TENSIOMETER-FOR JUDICIOUS USE OF WATER INPADDY

Rajan BhattAssistant Professor (Soil Scinece)Krishi Vigyan Kendra,Kapurthala

rajanbhatt79@rediffmail.com(98159-63858)

Of all the planet’s renewable resources water has a unique place. It

is essential for sustaining all forms of life, food production, economic

development and for general well being. Although water is a renewable source

its availability in appropriate quality and quantity is under severe stress due to increasing

demand from various sectors. Water resources consists of both surface water

and ground water resources. The main source of all the water resources is

the precipitation in the form of snow and rainfall. The surface water

available in the form of canal water is tapped by constructing dam and

reservoirs across the river at suitable locations. The surface and ground

water resources of the country plays a major role in  agriculture,

hydropower generation, livestock production, industrial activities,

forestry, fisheries, navigation, recreational activities, etc. Agriculture sector

is the largest user of water which consumes more than 80 per cent of the country’s

exploitable water resources. The over all development of agricultural sector and the

intended growth rate is largely dependent on judicious use of available water

resources. Punjab is one of the states where the ground water development

is maximum.

Punjab is one of the smallest states of India with total Geographical area

of 5.036 million hectare . During the last few decades there has been a

spectacular development  in agriculture in Punjab. Nearly 80% of the water

resources of Punjab are used by agriculture sector. It is quite evident from

the figures :

·        Cropped area                                                 = 86%·      Area under forests                                  = 6%·      Other                                                                         = 8%

·        Cropping Intensity                                = 189 %·       Irrigated area                                                 = 97% of cropped area·       Area irrigated by canals                  = 27%·      Area irrigated by tube wells       = 72%

Table 1 Punjab’s Share in World and India

S. No.

Particulars World’s ( % ) India ( % )

1 Punjab’s Land area 0.33 1.6

2 Punjab’s Rice contribution 1 42

3 Punjab’s wheat contribution 2 55

Scenario of irrigation resources in Punjab

Green Revolution has changed the overall scenario of Agriculture in

Punjab. As a result of all this the state‘s contribution in rice and wheat

production both nationally and internationally is remarkable as shown in

the table 1.With the advent of Green Revolution the state has developed

its water resources effectively and a mesh of irrigation canals has been

laid all over. The number of tube wells has increased to 11.68 lakhs in

2004-2005 from 1.28 lakhs in 1970-71.

Almost 100 % of irrigated area in central districts is irrigated by

groundwater. This has led to overexploitation of ground water resulting in decline of

water table in the fresh water zone of the state. Out of the total 138 blocks in the state, 84

blocks were categorized as dark (withdrawal more than 85%), 16 as gray (withdrawal 65-

85 %), and 38 as white blocks (withdrawal less than 65 %). Whole of the central Punjab

blocks are in dark zone. During 1997-2003 and 2005-06 the average fall in water table in

central Punjab was 0.53 and 0.74 cm/year respectively.

Due to decline in water table, water is to be pumped from lower depths that have

greater energy requirements. The decline in water table increased the energy requirement

by 20 % in 2005 compared to that in 2001 and it is estimated to undergo an extra increase

of 20% by 2023. Due to declining water table centrifugal pumps need to be replaced by

submersible pumps to lift water from deep soil layers which can cause an extra

expenditure of Rs. 5000 crores to Punjab farmers and can cause indebtedness.

To make the judicious use of water resources we should follow the

following steps:

1.       On Farm Water Management : It has been experienced that the over

all efficiency of the irrigation systems on the farmer’s field varies from

30 to 40% which can be increased to 60 to 70 % by adopting efficient

water management strategies.

a) Precision land leveling: Benefits of Laser leveling are

i)       More level and smooth surface.

ii) Reduction in time and water required to irrigate the field.

                        i i i) More uniform distribution of water in the field.

                        iv) More uniform moisture environment of the crops.

v) More uniform germination and growth of crops.

vi) Improved field traffic ability.

b) Irrigation scheduling: Irrigation scheduling of crops is an important

component of water saving technologies.

c) Improving the conveyance efficiency: By installing Under

Ground Pipe Line system 3-4% of land can be saved which can be

brought under cultivation.

d) Improved irrigation methods

i)       Furrow Irrigated Raised Beds: Irrigation is applied through

furrows between the beds. About 30-40% of water is saved in

this method.

ii)Furrow Irrigation method in wide row crops: Crops like

maize, cotton, Sun-flower, Sugar-cane and vegetables should

be grown on ridges and water should be applied through

furrows.

e) Micro Irrigation: Drip and sprinkler irrigation systems can be

used to save water.

f)  Mulching:     Application of straw mulch improves the water use

efficiency. It reduces the evaporation losses from the soil surface.

Mulching keeps the weeds down and improves the soil structure and

eventually increases the crop yield.

2. Timely Transplanting: Proper time of transplanting rice is the month of June. It

is worth mentioning that early transplanting of rice results in wastage of water equivalent

to 10 irrigations beside loss of 37 % energy in terms of electric consumption (440

KWH/ha)

3. Suitable Varieties: Timely or late sown short duration varieties of

crops should be encouraged over early and long duration varieties to

reduce evapo-transpiration losses.  

4. Conjunctive use of water: At present 30% of total canal water

available at the outlet is utilized in the central Punjab comprising about

49%of the total geographical area of the state. As a result there is

excessive withdrawal of ground water to meet the irrigation demand of the

crops.

5.Crop diversification: Replacing one million hectare area under rice with

pulses can save 0.2 million hectare meter of water.

6.Artificial recharge of Under Ground water : Various techniques being

adopted to recharge the ground water in Punjab are:

a)   Roof Top Water Harvesting

b)   Recharge from Village Ponds

c)  Recharge in Kandi Area

Rice- The Major culprit

Rice is grown both under lowland and upland conditions and throughout the year in some

parts of the country. Under lowland conditions the rice crop is generally transplanted in

the puddled soil. Puddling disperses the soil and reduces percolation losses. For lowland

rice practice of keeping the soil saturated or upto shallow submergence of 5 cm

throughout the growing period has been found to be most beneficial practice for

obtaining maximum yields. Shallow submergence is possible only if adequate care is

taken while leveling the field. When water resources are limited land should be

submerged at least during critical stages of growth i.e. tillering and flowering and

maintained only saturated at other stages thus economizing the use of water without

decreasing the yields. During kharif season when weather is humid and evapo-

transpiration rates are low then even maintaining the soil moisture near saturation is

adequate while when weather is hot and arid, the practice of submerging the land is

found to be advantageous.

The major portion of water applied to rice crop amounting to 50-75 per cent is

lost through deep percolation, which varies with texture of the soil during submergence

of land. Great economy in water use can be achieved in rice culture if suitable measures

are adopted to reduce the losses through percolation. The selection of heavy soils,

growing of rice in large and compact area instead of small and scattered area, providing

of impermeable layer below the root zone helps to minimize deep percolation losses in

rice fields.

Judicious use of irrigation water to Rice

I. Avoid excessive irrigation to rice: The table 2 shows that only 16 irrigations are

adequate to get good yield as with 24 irrigations to rice crop hence saving of eight

irrigations.

Table 2.Optimum irrigation requirements of Rice

Treatment No. of irrigations

Mean irrigation water, cm

Paddy grain yield, t/ha

Mean *IWUE, kg/ha/cm

Continuous flooding

24 190 5.51 29

1-day drainage 18 145 5.44 382-day drainage 16 125 5.53 443-day drainage 14 113 5.11 45 *IWUE-Irrigation water use efficiency

II. Timely transplanting of rice: Shifting the planting/transplanting time of crops from

high to low evaporative demand periods reduce withdrawal of irrigation water increasing

water use efficiency. For example ET demand of June 1 transplanted rice is 620 mm

against 520mm for June 21 transplanted rice (Table 3)

Table 3. Effect of transplanting date of Rice on water balance componentsTransplanting date

Water gain Water lossIrrigation

+rainET D S

June 1 (PAN-E= 621 mm)

2062 620 1384 +58

June 21 (PAN-E= 525 mm)

1834 520 1263 +51

III. Irrigation scheduling: Irrigation scheduling is a process to determine when

to irrigate and how much water to apply. Researchers have employed demand based

(meteorological) and supply based (soil water content) approaches for scheduling

irrigation to field crops. Prihar et al. (1974) suggested a simple meteorological approach

to schedule irrigation to crops based on the ratio between fixed depth (75mm) of

irrigation water and net cumulative pan evaporation since previous irrigation. In Rice it

has been demonstrated that higher yields can be maintained by irrigating crop at 2 days

drainage interval after soaking in of previous irrigation (after 2 week of continuous

ponding following transplanting). This helps in saving eight irrigations to rice (Sandhu et

al. 1980). Hira et al. 2002 used soil water tension as a criterion for scheduling irrigation

to rice and reported higher water use efficiency with irrigations at soil water tension value

of 1600+_200mm.

Tensiometer for measuring matric potential

TensiometerA tensiometer measures soil moisture. It is an instrument designed to measure the

tension or suction that plants’ roots must exert to extract water from the soil. This tension

is a direct measure of the availability of water to a plant. Tensiometers are most useful

when a crop’s water requirements are high and when any stress due to water shortage is

likely to damage crop potential. Tensiometers may be used in any irrigated crop.

Some facts

• Tensiometers continuously monitor soil water status, which is useful for practical

irrigation scheduling, and are extensively used on high-value cash crops where low water

tension is desirable.

• Tensiometers are ideal for sandy loam or light-textured soils.

• Tensiometers may be used in clay soils for crops that need low soil water tension for

maximum yield or high crop quality. Tensiometers are soil water measuring devices that

are sensitive to soil water change and useful for irrigation scheduling.

Time travel of Tensiometer

The earliest account of a tensiometer or a tensiometer-like device was reported by

Livingston (1908). It uses all the elements of a modern tensiometer to automatically

control soil water status of potted plants (Fig. 1a) . A liquid-filled porous cup was brought

into contact with the soil. The measurement capability of a similar device was

demonstrated by Pulling and Livingston (1915) who used an osmometer with a collodion

osmotic membrane backed by sugar cane solution (as depicted in Fig. 1b) to measure the

"water supplying power of the soil."

Fig. 1. (a) Livingston's (1908) auto-irrigator for maintaining constant matric potential in

Potted plant root zone.

(b) Tensiometer designed by Pulling and Livingston (1915) to measure the "water

supplying power of the soil."

 

 

Fig. 2. A hanging column for measuring soil capillary potential (Lynde and Dupre, 1913)

 

 Fg. 3. (a) Richards' (1928) tensiometer design.

(b) Haines' (1927) tensiometer design

Fig 4. Modern Tensiometer design

Parts of Tensiometer

Reservoir and cork: It consists of two acrylic transparent tubes of specific dimensions.

The inner tube is fitted with the narrow mouth of a ceramic cup of diameter equivalent to

that of the outer tube. The upper end of the outer tube is fitted with a silicon cork. The

cork on the reservoir must provide an airtight seal for the tensiometer. The body tube

works as a reservoir, and the cork directly seals the system.

Ceramic cups: The ceramic cup is porous, but the openings are so small that when

saturated with water, air cannot pass through within the range of soil water tensions to be

measured. Water moving out through the porous cup causes the reading to change

indicating the suction, or tension, at which the water is being pulled by the surrounding

soil. Both the tubes and the ceramic cup are filled with distilled de aerated water. Before

filing the whole tensiometer with water the cup is saturated overnight with water.

Coloured Strips: The upper portion of the outer tube is marked with three colored strips

which coincide with the different levels of soil matric potential, based on the water level

inside the inner tube. The irrigation to rice crop is recommended when the water level

inside the inner tube just crosses the green strip and enters the yellow strip.

Working of Tensiometer

Principle

The water in the inner tube of the tensiometer equilibrates with the surrounding soil

through the ceramic cup and its level indicates the soil matric tension and hence the water

status of the soil. The colored strips guide the farmers for scheduling irrigation to rice

crop. When buried in the soil the ceramic cup of the tensiometer allows water to move

freely in or out of the tube. As the soil dries out, water is sucked out through the porous

ceramic cup, creating a partial vacuum inside the tensiometer which causes the water to

move down. Soil tension increases as the soil dries out, the vacuum increases in the

tensiometer and the water level falls down. When the soil is wetted by sufficient rainfall

or irrigation, water flows back into the tensiometer, the vacuum decreases and the water

level starts rising.

Tensiometers measure how tightly water is held to the soil particles and not how

much water is left in the soil. A sandy soil will reach a high tension sooner than a clay

loam because sandy soils cannot supply as much water to the plant and it is used up more

quickly. Tensiometers do not operate in dry soil because the pores in the ceramic tip drain

and air is sucked in through them breaking the vacuum seal between the soil and the

gauge on top of the tensiometer.

Installation of Tensiometer

Depth selection. The number of tensiometer installation sites required will

depend on the crops grown and field conditions. Fewer sites of tensiometers are needed

when a single crop is grown in large blocks of uniform soil. If the soils are varied or

different crops are to be grown, more sites are necessary. Sites need to be selected to

represent an area, and care should be taken not to cause excessive compaction or

destruction of plants around during installation, which may alter the condition.

Site selection. Location of the tensiometers in the field generally depends on the

type of irrigation system used. If the tensiometers are installed in a flood-irrigated field,

locations should be at the top and bottom of the first and last sets. Each location should

be far enough from the top or bottom of the field so that it is not affected by initial

wetting effects or by ponding of water. Placement should be in a crop row to avoid

traffic. Ceramic cups of the tensiometers must be kept wet until installed. A brightly

painted wooden stake or a metal rod with a colored flag attached are good markers.

Remove the silicon cork from tensiometer body and keep the tensiometer cup in a

container filled with distilled water and let it remain as such overnight till the

water level inside the tube is same as that of water outside in the container. Fill

the inner tube of tensiometer with distilled and de aerated water and keep it as

such over night.

Next day fill both inner and outer tubes of tensiometer with distilled water. Make

a hole in the field with steel iron tube of similar diameter to the depth of 20 cm.

The diameter of the hole should be slightly bigger than that of ceramic cup of

tensiometer.

Put the tensiometer into the hole and make slurry of soil and water in the ratio of

1:2 and put this into the hole around tensiometer cup. The remaining portion of

the hole can be filled with soil taken out of the hole.

Fit the silicon cork tightly. Tensiometer reading should be taken in morning hours

around 8..00 a.m. or so.

When the water level in the inner tube is within the green portion, there is no need

to irrigate the rice field and once it enters the yellow zone, rice field should be

irrigated. Don’t let the water level enter the red zone as it may cause stress to

crop.

When the field is re irrigated the water level in the inner tube will rise. If the

water level in tensiometer tube is less than 3 cm after irrigation, remove the cork

and refill the inner tube of tensiometer.

Irrigation timing with tensiometers

Tensiometers placed at about the mid-point of the main fibrous root system are used

to determine when to irrigate. This is particularly important during the period when the

water requirement of the crop is highest and yields are most sensitive to water shortage.

During this period tensiometers should be read daily. Sufficient amount of water should

be applied to re-wet the root zone. Following irrigation the reading on the tensiometer

will be reduced. Daily readings should continue to determine when irrigation is required

again.

When to irrigate will be determined largely by the amount of water applied and

stored in the root zone at the last irrigation. If only a light irrigation was applied, or a

small section of the root zone wetted, then the soil will dry faster and a high tensiometer

reading reached sooner than if a heavy irrigation was applied and all of the root zone

wetted. Climatic conditions and the leaf development of the crop will also affect the rate

of soil drying.

Conclusion

The use of soil auguring to feel the soil moisture and evaporation readings will

increase the accuracy of tensiometer irrigation scheduling. Pan evaporation readings are

particularly important as they are closely linked to the rate at which soil moisture will be

used. The combination of evaporation and tensiometer readings gives the irrigation

measurements of both climatic conditions and soil moisture, therefore enabling accurate

determination of irrigating timing and amounts.

Priority areas

In situ and ex situ conservation of rain water and its efficient recycling

Multiple use of water for increasing water productivity.

Conjunctive use of rain, surface and ground water for maintaining sustainable

hydrologic regime.

Increasing water use efficiency through efficient utilization of available irrigation

water in dry areas through promoting micro irrigating techniques.

Ground water recharge and management

Conjunctive use of poor and good quality waters.

Sources

Hira GS, Rachhpal Singh and SS Kukal (2002) Soil matric suction: a criterion for scheduling irrigation to rice. Indian Journal of Agricultural Sciences 72:236-37.

Hira GS, SK Jalota and VK Arora (2004) Efficient management of water resources for sustainable cropping in Punjab. Research Bulletin : Department of Soils, PAU, Ludhiana.

Prihar SS, PR Gajri and RS Narang (1974) Scheduling irrigation to wheat using pan evaporation. Indian Journal of Agricultural Sciences 44:567-71.

Sandhu BS, KL Khera, SS Prihar and Baldev Singh (1980) Irrigation needs and yield of rice on a sandy loam soil as affected by continuous and intermittent submergence. Indian Journal of Agricultural Sciences 50:492-96.