Soil moistur

SOIL MOISTURE

Prepared by Mohammed Kareem

WHAT IS SOIL ?

Soil is a natural body that consists of layers (soil

horizons), composed primarily of minerals , which differ

from their parent materials in their texture, structure,

consistency, color, chemical, biological and other

physical characteristics.

Soil is the end product of the influence of the climate

(temperature, precipitation), relief (slope), organisms

(flora and fauna), parent materials (original minerals),

temperature, and time.

A represents soil; B represents laterite, a regolith; C represents saprolite, a less-weathered regolith; the bottommost layer represents bedrock

SOIL MOISTURESoil moisture is the water that is held in the spaces between soil

particles. Surface soil moisture is the water that is in the upper

10cm of soil, whereas root zone soil moisture is the water that is

available to plants, which is generally considered to be in the

upper 200 cm of soil.

Soil Moisture Estimation

Direct method:  Measurement of moisture content in the soil (wetness)       Indirect methods: Measurement of water potential or stress or tension under which water is held by the soil.

A  DIRECT METHODS

Gravimetric Methods: Soil sample from the desired depths are

collected with a soil auger in air tight aluminum containers.The soil

samples are weighed and they are dried in an oven at 105 degree C for

about 24 hours until all the moisture is driven off. After removing from

oven, they are cooled slowly to room temperature and weighed again. the

difference in weight is amount of moisture in the soil. The moisture content

in the soil is calculated by the following formula:

Moisture content            Wet weight –Dry weight

on weight basis     = ----------------------------- X 100

                                            Dry weight

Gravimetric Techniques

• Oven drying a soil sample at 1050C for about 12 hours.

Volumetric Method: Soil sample is taken with a core sampler or with a tube auger whose volume is known. The amount of water present in soil sample is estimated by drying it in the oven and calculating by following formula:

Moisture content = Moisture content (%) by weight x Bulk Density (%) by volume

Volumetric Soil Moisture (gm/cm3)

INDIRECT METHODS

• No water content in the soil is directly measured but the water potential or stress or tension under which the water is held by the soil is measured. 1)  Tensiometer2)  Gypsum block3)  Neutron probe

The Tensiometer for columns, remote locations,limited access - 1/4 in (6mm) Dia. Sensor

Laboratory research in soil columns, in tight corners, or near the soil surfaces cannot be done with a large inflexible standard tensiometer . The Model 2100F Soil moisture Probe is a specialized unit designed for investigation of soil suction in such small regimes. Ideal for laboratory work such as measuring soil suction values at various levels in soil columns. In getting measurements near surface conditions with minimal disturbance or in places where the readout must be remotely located.

Advantages : easy to design, cost little, at any conditions in real time, placed in soil easily,

Disadvantages : Only measures soil water suction,but only indirect measurement of soil moisture content; during installation, it may break.

Remote Sensing Methods• Visible & near IR – Reflected Solar• Thermal IR – Surface Temperature• Passive Microwave – Microwave• Emission/Brightness Temperature• Active Microwave – Backscattering• coefficient/dielectric properties

Visible Technique: Reflected solar energy is measured. (0.4 – 1.7 mm) • Relationship between Reflectance and SM Depends on reflectance of dry soil, roughness, colour, illumination, organic matter, soil texture.

Thermal infrared remote sensing for soilmoisture estimation

• Thermal inertia method Variations in soil moisture have a strong influence on the thermal

properties of the soil, which is an intrinsic factor of soil surface temperature change. The amplitude of the diurnal range of soil surface temerature has been found to be highly correlated with the surface soil moisture content Areas having higher soil moisture content are cooler during the day and warmer at night

Diurnal range of Surface Temperature ( T max–T min )• (T max-T min) depends on internal and external factors• Internal factors : Thermal conductivity(K) and heat capacity

(C ) where P = (KC)1/2 is known as Thermal Inertia. K and C increases with Soil Moisture.

• External Factors : solar radiation, air temperature, RH, cloudiness, and wind.

Microwave remote sensing for soilmoisture estimation

• Microwave remote sensing provides a unique capability for soil moisture estimation by measuring the electromagnetic radiation in the microwave region between 0.5 and 100 cm. The

fundamental basis of microwave remote sensing for soil moisture is the large contrast between the dielectric properties of water (~80) and soil particles (<4).

As the moisture increases, the dielectric constant of the soil-water mixture increases, and this change is detectable by microwave sensors .

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