Assessment of Soil Moisture

 WATER

n  Water is the most important resource and gift of Allah to the human being.

n  No life with out water

n  No Substitute for water

n  It is universal solvent

n  Agri-Industry-Muncipal-Domistic

n  Population increase

n  Water problems also increase

n  Disputes

n  Within country- between country

Although the total amount of water on the planet is high, 97% is in the oceans, and 2% is locked up in ice, caps, which leaves only 1% ( or approximately ( 40, 000 Km3) on the land. Two-third of this water flows back into the Sea, leaving about 14,000 Km3 available for use of which agriculture consume more than a further two-third. As population rise so too dose demand for water and shortage of water a serious constraint to development.

                                Oceans:                               97%

                                Ice Caps:                              2%

                                On land:                            1% = 40,000 Km3.

                                Flow back to sea:             26,000 Km3"

                                Leaving for use:                14000 Km3

                                Agriculture:                        9300 Km3

                                Domestic and other:       4700 Km3

WHY MOISTURE CONTENTS ARE MEASURED

Ø  To restore soil water status in root zone.

Ø  Engineering purpose (Road, building, DAMS, etc.)

Ø  Direction of water movements.

WHY IRRIGATION IS ESSENTIAL 

Ø  To add water to soil to supply moisture   essential for plant growth.

Ø  To cool soil atmosphere (Better environment    for P. growth).

Ø  To washout dilute salts in the soil.

Ø  To desolve mineral nutrients (uptake).

Sources and storage of irrigation water

Ø  Precipitation

Ø  Snow fall

Ø  Rivers and reservoirs (DAMS)

Ø  Ground water storage (Recharging)

                        

Field methods for soil water measurement

        I)             Direct methods

      II)             Indirect methods.

    III)            Thermo Gravimetric method.

• Removal of soil sample from the field by an auger from
•  Several locations.
• Fresh. Weighing        Oven drying (105-110C⁰)
• Original weight – oven dry wt = water loss
• %age of water =            water loss x 100
• in the soil sample        wt of oven dry soil
• on volume basis   = percent water in the sample x B.D
•  For example Fresh wt.   oven dry wt.

                        (100 g     –     80 g)            = 20 g

• Moisture % content in the soil       =        20 x 100 = 25 %

                                                                                     80

• On volume basis =                                          25 x  1.6 = 40 %

Advantages. 1st and  basic method

Ø  Most accurate

Ø  No complicated equipments are required (simple)

Dis. Advantages              

Ø  Laborious and time consuming

Ø  Transportation, drying, repeated weighing.

Ø  Water content of stony soil is misleading.

Ø  Destructive method (damage crops)

NUCLEAR METHODS

                    Neutron Moderation Method

i) In this method, measurement is made of the number of Hydrogen nuclei that are present per unit volume of soil and therefore water content by volume is measured.

1. Neutron production.
2. Neutron Moderation.
3. Neutron Detection

Nuclear Reaction

₈₈Ra²²⁶ ___________________    ₈₆Rn²²²   +   ₂He⁴      + ^

₂He⁴                       +             ₄Be⁹       _____________     ₆C¹²       +   _on¹

                (alpha particle)                                  (fast neutron).

Production

_On¹         +             ₅B¹⁰ _________________      ₃Li⁷             +             ₂He⁴

 (slow neutron)                                       (alpha particle)

Detection

Neutron (uncharged particle) Equal in mass to proton or Hydrogen nucleus

Fast Neutron 2.8 MEV emitted from a source (Am-Be) collide with Atomic nuclei of H neutron, thermolised or slow down to .023 EV energy.

Slow down neutron are detected by a counter (BF3 ) or B10 placed near the source

The percentage of count registered will be proportional to the amount of “H” (water in the soil).

Sources:

Radium – Beryllium Half Life: 458 Years

Americium - Beryllium (Pellets)

Emission 25000 neutron/sec

Calibration

Reading of the probe are recorded at various depth, i.e. 30 cm, 60 cm, 90 cm etc.

 At the same time soil sample are  collected from above depth

 Their moisture % age are determined by the gravimetric method.

These values are fixed in computer of the probe.

Advantages

Wide application.

Reliable.

Rapid-30 sec to  4 minute /reading

Non destructive – minimum soil disturbance   and repeated reading.

Direct water measurement on volume basis.

Water Movement

Dis. Advantages

Cost of the equipment.

Separate calibration for each soil type.

Health Hazard

Results

n  Crop residues retained on soil surface along with zero tillage practiced improved the soil organic matter and thus reduced the N-fertilizer requirement of rainfed wheat crop by 10-12% as compared to conventional tillage practices

n  Legume based sequence enhanced the yield and WUE of subsequent wheat crop as compared to wheat-wheat sequence and provided an additional income of Rs.6020/- ha-1 to farmers.

Gamma Ray Method

The absorption of gamma rays is essentially density dependent

Phenomena. The degree to which a bean of gamma rays is

Reduced in intensity in passing through the soil column depend

Upon the over all bulk density of the column.

INDIRECT METHOD

Tensiometers

Inverse relation between water content of soil and the suction or tension with which water is held. Water is more opt to flow out of a wet soil than from one low in moisture.

More pressure, less water content. 

Advantages

n  It is very cheap compared to other methods.

n  Easy to operate

n  Water movement

n  Moisture content