Irrigation

Distribution of rainfall in North East India is highly uneven. During the period from October-February, rainfall is scanty and the quantity varies from 5 to 10% of the annual rainfall. In this period average evaporation exceeds average precipitation by 8 to 357 mm. Therefore, conservation and supplementation of soil moisture by irrigation becomes necessary during this period of moisture stress.

Field Capacity (FC), Permanent Wilting Point (PWP) & Available moisture

The amount of water retained after drainage of saturated soil is called field capacity (FC) moisture. At field capacity a loam or clay soil retains moisture at about 1/3 rd atmospheric pressure, whereas in sandy soil it may be as low as 0.1 atmosphere. At permanent wilting point, the soil moisture tension reaches about 15 atmosphere. Field capacity moisture and wilting point moisture are influenced by soil texture. Finer the texture higher is FC and PWP moisture content in soil. The range of moisture between field capacity and wilting point is available to plant roots. Fifty percent of this available moisture is considered as readily available moisture for working of irrigation schedule.

Method of determination of Irrigation Requirement

Plant water use may be measured or estimated. Direct measurement requires sophisticated apparatus. Therefore it is usual to rely on indirect measurements like monitoring of soil moisture status, or on estimates based on meteorological observations. Less sophisticated equipment like tensiometer can be used to monitor soil moisture status. However, the meteorological observations seem to be the only practical approach for large tracts under tea. The environmental factors determine the potential evapotranspiration, the plant factors determine the water requirement for growth, and the soil factors determine the water holding capacity and moisture release characteristics. There are several models available for indirect measurement of evapotranspiration (ET). Estimates of Penman’s ET based on meteorological data have been worked out for different tea growing regions of North East India:

 
Regions Oct Nov Dec Jan Feb Mar Apr
South Bank 127 89 68 64 86 135 158
North Bank 128 89 66 64 87 139 153
Cachar 144 111 83 82 108 157 174
Dooars 138 100 76 75 95 150 164
               
 

The irrigation requirement should also take into account the rainfall during the period of moisture stress. In North East India high intensity rainfall causing surface runoff occurs during monsoon. The intensity, duration and rainfall-runoff analysis during November to April shows that almost the entire rain received during this period can be taken as effective rainfall (ER) from irrigation point of view. Regional analysis of effective rainfall for different return periods during droughty months was carried out at Tocklai from long-term rainfall data. The difference of ET and ER for the period October to April can be considered as the total irrigation requirement.

Estimates of irrigation requirement

 
Regions of North East India Oct Nov Dec Jan Feb Mar Apr
South Bank              
ET, mm 122 66 42 35 43 65 84
ER, mm (1 in 5 yrs) 39 2 0 5 17 18 84
Net irrigation requirement, mm 83 64 42 30 26 47
North Bank              
ET, mm 117 65 37 30 42 86 144
ER, mm (1 in 5 yrs) 34 0 1 1 2 12 58
Net irrigation requirement, mm 83 65 36 29 40 74 86
Cachar              
ET, mm 137 83 48 38 53 97 137
ER mm (1 in 5 yrs) 68 3 0 0 2 19 117
Net irrigation requirement, mm 69 80 48 38 51 78 20
Dooars              
ET, mm 105 65 43 36 45 90 118
ER mm (1 in 5 yrs) 44 0 0 1 5 5 48
Net irrigation requirement, mm 61 65 43 35 40 85 70
               
 

Soil moisture measurement

Time of irrigation can be determined by judging the soil moisture content. Soil moisture can be determined by gravimetric method, resistance meters, neutron probe and tensiometers. Out of these equipments the less sophisticated equipment viz. tensiometers can be used for timing of irrigation.

Tensiometer Reading B. Range of soil moisture
0.0 – 0.1 Soil is saturated
0.1 – 0.2 Soil is in field capacity range
0.2 – 0.5 Indicates soil moisture availability to the plant
0.5 – 0.6 Usual range for starting irrigation
0.6 and above Moisture stress range
   

For young tea areas the tensiometer should be installed at 20 cm depth in the year of planting, and subsequently lowered every year by 10 cm as the root system develops.

For mature tea, the tensiometer should be installed with its tip down to 90 cm depth below the ground surface. It will be desirable to install an additional tensiometer at ΒΌ the depth of root zone i.e. at 25 cm to provide information on soil moisture in the most active root zone. When shallow tensiometer indicates suction equal to 0.5 then irrigation should be started.

Methods of Irrigation

Sprinkler irrigation is the most widely accepted method in tea gardens of North East India. Drip irrigation is confined to seed baris.

Sprinkler system

This system simulates the natural rainfall and is easy to operate. However, it is capital intensive and requires high amount of energy. The rotating-head sprinkler is the most commonly used system. It consists of a head with one or two nozzles, which is rotated slowly by the action of water passing through it. It irrigates roughly a circular patch of land around the sprinkler. Rotating sprinkler operates under wide range of pressure and discharge. Sprinklers are classified broadly into three main groups according to the operating pressure.

Characteristics
Low pressure
Medium pressure
High pressure
Operating pressure (atm)
1-2
2-5
5-10
Nozzle diameter (mm)
1.5-6
6-20
20-40
Discharge (l/s)
0.6-1
1-10
10-50
Diameter of coverage (m)
6-35
35-80
80-140
Sprinkler spacing (m)
9-18
18-54
54-100
       

A sprinkler irrigation system is designed to provide a calculated depth of water at a fixed rate of application, which is determined from the infiltration rate of soil:

Soil type
Infiltration rate (mm/hr)
Clay loam
6-8
Silt loam
7-10
Sandy loam
8-12
Loamy sand / sand
10-25
 

The ratio of effective water application to total application from the sprinklers is known as the application efficiency, and this is usually around 70-80%. It is necessary to seek professional advice in choosing the appropriate equipment and designing a sprinkler layout.

Irrigation in Young Tea

In planning irrigation first priority should be given to the newly planted young tea. Among tea areas those having coarse texture soil and in the hills especially the south facing slopes should also get priority.

Mature Tea

Maximum response of irrigation is generally obtained in the best sections of existing mature tea areas. For this it will be also essential to identify and remove other limiting factors. The best results are expected to come from irrigating unprune or early light skiffed teas. In general, depending upon rainfall received in October irrigation should commence from November and continue till March/April. The first application in November can be a little more than the estimated field irrigation requirement followed by five more applications, each at an interval of three weeks. In severely drought prone years, irrigation in April may be necessary but after the rainfall in April exceeds 75-125 mm, irrigation can be discontinued. As in the case of young tea, irrigation schedule should not be interrupted except after heavy rainfall exceeding 38 mm and then for not longer than 2-3 days. The quality of the water used for irrigation should be checked prior to use.