenman-

Monteith method is recommended as the

sole method for determining ETo.


45/61

TABLE 2. Average ETo for different agroclimatic regions in

mm/day

Mean daily temperature

C0

Regions Warm30C >

Moderate20C

Cool~10C

Tropics and subtropics

5-73-52-3- humid and sub-humid

6-84-62-4-arid and semi-arid

Temperate region

4-72-41-2- humid and sub-humid

6-94-71-3-arid and semi-arid


46/61

Calculation of ETo

Penman-Monteith equation

Blaney- Criddle equation

Turc

Modified penman

Radiation method

Pan evaporation


47/61


48/61

2- Blaney-Criddle equation:

ETo = C {P (0.46 t + 8.13)}

Where:

ETo = reference evapotranspiration in mm for the period

considered.

t = mean daily temperature in c over the period

considered.

P = mean daily percentage of total annual daytime hours

C = adjustment factors which depends on minimum

relative humidity, sunshine hours and daytime wind

estimates, according to Doorenbos and Pruitt (1984).


49/61

Crop water requirements

where

ETc = Kc x ETo

ETc crop evapotranspiration [mm d-1],

Kc crop coefficient [dimensionless],

ETo reference crop evapotranspiration [mm

d-1].


50/61

maximum rate when soil water is at field

capacity. When soil moisture decreases,

crops

have to exert energy to extract water from

soil. Usually, the transpiration rate does

not

decrease significantly until the soil

moisture falls below 50% of field capacity.

The evapo-transpiration (Etc in mm) of acrop under irrigation is obtained by the

following equation:


51/61


52/61

The growing period can be divided

into four distinct growth stages:

Initial stage

crop development stage,

mid-season stage

late season stage.


53/61

Following four stages of crop growth:

i. Initialstage: Germination period and early

growth of crop when the soil cover by the crop is

less than 10%.

ii. Crop development stage: The end of initial

stage till the soil cover by the crop is about 70-80%.

iii. Mid-season stage: From the end of the crop

development stage to the start of maturing, for

most crops this shall be beyond flowering stage.

iv. Late-season stage: From the end of mid-

season stage to the full maturity or harvesting.


54/61


55/61

Kc for maize

Late-

season

Mid -

season

developmentinitial

0.61.20.70.4


56/61

Water requirement liter/ day / plant.Crops

ConventionalDrip

200-30075-100Coconut

90-10025-35Grapes

90-10030-40Mango

70-10022-30Guava

70-10020-30Sapota

60-10020-40Pomegranate

30-408-12Banana

20-6510-20Lemon

16-265-6Papaya

4-61-2Vegetables and Flowers

3-41.5-2Tapioca

3-51.5-2.5Cotton h brid

Water requirement of crops in drip and conventional system


57/61

Irrigation scheduling

1 irrigation water quantity by irrigate

RAW= p(v fc - v wp) x zr

Where: p is the allowed depletion

2-irrigation intervals = RAW/ ETc


58/61

*Calculate Crop water requirements for maize

crop which cultivated in Sand soil ( FC= 12%,WP= 4% ) and Clay soil ( FC= 42 % , WP = 22%),

and the effective roots of maize are 20,40,60,80

cm, ETo from May to August are 6.4,7.3, 6.8 and6.1 mm/day Crop coefficient are 0.4, 0.8,1.2,and

0.6 ,respectively calculate the depth of applied

water irrigation for the types of soil and theirrigation interval of each month .allowed

depletion is 50%


59/61

RAW

mm

ZrPw.pF.CMonth

202000.50.220.42May

404000.50.220.42June

606000.50.220.42July

808000.50.220.42August


60/61

Irrigation

interval

(day)

RAW

mm

ETc

mm/day

KcETo

mm/day

Month

10202.00.405.0May

8404.80.806.0June

66010.01.258.0July

20804.00.508.0August


61/61

Date post:	06-Apr-2018
Category:	Documents
Upload:	ahmed-oraby
View:	219 times
Download:	0 times

Soil WaterNEW

Documents