HORTICULTURE
General Horticulture
Water Management
Dr. S.S. Magar Ex-Vice Chancellor
Dr. Balasaheb Sawant Konkan Krishi Vidyapeeth Dapoli – 415712
Maharashtra
(24.12.2007) CONTENTS
Introduction Basic Principles Water Functions Sources of Water
Soil-Water-Plant Relationship Water Requirements
Scheduling of Irrigations Irrigation Method
Surface Irrigation Sprinkler Irrigation Micro-Irrigation Drainage
Keywords
Border irrigation, Furrow irrigation, Basin irrigation, Sprinkler irrigation, micro irrigation
Introduction
Water is a life sustaining and natural renewable resource. Adequate, timely and assured availability of water is critical to agriculture, horticulture and plantation crops for ensured yield. In general, water management assumes paramount importance to reduce the wastages of water. It is also necessary to increase the water use efficiency (WUE) and ensure equitable water distribution. The country receives about 400 million ha m annual precipitation. The average annual natural flow is about 188 million ha m. At the same time, the fresh water requirement is estimated at about 105 million ha m by 2025 AD. Out of this, 77 million ha m has been considered for irrigation purpose. The ultimate irrigation potential of the country has been assessed at 155 million ha. Although India has the largest irrigation system in the world its water use efficiency is not more than 40 %.
Out of the total water available on the earth nearly 97.3% is the salt water in the form of seas and oceans, while 2.7% is in the form of fresh water. Globally, of the total fresh water 69% is used in agriculture sector, 8% in domestic, 23% in industrial and other sectors. However, 82% of water is used in agriculture sector in India which covers around 80 million hectare area under irrigation. The demand for water in industrial and domestic sectors is likely to increase due to liberalization of industrial policies and other developmental activities and consequently, smaller quantity of water would be diverted for agriculture sector. Currently hardly 38% of cultivable area is irrigated. The current level of irrigation, even after exploitation of all the available resources can cover only 50% of cultivated area and more than 50% area may remain rainfed. More attention needs be diverted to increase the productivity and water use efficiency.
Basic Principles
The term water management refers to artificial ways and means to provide the specific quantity of water at an appropriate time to the effective root zone of crops deriving maximum water application efficiency and water use efficiency. However, it is always supplementary to precipitation received during crop growth period. The function of water is to meet the evapo-transpiration (ETc) demand of crop plants. The water is also required for metabolic activities of plant (ETc <1.0%) and for special activities of land preparation including unavoidable deep percolation losses below the effective root zone. The water requirement (WR) is expressed as the quantity of water which is the summation of ET and water used for metabolic purpose plus special purposes including deep percolation losses, during crop growth period (Equation 1).
WR = E + T + M + DP + SP……….(1)
E = Evaporation from soil surface, mm T = Transpiration from the leaves, mm
M = Water required for metabolic activities, mm DP = Deep percolation losses, mm
SP = Special purposes such as for tillage, leaching etc., mm
The absolute or net water requirement includes the quantity of water lost through the evapotranspiration and water used by the crop for plant metabolic activities including that retained in plant body. It is synonymous with consumptive of water (CU). The
components of hydrological parameters and water budget between two successive irrigations are depicted in fig. 1
H y d r o l o g i c a l p a r a m e t e r s a n d w a t e r b u d g e t b e t w e e n t w o s u c c e s s i v e i r r i g a t i o n s
•E v a p o r a t i o n f r o m W a t e r B o d y
•E v a p o t r a n s p i r a t i o n
•P r e c i p i t a t i o n
•R u n o f f
•D y n a m i c s o f S o i l w a t e r
•D e e p P e r c o l a t i o n
•U s e a b l e R e t u r n f l o w
•C a n a l L o s s e s
•O p e r a t i o n a l L o s s e s
Fig. 1 Components of Hydrological Parameters
Water management consists of three basic principles: i) the time of irrigation (when to irrigate?) II) The method of irrigation (How to irrigate?) III) The quantity of water to be irrigated (How much to irrigate?) The first principle refers to scheduling of irrigation; second principle reflects the irrigation layout and third principles indicates the quantity of water to be given at each irrigation or the total, in a given period of time.
Water Functions
Soil is the medium for water storage, and helps in continuous release of available water between two consecutive irrigations during the crop growth period. The microbial activities are maximized in the presence of optimum available water and good aeration. The fertilizer compounds are converted into available forms and dissolved into water. The nutrients are being absorbed by the roots through the water solution due to water potential gradient of plant. Water is the prime requirement of physical, chemical and biological activities of soil and plant, in addition to plant metabolic activities and ET requirement. Hence, water is life sustaining and renewable source. The liquid form of water, with dissolved nutrient concentration and suspended particles is known as 'soil solution'. Water and specific microbes are responsible for fertilizer chemical reactions converting them into available forms in the vicinity of plant roots. This translocation occurs due to hydraulic conductivity or water potential gradient in the soil profile. Soil water potential (w) and leaf water potential (f) are important components in transport of dissolved nutrients in water.
Sources of Water
The global water resource availability in the sea as salty water, south and north poles as solid ice form and fresh water on the land, rivers or lakes etc. is depicted separately Fig. 2).
Fig. 2: Water Management Strategies in Horticultural Crops
In India, monsoon winds are mostly responsible for precipitation, termed as monsoon rains. Out of fresh water available on the earth, India contributes to the extent of 4.5% with 2.4% geographical area and about 16% of world population. The per capita available water of 6000 m3 / year has declined to 2300 in the year 2000 and will be declined to 1600 m3 / year in the year 2010 (Fig. 3).
Fig. 3: Per capita water availability
Out of the total fresh water available, about 82% of water is used for agriculture sector. Recently it is estimated that 38% of cultivable area has been brought under irrigation in the country. The annual precipitation was estimated to the tune of 400 million ha m out of which 115 million ha m classified as surface flow or runoff losses.
Water storage in reservoirs and tanks is about 15 million ha m.
Soil-Water-Plant-Relationship
Soil water plant relationship includes the components of physico-chemical properties of effective root zone, plant response and the climatic fluctuations in terms of temperature, relative humidity and wind velocity etc. In brief, this complex phenomenon refers to simple terminology as 'SPAC' (Fig. 4).
S o i l P l a n t A t m o s p h e r e C o n t i n u u m ( S P A C )
P r i n c i p l e s o f i r r i g a t i o n w a t e r m a n a g e m e n t
A t m o s p h e r e
L i t h o s p h e r e H y d r o s p h e r e
Fig. 4: Soil Plant Atmosphere Continuum (SPAC) Where
S = Pertinent physico-chemical and hydraulic properties of soil
P = Plant response in terms of water and nutrients absorption translocation and transpiration mechanism
A = Atmospheric fluctuation related to temperature (T) relative humidity (RH) and wind velocity etc.
C = Continuity or continual processes
Physical Properties of Soil: Water is retained under the influence of capillary forces in the pore space which is a function of particle density (PD) and bulk density (BD) of soil as below (equation 2):
e = ( 1 - BD ) x 100 ……….(2)
PD
e = Total porosity expressed in percentage BD = Bulk density of soil, g/cc
PD = Particle density, g/cc
The porosity depends upon the soil texture and ranges from 35 to 55 % for coarse textured to clayey soils. At saturation, all the pore space is occupied by water which is referred to as maximum water holding capacity.
Following saturation, water under the influence of gravitational forces materially ceases to go down, when evaporation from soil surface is checked and water attains equilibrium, this quantity of water retained is termed as field capacity (FC), expressed in term of percentage.
Permanent wilting point (PWP) refers to the water content of soil at which plants do not get enough water to meet the transpiration demand and wilt permanently and it does not recover. The moisture content at this stage is known as Permanent wilting point.
Maximum Water Holding Capacity (MWH), Field Capacity (FC) and Permanent Wilting Point (PWP) are designed as soil moisture constants. The difference between FC and PWP is termed as 'Available Soil Moisture' (ASM). As a thumb rule, irrigations are scheduled when 50% ASM is depleted from the effective root zone.
This level of soil moisture depletion is called as 'Management Allowable Deficit' (MAD). The 'MAD' values are different for crops depending upon drought resistance or tolerance capacity of that crop. Generally it ranges from 25 to 66 % from vegetable to drought resistance crops like sorghum, pearl millet and safflower.
Fig. 5: Kind of water in the soil and difference in available moisture content Source: USDA SCS Handbook Sec. 15, 1964
Soil moisture content in the soil profile is related to soil water potential which is expressed in terms of work that should be done in order to transport inestimable water quantity irreversibly and isothermally from a common resource of water to specified condition. The relationship between soil moisture content and total soil water potential is known as soil moisture characteristic curve and it is depicted in fig. 6.
Chemical Properties: Salt concentration in the soil solution is represented by the electrical conductivity and soil PH. Moreover, sodium relationship with calcium and maganize is also reported by Sodium Absorption Ratio (SAR) and carbonate bicarbonate relationship with Residual Sodium Carbonate (RAC).
Fig. 6: Soil moisture tension curve for soils of different textures Water Requirements of Horticultural Crops
It is a fact that different crop species or horticultural, plantation crops have variable degree of soil moisture stress tolerance limit hence water requirement of different crops will vary under similar soil and climate conditions. The rainfed horticultural crops have different degree of drought tolerance or drought resistance.
Direct Method: Water requirements of crops (WR) are decided either by direct or indirect methods. The direct method is classified as lysimetric technique or soil moisture or soil water balance method. The lysimetric technique is precise for shallow root system, close spaced, short duration vegetable crops.
WR = P + IR + ∆SW - (r + PW)………..(3)
P = Precipitation, cm
IR = NIR, cm
∆SW = Soil water contribution, mm
r = Surface funoff, mm
PW = deep percolation, mm
The soil moisture monitoring to a desired depth either by nucleic technique or neutron probe are necessary in case of widely spaced, long duration horticultural crops. Water requirement of mostly rainfed horticultural crops such as mango, sapota, pomegranate, citrus are difficult to predict due to their deep root system and ground water contribution.
Water requirement is also determined by the field experimentation method.
CU = ER + IR + ASW ……….(4)
CU = Seasonal consumptive use, cm ER = effective rainfall, cm
IR = net irrigation water, cm ASW = soil water contribution, cm
Estimation of effective rainfall under this method is very crucial. There are other methods of estimating water requirement like soil water depletion method and inflow - out flow method but precision in soil moisture monitoring is very important in estimating the requirement.
Indirect Methods: Evapotranspiration rates of various crops are estimated by pan evaporation multiplied by a pan factor (Kp) and crop coefficient factor (Kc). The crop coefficient values increase with age of the crop approaching grand growth period nearly stabilizes at the grand growth reaching its maximum at flowering and then declines with senescence.
Net irrigation requirement of horticultural crops by using daily pan evaporation data is estimated and used for micro-irrigation purpose. The equations for estimations of ETc and ETca are as follows:
Eo = Ep x Kp ……….(5)
Where
Eo = Potential evapotranspiration, mm/day Ep = USWB class A pan evaporation mm/day
Kp = Pan factor
Evapotranspiration (ETc) is again estimated using the crop coefficient constants (Kc)
ETc = Eo x Kc ……….(6)
The values of crop coefficient are reported in FAO bulletin No. 31, 1975 by Doorenbos and Pruitt.
Blaney - Criddle (1950) developed a formula for estimating Cu based on mean monthly temperature, day light hours and locally developed crop coefficients.
Doorenbos and Pruitt (1975) proposed a modified Penman method for estimating the reference crop evapotranspiration ETo and gave tables to facilitate the necessary computations.
Scheduling of Irrigations
Plants are subjected to moisture stress below the soil moisture status of field capacity.
The crops or plants can sustain the water stress up to certain level, which was earlier, referred as management allowable deficit. The available water deficit needs to be restored by application water at that period. It is expected that depleted water quantity is to be applied at each irrigation. The interval between two irrigations should be decided by the soil moisture depletion level in the effective root zone, without adversely affecting the growth and yield. Water needs of the crop are however, the prime consideration to decide the time of irrigation. Hence pre-decide frequency of irrigation depending upon soil type, crops and climate to restore depleted available soil moisture which is referred to as 'scheduling or irrigations'.
Criteria for Scheduling of Irrigations: Irrigation need of the crops is decided by the evaporative demand of the ambient atmosphere, soil water status and plant
characterizations. The criteria for scheduling of irrigations are mainly grouped into three categories.
i) Plant criteria
a) Critical crop growth stages approach b) Indicator plant
ii) Soil water status criteria
Soil moisture depletion approach iii) Climate criteria
Climatological approach
i) Plant Criteria: The plant physiologists have identified the critical growth stages of plantation crops and vegetable crops. This approach is used in cereals, pulses, oil seeds and vegetable crops on large scale as these crops are seasonal short duration crops. The limitation for adoption of this approach is the type of soil and its soil water retentivity and its release. The coarse textured and shallow soils are not suitable to adopt this approach.
Plant appearance at temporary wilting stage, stunted plant growth, plant water potential, indicator plant, leaf diffusion resistance, stomatal aperture, plant temperature and critical growth stages are also ways and means available to schedule irrigations under 'Plant Criteria Approach'. The plant shows certain characteristic changes in their constitution, appearance and growth behaviour with changes in available water. The plant physiologists have decided the critical growth stages for all crops when optimum soil moisture availability is necessary. Hence indicator plant and critical growth stages approaches are common for scheduling purpose.
b) Soil Moisture Status Criteria: Some plants are very sensitive to soil moisture stress. Moreover, early soil moisture stress condition is created in observation plot be one square meter area by modifying soil profile, changing its water holding capacity by sand application. The pre - indication of temporary wilting is reported in that plot for scheduling of irrigation. Moreover moisture stress sensitive crop like sunflower is planted in the observation plot.
Soil water status criteria are dependent on soil moisture measurement and soil moisture tension measurement in the effective root zone. It is very precise and depth of water to be applied could be adjusted economically. The soil moisture is monitored by either direct or indirect methods. In general, irrigations are scheduled when 50% available soil moisture is depleted in the effective root zone. This is also termed as management allowable deficit (MAD). It differs from crop to crop depending upon their moisture stress tolerance or moisture availability index. The vegetable crops have hardly 30% available moisture deficit or 30% MAD value to avoid the stress and maximization of water use efficiency. It requires huge periodical soil moisture sampling for estimation of ASM levels. It is cumbersome and time consuming. Moreover soil moisture status could be measured by the nucleonic neutron probe, which is accurate, but the initial cost is very high.
The irrigations are scheduled after reaching specific values (0.5 to 0.6) of IW / CPE ratio. This cumulative Pan evaporation can be converted into number of days or interval in days between two irrigations.
iii) Climatological Approach: The climatological approach is categorized under empirical formulae. Penman (1968), Thornthwaite (1948), Blaney - Cirddle (1950), and Christiansen (1968) developed the formula for estimating potential evapotranspiration. These ETO values were subsequently used for determination of consumptive use (CU) and deciding schedule for water application. The USWB - class A pan evaporimeter was used on large scale. The pan factor was another parameter. The values of crop coefficient were tested with the help of lysimeter technique. Simple empirical quotient IW / CPE was used for scheduling purpose, where IW is the depth of irrigation, mm and CPE is the cumulative pan evaporation, mm.
Fig. 7: Open pan evaporimeter (US WB type)
The reference ETO is defined as evapotranspiration rate of uniform 15 cm tall green alfa-alfa grass cover, actively growing, completely shading the ground when soil moisture existed at field capacity (not short of available water),. Similarly, Kc refers to the ratio of ETc/ETo where ETc and ETo are actual evapotranspiration rate at a particular critical growth stage and reference evapotranspiration rate respectively.
Some universal methods estimating evapotranspiration on the basis of meteorological parameters are given below:
I) Modified Penmen Method
ET*o = { W . Rn + ( I - W) . f(u) . ( ea - ed ) } ………(7) (radiation term ) (aerodynamic term )
ETo = CET*o
where,
ET*o = the reference crop evapotranspiration in mm/day (unadjusted) ETo = the reference crop evapotranspiration in mm/day (adjusted)
W = temperature related weighting factor for the effect of radiation on ETo
Rn = net radiation in equivalent evaporation (mm/day)
= Rns - Rnl
where,
Rns = the net incoming short wave solar radiation = RA (1 - r ) ( 0.25 + 0.50 n/N ) in which RA is extra-terrestrial radiation expressed in equivalent
evaporation in mm/day, n/N is the ratio between n = actual duration of bright sunshine hours and N = maximum possible duration of bright sunshine hours and r is the reflection coefficient
Rnl = net long wave radiation = f(t) . f(ed) - f (n/N)
ea = saturation vapour pressure in mbar at the mean air temperature in oC ed = mean actual vapour pressure of the air in mbar = ea x RH/100 in which, RH = mean relative humidity. This can be determined from dry and wet bulb temperatures or dew point temperature f(u) = wind related function
( I - W ) = a temperature and elevation related weighting factor for the effect of wind and humidity on ETo
( ea - ed ) = difference between saturation vapour pressure at mean air temperature and mean actual vapour pressure of air (mbar)
C = adjustment factor to compensate for the day and night effects.
II) Modified Blaney & Criddle Method (1950) U ( or CU ) = cu (or ΣCu ) = KF = Σkf = Σ ktp
100 where,
U ( or CU ) = seasonal consumptive use for a given period, inches u (or Cu) = monthly consumptive use, inches
t = mean monthly temperature, oF
p = monthly daylight hours expressed as percentage of daylight hours of the year
f = t x p/100, monthly consumptive use factor
F = sum of the monthly consumptive use factors (f) for the growing season ( = Σf)
k = empirical consumptive use crop coefficient for the month ( = u/f), dimensionless
K = empirical seasonal consumptive use crop coefficient for the growing season, dimensionless
The value of pan factor (0.68) and crop coefficient (0.4 to 0.9) are two parameters for estimating consumptive use (ETc) for a particular period. The irrigations are scheduled after reaching specific values (0.5 to 0.6) of IW / CPE ratio. This cumulative Pan evaporation can be converted into number of days or interval in days between two irrigations.
Irrigation Method
The methods used for water application to crops or plantation crops have an important bearing on water management. These methods include the flooding (ridges &
furrows, check basin and border irrigation) applying it beneath the soil surface or spraying it under pressure as well as by applying it on the surface or subsurface drop by drop. The conventional system of irrigation revolves around the concept of replenishing the moisture level to the field capacity (FC) after 50-60% depletion. The system does not permit the restricting of water to meet the requirement at the root zone. It results into deep percolation losses or sometimes run off. The problems of water logging and soil salinity are reported in command areas. The crops have suffered serious moisture due to uneven distribution of water. This necessitates the
use of modern irrigation methods which will irrigate the plant or the crops rather than whole field. The effective root zone is brought to field capacity by subsequent shallow ponding and advancement of water front in border or furrow basin. This is possible by adjusting the duration and the rate of stream size, the gradient, the soil structure, infiltration rate, and soil texture. The application efficiency of irrigation water is the relationship between the quantity of water that has wetted the effective root zone of the crop and the quantity of water released from the source.
Water management refers to efficient use of water for best possible crop production.
The term efficient use includes the optimization of water use and maximization of crop yields. The water losses either through deep percolation beyond the root zone and run off is to be minimized or avoided to the maximum extent. It is important to know the irrigable area, stream size, irrigation depth and opportunity time for uniform water application. Irrigation methods are classified under surface, subsurface and pressurized irrigation.
1. Surface Irrigation Methods: Surface irrigation methods refer to a scientific and systematic approach to spread the water over the surface crop area as uniform as possible to satisfy the demand of replenishable soil moisture and bring it to the field capacity maintaining the gradient, design discharge and duration or opportunity time to infiltrate the water. Although there are many ways of water application, the following are the three main methods (Fig. 7):
i) Border Irrigation ii) Furrow Irrigation iii) Basin Irrigation
S u r f a c e i r r i g a t i o n m e t h o d s F l o o d i r r i g a t i o n : U n e v e n W a t e r F l o w &
D i s t r i b u t i o n
V a r i a b l e O p p o r t u n i t y T i m e
F u r r o w I r r i g a t i o n :
L a y o u t D e s i g n e d f o r 1 . L e n g t h
2 . B r e a d t h 3 . S l o p e 4 . D i s c h a r g e
Fig.7: Surface Irrigation Methods
In addition to above, flood irrigation is generally adopted for rice where standing water is required for long duration of crop growth and cultivated on low level plots.
Many times, the field to field irrigation is advocated in low-line rice fields.
i) Border Irrigation: The borders are prepared by dividing the land into strips by small earth bunds to irrigate similar crops and soils. The borders usually have uniform slope away from the farm channel, in the direction of water flow. The slope
of border is maintained within the range of 0.3 to 0.6 % depending upon the soil types and infiltration rate. The length of border is also decided on size and slope of field, soil type crops and depth of irrigation. The water discharge in the border is dependent on above parameters. However, the discharge of 2 liters / second / meter width of border are maintained to have opportunity time and irrigation depth.
ii) Furrow Irrigation: The ridges and furrows are opened in the field before planting or transplanting the crops or vegetables or cash crops like sugarcane. The water is led through the furrows for irrigating the crops. This method is the most widely used method for irrigating row crops. Water flows through the furrows and does not wet the entire field. The water moves vertically downwards as well as horizontally under the influence of hydraulic gradient. It is important to select the furrow spacing and length. The stream size, type of soil and crops are other parameters to achieve the maximum water application efficiency. Alternate furrows are irrigated under minimal or deficit irrigation system under water scarcity irrigation. This system has some limitation such as advance time too long, stopping inflow too soon, maintenance of low discharge difficult and no return flow (Fig. 8).
F u r r o w i r r i g a t i o n
S p e c i f i c D i s a d v a n t a g e s
¾
¾
¾
¾
¾
¾
A c c u m u l a t i o n o f s a l i n i t y - L o c a t i o n S p e c i f I n c r e a s e w a t e r l o s s e s
D i f f i c u l t y i n f a r m o p e r a t i o n s
E x p e n s e a n d t i m e o v e r t i l l a g e p r a c t i c e H i g h e r l a b o u r c o s t
L o w u n i f o r m i t y C o e f f . C o m p a r e t o S p r i n k l e r a n d D r i p I r r i g a t i o n
Fig.8: Furrow irrigation of a vegetable crop
iii) Basin Irrigation: Basins are the simplest and most widely used by the farmers.
The field is divided into basins depending upon field size, soil types and crops grown.
Each basin is a level area of the land surrounding by the earth bunds, in which water can be impounded until it infiltrates into the soil. Basin irrigation can be adopted to suit many crops, soils and farming practices. The basins can be much larger on clay soils than sandy soils. The stream size can be adjusted as per the size of basins and irrigation depth. The water supply is done by direct or cascade supply.
In case of direct supply, a farm channel is constructed alongside every basin in the fields. Each basin is supplied directly from the farm channel. Cascade supply is used on slopping land where basins are constructed on terraces. Water flows from one basin to the next down the slope. However cascade supply has limitations of soil erosion, leaching and poor drainage. More ever, irrigation schedule is common for all basins.
Fig.9 : Schematic sketch illustrating typical layout of check basin method of irrigation
Surface irrigation pertains to water application over crop area from a supply channel at upper reach of the field. Surface irrigation method is further divided into full flooding and partial flooding of soil surface. This is possible due to manipulation of soil surface. The primitive method of wild flooding of crop area needs to be banned from water scarcity point of view. However, border strip irrigation (straight - Contour) and check basin irrigation are common for food grains and horticultural crops respectively. The furrow irrigation, basin and ring irrigation are predominant methods under partial flooding. The field is divided into small plots or strips to irrigate the soil surface uniformly to a desired depth. The size of stream should be maintained properly so as to have adequate control of water. The deep percolation and run off are minimized to achieve high water application efficiency. The land slopes from both directions are to be kept precise; (< 0.6 %).
Surface irrigation methods are mostly followed for field crops in command areas of irrigation projects. Major advantages of surface irrigation are as follows:
¾ Variable stream size could be managed efficiently
¾ Cost of water application is less
¾ Could be managed with unskilled persons
¾ Water is conveyed by field channel
¾ No energy is required
2. Sprinkler Irrigation: The water is convened under pressure through aluminum or high density polyethylene pipes to the fields. The water is sprinkled over the crop through the rotating nozzles at a pressure of 3-4 kg / cm2. The riser and nozzles are installed on the lateral pipes. The conventional types of sprinkler system are depicted in fig. 10. Moreover, modern centrally pivoted sprinkler irrigation system is shown in fig. 11.
S p r i n k l e r i r r i g a t i o n m e t h o d s
•U n i f o r m D i s c h a r g e
•H i g h U n i f o r m i t y C o e f f .
•W a t e r S a v i n g
•I n c r e a s e i n y i e l d a n d Q u a l i t y
•H i g h E n e r g y R e q u i r e m e n t
•H i g h I n i t i a l C o s t
•T r a i n e d M a n p o w e r
•R e g u l a r M a i n t a i n a n c e
•P r e s s u r e R e q u i r e m e n t – 3 - 4 K g / c m 2
Up
DN
S p r i n k l e r i r r i g a t i o n m e t h o d s
R a i n g u n
S p r i n k l e r i r r i g a t i o n m e t h o d s
S i d e r o l l s p r i n k l e r i r r i g a t i o n
Fig. 10: Types of Sprinkler Irrigation Methods
S p r i n k l e r i r r i g a t i o n m e t h o d s
C e n t r a l l y p i v o t e d s p r i n k l e r i r r i g a t i o n
Fig. 11: Modern centrally pivoted sprinkler irrigation
The design discharge of sprinkler system should not exceed the infiltration rate.
Steady state in filtration is known as basic infiltration which is very similar to hydraulic conductivity of the soil. The water is conveyed through the pipe system with desired pressure and spread uniformly on surface area through the sprinkler nozzles maintaining high pressure of about 4 kg/cm2. The uniformity coefficient of sprinkler should be > 80 % which is a function of wind velocity and the pressure existed at the nozzle. The principal advantages are:
¾ water saving of about 30 - 40% over traditional methods
¾ small quantity and frequent applications of water
¾ water application efficiency is high
¾ uniform depth of irrigation could be maintained, land leveling not necessary i.e. undulating and sloppy lands could be irrigated
¾ application of water with fertilizers, (fertigation)
¾ crops are protected from frost damage.
However, sprinkler irrigation system adoption is restricted due to its high cost, and capital availability, high operating cost, paucity of technical and skilled man power, frequent maintenance and high wind velocity etc.
The components of sprinkler system are depicted in Fig. 12.
Fig. 12: Components of a Sprinkler Irrigation System
Amongst different types of sprinkler irrigation systems, nozzle line sprinkler system is very common in Indian agriculture. One or more pipes of relatively smaller diameter having a single row of fixed nozzles spaced at uniform intervals along their entire length are used in the system. The nozzles are rotated covering water spread circle of 6 to 15 m radius. This rotary head sprinkler system is mostly advocated due to its slow rate spread and high uniformity coefficient. A working pressure of about 3 kg / cm2 is appropriate from energy point of view. These systems are categorized under portable shifting types. Now days, the rain-gun is commonly used for the sugarcane crop. Advanced sprinkler system consisting of portable wheel sprinklers are being used for large farming system in USA and other countries. An appropriate design of sprinkler irrigation is absolutely necessary for determining sprinkler discharge, spacing and spread at sprinklers and the capacity of the sprinklers. The rate of water application by an individual nozzle is decided by the rate of discharge of sprinkler and wetted area of sprinklers. Radius of the wetted area covered by sprinkler is also determined from pressure head at nozzle and diameter of the nozzle.
3. Micro Irrigation: The micro-irrigation system consists of a network of pipes along with water filtration provisions and suitable emitting devices, which could maintain high frequency application of water in and around the root zone of plants as per daily net irrigation requirement (NIR) with very high irrigation efficiency (> 95%). The system is based on the fundamental concept of irrigating only the root zone of the crop, which would maintain excellent soil-water-plant relationship. The root zone is always kept at field capacity (FC) due to which the microbial activities are enhanced to the maximum extent with best aeration properties. The system is known to be able to achieve high water use efficiency (WUE), water saving to the extent of 70-80 % and increase in the quality and yield to the extent of 60 %. The conveyance and deep percolation loss beyond root zone are totally avoided. In conventional irrigation system, the crop / plant is subjected to moisture stress before irrigation and oxygen stress after irrigation (Fig. 13).
S u r f a c e i r r i g a t i o n D r i p
S p r i n k l e r
D u r a t i o n , d a y s F i e l d c a p a c i t y
( 1 / 3 a t m )
Moisture content
W i l t i n g p o i n t ( 1 5 a t m )
S a t u r a t i o n m o i s t u r e c o n t e n t
M o i s t u r e a v a i l a b i l i t y o f c r o p s u n d e r d i f f e r e n t m e t h o d s o f i r r i g a t i o n
Fig.13: Saturation Moisture Content
However, excellent soil-moisture-air (oxygen) balance is retained as macro pores are categorized under aeration porosity. The system has got advantages of fertigation (Fig.14) and use of unleveled, undulating, sloppy lands. The system is technically feasible and economically viable particularly for horticultural crops due to its comparative low cost and high uniformity coefficient.
F e a t u r e s o f d r i p i r r i g a t i o n
F e r t i g a t i o n
Fig. 14: Fertigation System (Component of micro-irrigation)
Such direct application of water to the root zone in controlled quantities as per NIR results in uniform growth of plants, higher crop yields with excellent quality ensuring substantial saving in irrigation water. The withdrawal of moisture and nutrients by the plants are replenished almost immediately ensuring that crops never undergo water and nutrient stresses. The typical drip irrigation system contains components of water pumping arrangement, filtration system, control head system, distribution lines, fittings and emitters, etc.(Fig. 15)
A t y p i c a l l a y - o u t o f m i c r o - i r r i g a t i o n s y s t e m
Fig. 15:Micro-irrigation System
This is now very versatile subject to farmers, scientist and horticulturists in the country. Hence its original history, development, etc. have not been covered in this article. However, it is necessary that emphasis needs to be given on its operational steps and principles behind it.
Basic Concept of its Operation: At the outset, it is necessary to know the uniformity coefficient, emitter discharge and pressure that existed at the emitter. The type of emitter is decided on the basis of crop spacing, type of soil, wetted area and its daily or alternate day net water requirement. The pertinent physico-chemical properties, such as texture, structure, structural class of soil, porosity, MWHC, FC, PWP, available water and management allowable deficit (MAD) are to be assessed precisely for design of drip system. Moreover, it is necessary to know hydrological properties of soil such as Ko, Do and water capacity, etc. The water requirement in terms of litres per day, per plant could be worked out from daily pan evaporation pan factor, crop coefficient value and wetted area. Precise formulae are available which were tested under Indian conditions (Magar et al., 1990; Sivanappan, 1994). Moreover, effective rainfall and rainy days are considered in these calculations from time to time. It is also necessary to identify the critical growth stages and moisture stress period for horticultural crops. The grape, ber and mango crops are subjected to moisture stress before flowering. The micro-irrigation for plantation crops are depicted in Fig. 16
M i c r o - i r r i g a t i o n m e t h o d s
M i c r o - s p r i n k l e r i r r i g a t i o n
L o w d i s c h a r g e , u s e d f o r c l o s e g r o w i n g c r o p
M i c r o - i r r i g a t i o n m e t h o d s
M i n i - s p r i n k l e r i r r i g a t i o n
H i g h d i s c h a r g e , u s e d f o r w i d e s p a c e d c r o p s
Fig. 16: Micro-irrigation operations for different crops
Design and Operational Limitations: The perfect design of micro-irrigation mostly characterized by uniformity coefficient (UC), NIR, wetted area and ultimately higher production and productivity. On the other hand, UC is a function of pressure existing in the system at extreme point of lateral pipe. The pressure or head loss is the function of emitters, frictional losses, etc. The type of laminar or turbulent flow could be regulated by diameter of pipe and pressure. Normally, PVC pipes are used for conveyance of water from well to field. Subsequently, micro-irrigation system in field consisting of HDPE (main line), low density polythene pipe LDPE (sub-line) and drippers (LLDPE). These LDPE or LLDPE pipes are well protected from ultra- violet rays by the carbon black content to the extent of 2.5 + 0.5 %.
The design has minimum operational trouble. However, irrigation water quality (saline water), manually operated filtration system, several elbow joints and sudden contraction of water conveyance pipes creating hammering action will be the key factors for abnormal operation. The clogging of pipes by deposits of Fe2SO4 and MgSO4 and development of algae is possible. The most significant factors in operation are the chemical clogging of emitters and its cleaning by chlorination or acidification processes. The physical impurities are cleared by filtration system. The physical suspending material in the water could be removed by increasing pressure in the system and opening the end plugs of lateral pipes, but chemical compounds combined with biological microbes show difficult interactions which need to be dissolved by acidification and chlorination processes at a regular interval depending upon water quality and temperature. Back flushing of water in sand filtration tank is necessary before the system is put into operation. These are some of the reasons why soil and water analysis is necessary. The operation becomes still critical because of its high water application frequency almost daily (sandy soils) or on alternate days (loamy soils) or once in three days in black cotton soils.
Micro-irrigation achievements: It is needless to say that our country was at crawling stage in 1980 when Israel, USA and Australia had achieved rapid progress in micro- irrigation. The research on micro-irrigation was mostly initiated by TNAU, Coimbatore, MPKV., Rahuri and WTC, IARI, New Delhi, Haryana State was leading for sprinkler irrigation on large scale due to sandy soils in Bhiwani district. A major boost to the drip irrigation technology came in India when a private corporation, Jain Irrigation System Ltd. (JISL) entered into this profession. Micro-irrigation has received the government support when the Government of Maharashtra had declared the significant subsidy in 1988. The financial support was rendered subsequently by NCPA, NCPAH and Agriculture Ministry, Government of India. Few hundred hectares of micro-irrigation mostly experimental areas has grown to few million hectares due to joint efforts of policy makers, scientists, technologists, NGOs and mostly over-whelming response from farmers. The famine and droughts have also taught us the lessons in spreading micro-irrigation technology.
The achievements of micro-irrigation are of two types – qualitative and quantative.
The qualitative achievements are those which address production of quality agricultural produce and that increase WUE and FUE. Excellent soil health could be maintained due to optimum soil moisture content, accelerated microbial activities and maximization of biomass decomposition converting it into humus. The cycles of high temperatures and low humidity could be avoided mostly under summer conditions. It
is also worthwhile to mention that undulating, sloppy land terrain also could be brought under horticultural crops with technological innovations.
Quantitative parameters are related to growth of micro-irrigation system in terms of area coverage and overall increase in the productivity. However, quantity of water saved or fertilizer saved due to imposition of drip system, the energy conservation and reduced labour cost, etc. are also other parameters form the quantitative achievements.
The data on research work carried by SAUs, IARI; WALMIs and AICO on Water Requirement (WR) and yield, and WUE were compiled by INCID and presented in Table 1(Singh 2005).
Table1. Result of studies on micro-irrigation
Yield (q/ha) Irrigation (cm)
WUE (q/ha/cm)
Advantage of MI (%) Sr.
No.
Crop
Surface Drip Surfac e
Drip Surf ace
Drip Saving Increa se in yield
1 Beet 5.7 8.9 86.0 18.0 0.07 0.50 79.10 56.10
2 Bitter gourd 32.0 43.0 76.0 33.0 0.42 1.30 56.60 34.40 3 Brinjal 91.0 148.0 168.0 64.0 0.55 2.30 61.90 62.60 4 Broccoli 140.0 195.0 70.0 60.0 2.00 3.25 14.30 39.30 5 Cauliflower 171.0 274.0 27.0 18.0 6.30 15.20 33.30 60.20
6 Chilli 42.3 60.9 109.0 41.7 0.39 1.50 61.70 44.00
7 Cucumber 155.0 225.0 54.0 24.0 2.90 9.40 55.60 45.20 8 Lady's
finger
100.0 113.1 53.5 8.6 1.87 13.20 84.00 13.10 9 Onion 284.0 342.0 52.0 26.0 5.50 13.20 50.00 20.40 10 Potato 172.0 291.0 60.0 27.5 2.90 10.60 54.20 69.20 11 Radish 10.5 11.9 46.0 11.0 0.23 1.10 76.10 13.30 12 Sweet potato 42.4 58.9 63.0 25.0 0.67 2.40 60.30 38.90 13 Tomato 61.8 88.7 49.8 10.7 1.24 8.28 78.50 43.50 14 Banana 575.0 875.0 176.0 97.0 3.27 9.00 45.00 52.20 15 Grape 264.0 325.0 53.0 28.0 5.00 11.60 47.20 23.10 16 Papaya 130.0 230.0 228.0 73.0 0.60 3.20 6.90 76.90 17 Pomegranate 34.0 67.0 21.0 16.0 1.62 4.20 23.80 97.00 18 Water melon 82.1 504.0 72.0 25.0 5.90 20.20 65.30 513.9 Source : Past efforts for promoting micro-irrigation in 'Report of Task Force on Micro-
irrigation' 2004. Chapter V, pp. 122 (Singh HP, 2005)
Micro sprinkler: Water pumping, filtration and water conveyance system are similar under micro-irrigation system. Water is applied on specified area with the help of rotating type of mini sprinklers or spray jets. The discharge rate of each micro- sprinkler is about 30-40 litres / hour covering large spread of water around the plant.
It also requires 2 kg/cm2 pressure at sprinkler unit to achieve high uniformity coefficient. Micro-irrigation system was proved to be the best for widely spaced horticultural crops and closely spaced vegetable crops.
Drip / Trickle Irrigation: Drip irrigation refers to the water application at a slow rate i.e. drop by drop through emitters or drippers or perforations in pipes to have desired wetted area. Drip system consists of following components :
¾ Electric motor, pumping unit (suction and delivery pipes)
¾ Control system, sand filter, pressure gauges back flush system, screen filter, fertilizer tank and ventury.
¾ Water Distribution System
¾ PVC main and sub main pipes IDPE, Lateral pipes, on-line dripper, connector and end plugs etc.
The pumping unit is creating sufficient pressure to mitigate the head losses occurred due to filtration, system frictional loss, PVC pipe connection and bends etc. Thus the uniformity coefficient should be more than 90% in drip irrigation system.
The schematic plan of drip irrigation showing water resource pumping unit, control unit filtration system with back wash and valves, main and sub main pipes, laterals and dripper around the trees are depicted in fig. 17.
Fig. 17: Design and set up of Drip Irrigation System
In general lateral LDPE or LLDPE pipes of 10, 16 and 32 mm are manufactured by the drip manufacturing company. The design of network of main and sub main PVC pipes, LDPE lateral pipe spacing and distance with drippers depends upon to topography of plot, type of soil and crop geometry. The size of electric motors, pressure created at control head and head loss gradient in the system are major parameters. The on-line emitters have been designed for pressure compensating characteristics with the discharge rates of 2,4 and 8 liters / hour.(fig. 18)
D r i p i r r i g a t i o n m e t h o d s
O n l i n e d r i p i r r i g a t i o n
O n l i n e e m i t t e r s E a s y f o r i n s p e c t i o n
2 l i t / h r
4 l i t / h r
8 l i t / h r
Fig. 18: Conventional online drippers
Drip irrigation system has certain specific advantages :
¾ Water saving to the extent of 60 % as compared to traditional surface irrigation method.
¾ High water application efficiency
¾ Increase in the yield to the extent of 25 - 30 % over traditional irrigation methods.
¾ Use of water soluble fertilizer, very high FUE with fertilizer saving to the extent of 25 to 30%
¾ Use of saline water is possible
¾ Reducing inter-culturing and weeding cost.
¾ Excellent soil health and maximum soil microbiological activities
¾ Saving in labour cost, due to atomization
¾ Excellent and cost effective for horticultural crops - widely spaced, low cost and long duration crop.
¾ Adoptable for undulating topography, variable soil types and all crops.
However drip irrigation system has also limitations as below :
¾ High initial cost due to solid system
¾ Skilled man-power is required for design, operation and maintenance.
¾ Availability of electricity as system is to operate daily or alternate day deposits
¾ The problem of clogging of emitters due to physical impurities, chemical and biological residues of bacteria and algae.
¾ Periodical maintenance such as cleaning of filters, flushing of piping network and pressure regulation in the system.
¾ Treatment of back water flush system, (Sand filter cleaning), acidification (Chemical clogging) and chlorination removal of algae and bacteria are tedious and time consuming.
¾ Breaking of lateral pipes due to cultural operation, rats and other animal troubles.
In spite of the above limitations and some problems the micro-irrigation system has proved to be the best system amongst all irrigation methods. The subsurface irrigation through porous pipes and inbuilt drip bi-wall LLDPE system were also used by many farmers. The pitcher irrigation also exists where water scarcity and abundant labour are available in many parts of the country. However pitcher irrigation limits its utility due to the filling of earthen pots every day or alternate day.
Drainage
Whatever may be the irrigation system method, it is bound to have deep percolation losses beyond root zone. Continuous irrigations may accumulate the water and may build up water table. This water table may rise slowly. Existence of water table just below the effective root zone may contribute to the water but the quality of water is doubtful. The salt accumulation continues for long time as a result soils become water logged in the first instant and turn to saline conditions. The water and nutrients availability are reduced considerably. Such poor aeration status with high water content is known as ill-drained condition of soil. Most of the times, excess irrigations are given for crop such as sugarcane resulting into poor soil health and ill-drain conditions. Drainage is artificial process of removing excess water, lowering the water table beyond 1.2 m depth and reducing toxic salt concentrations within effective
root zone. The process of desalinization is also called as reclamation of saline soils.
It is physico-chemical process but chemical reactions will not be accelerated without physical process of drainage.
