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CHAPTER TWO: BASICS IN IRRIGATION ENGINEERING. 2.1. IRRIGATION ENGINEERING: This involves Conception, Planning, Design, Construction, Operation and Management of an irrigation system.

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chapter two basics in irrigation engineering
  • 2.1. IRRIGATION ENGINEERING: This involves
  • Conception,
  • Planning,
  • Design,
  • Construction,
  • Operation and
  • Management of an irrigation system.
  • An irrigation engineer is one who has a long theoretical and practical training in planning, design, construction, operation and management of irrigation systems.
considerations in planning irrigation systems
Considerations in Planning Irrigation Systems
  • i) Location: The main point to consider in locating an irrigation project is the need to investigate available resources in the area e.g.
  • Climate,
  • Adequate water in quality and quantity,
  • Land with good agricultural potential and
  • Good topography,
  • Availability of labour (sophisticated or not),
  • Land tenure,
  • Marketing,
  • Transport facilities etc.
considerations in planning irrigation systems contd
Considerations in Planning Irrigation Systems Contd.
  • ii) Crops to be grown: Should be determined by available resources as well as marketability of the crops especially in terms of what people like to eat.

iii) Water Supply: Consider

(a)Sources of water

(b) Quantity and quality of water

c) Engineering works necessary to obtain water e.g. if underground, pumping is needed

  • d) Conveyance System: can be by gravity e.g. open channels or canals or by closed conduits e.g. pipes.
  • (e) Water measuring devices e.g. weirs, orifice, flumes, current meters
other considerations
Other Considerations
  • iv) Systems of Applying Water:

e.g. Surface (90% worldwide),


Trickle and Sub-irrigation(5%).

  • v) Water Demand: The water requirement for the given crop has to be determined. This is by calculating the evapotranspiration (to be treated later)
  • vi) Project Management: Consider how to manage the irrigation system
2 2 crop water and net irrigation requirements
  • In irrigation, it is essential to know the amount of water needed by crops.
  • This determines the quantity of water to be added by irrigation and rainfall and helps in day to day management of irrigation systems.

Total water demand of crops is made up of:

  • i) Crop water use: includes evaporation and transpiration (evapotranspiration described in section 2.3 below)
  • ii) Leaching requirement:
  • iii) Losses of water due to deep seepage in canals and losses due to the inefficiency of application.
  • a) Evaporation: The process by which water is changed from the liquid or solid state into the gaseous state through the transfer of heat energy.
  • b) Transpiration: The evaporation of water absorbed by the crop which is used directly in the building of plant tissue in a specified time. It does not include soil evaporation.
  • c) Evapotranspiration, ET: It is the sum of the amount of water transpired by plants during the growth process and that amount that is evaporated from soil and vegetation in the domain occupied by the growing crop. ET is normally expressed in mm/day.
factors that affect evapotranspiration

 Weather parameters, Crop Characteristics, Management and Environmental aspects are factors affecting ET

  • (a)Weather Parameters:
  • The principal weather conditions affecting evapotranspiration are:
  • Radiation,
  • Air temperature,
  • Humidity and
  • Wind speed.
crop factors that affect et
Crop Type

Variety of Crop

Development Stage

Crop Height

Crop Roughness

Ground Cover

Crop Rooting Depth

management and environmental factors
Management and Environmental Factors
  • (a)Factors such as soil salinity,
  • Poor land fertility,
  • Limited application of fertilizers,
  • Absence of control of diseases and
  • Pests and poor soil management
  • May limit the crop development and reduce soil evapotranspiration.
  • Other factors that affect ET are ground cover, plant density and soil water content. The effect of soil water content on ET is conditioned primarily by the magnitude of the water deficit and the type of soil. Too much water will result in waterlogging which might damage the root and limit root water uptake by inhibiting respiration.
evapotranspiration concepts
  • (a) Reference Crop Evapotranspiration (ETo): Used by FAO.
  • This is ET rate from a reference plant e.g. grass or alfalfa, not short of water and is denoted as ETo. The ET of other crops can be related to the Et of the reference plant.
  • ETo is a climatic parameter as it is only affected by climatic factors.
  • The FAO Penman-Monteith method is recommended as the sole method for determining ETo. The method has been selected because it closely approximates grass ETo at the location evaluated, is physically based, and explicitly incorporates both physiological and aerodynamic parameters.
crop et under standard conditions etc
  • This refers to crop ET under standard conditions, i.e. ET from disease-free, well-fertilized crops, grown in large fields, under optimum soil water conditions.
  • ETc can be derived from ETo using the equation:

ETc = Kc . ETo where Kc is crop coefficient

  • Crop Evapotranspiration under non- standard conditions as mentioned above is called ETc (adjusted). This refers to growth of crops under non-optimal conditions.
determination of evapotranspiration
  • Evapotranspiration is not easy to measure. Specific devices and accurate measurements of various physical parameters or the soil water balance in lysimeters are required to determine ET. The methods are expensive, demanding and used for research purposes. They remain important for evaluating ET estimates obtained by more indirect methods.
energy budget method
  • This method like the water budget approach involves solving an equation which lists all the sources and sinks of thermal energy and leaves evaporation as the only unknown. It involves a great deal of instrumentation and is still under active development. It is data intensive and is really a specialist approach.
water balance method
Water Balance Method
  • The Water Balance or Budget Method is a measurement of continuity of flow of water.
  • This method consists of drawing up a balance sheet of all the water entering and leaving a particular catchment or drainage basin.
  • The water balance equation can be written as:
  •  ET = I + P – RO – DP + CR + SF + SW
  • Where: I is Irrigation, P is rainfall, RO is surface runoff, DP is deep percolation, CR is capillary rise, SF and SW are change in sub-surface flow and change in soil water content respectively
lysimeters for water balance method
Lysimeters For Water Balance Method
  • Lysimeters are normally adopted in water balance studies.
  • By isolating the crop root zone from its environment and controlling the processes that are difficult to measure, the different terms in the soil balance equation can be determined with greater accuracy.
  • Using Lysimeters, crop grows in isolated tanks filled with either disturbed or undisturbed soil.
  • In weighing lysimeters, water loss is directly measured by change in mass while
  • In non-weighing ones, the ET for a given time is determined by deducting the drainage water collected at the bottom of the lysimeters, from the total water input.
et computed from meteorological data
ET Computed from Meteorological Data:
  • ET is commonly computed from weather data. A large number of empirical equations have been developed for assessing crop or reference crop evapotranspiration from weather data. Some of these methods include the Blaney-Criddle, Penman, Thornthwaite, Radiation, Hargreaves, Turc and many others. Most of these methods have been found to only work in specific locations.
  • Following an Expert Consultation by Food and Agriculture Organization in May 1990, the FAO Penman-Monteith method is now recommended as the standard method for the definition and computation of the reference evapotranspiration. The FAO Penman-Monteith equation is described in the Notes.
et estimated from evaporation pans
ET Estimated from Evaporation Pans:
  • Evaporation from an open water surface provides an index of integrated effect of radiation, air temperature, air humidity and wind on evapotranspiration. However, differences in the water and cropped surface produce significant differences in the water loss from an open surface and the crop. The pan is used to estimate reference ETo by observing the evaporation loss from a water surface (Epan) and applying empirical coefficients (Kpan)to relate pan evaporation to Eto thus:
  • ETo = Kp x Epan
standard pan united states class a pan
Standard Pan: United States Class A Pan
  • The most common Evaporation Pan used is the United States Class A pan. This is made up of unpainted galvanized iron, 1.2 m in diameter and 25.4 cm deep. The bottom supported on a wooded frame, is raised 15.24 cm above the ground surface. The water surface is maintained between 5.0 and 7.6 cm below the rim of the pan and is measured daily with a gauge. The daily evaporation is computed as the difference between observed levels corrected for any precipitation measured in an adjacent or nearby standard rain gauge. A pan coefficient of 0.7 (0.6 - 0.8) is normally used to convert the observed value to an estimated value for lake or reservoirs. This is because the rate of evaporation in small areas is greater than that from large areas.
us class a evaporation pan
US Class A Evaporation Pan

Heat Transfer Mechanisms Involved In Heating Of Water In The Standard Pans (diameter D) And Their Walls (After Jagroop,2000).

Incoming Radiation q’ Absorbed By Water


Air Flow

Incoming Radiation Heats Pan Wall q’’ rad

q’ conv absorbed by the water

Conduction Through Walls of pan

Convection q”conv heats uppan walls

2 4 leaching requirement
  • Most irrigation water contain dissolved salts.
  • Evaporation removes pure water leaving a concentration of salt in soil.
  • Salt concentration may reach a level that is detrimental to the growth of the crop and should be controlled. The only practical way of achieving this is by leaching.
  • Leaching requirement is an extra water needed to pass through the root zone in addition to the normal requirement to ensure that salts are placed below the root zone.
leaching requirement contd

Ec acceptable = 4 mmhos/cm. For water quality, Ec of 0.8

Mmhos/cm is medium, quality while Ec of 4 mmhos/cm is saline.

2 5 effective precipitation
  • This is the component of rainfall that is available to crops ie. does not runoff.
  • It can be estimated as 65% of total rainfall.
  • It can also be estimated as the rainfall value, which has 80% probability of being exceeded (D80).
2 6 net irrigation requirement nir
  • This is the moisture that must be supplied by irrigation to satisfy evapotranspiration plus that needed for leaching and not supplied by off-season storage, and the effects of precipitation and groundwater storage.
  • Nir = ET + Wl - Ws - Re
  • Where: Nir is the net irrigation;
  • ET is evapotranspiration,
  • Wl is leaching requirement;
  • Ws is off-season soil moisture carry-over.
  • All parameters are in mm of water.
2 7 gross irrigation requirement gir
  • Gross Irrigation Requirement is equal to:
  • Net Irrigation Requirement Divided by Irrigation Efficiency
  • Irrigation efficiency accounts for losses in storage and distribution systems, losses in application systems as well as operation and management losses.
  • Irrigation Efficiency depends on the Method of Applying Irrigation Water
2 8 irrigation terms
  • 2.8.1. Depth of Irrigation: This is the depth of the readily available moisture. This is the net depth of water normally needed to be applied to the crops during each irrigation
example 1
Example 1
  • The Moisture Content at Field Capacity of a Clay Loam Soil is 28% by Weight While that at Permanent Wilting Point is 14% by Weight. Root Zone Depth Is 1 m and the Bulk Density Is 1.2 g/cm3 . Calculate the Net and Gross Depth of Irrigation Required If the Irrigation Efficiency Is 0.7.
  • Solution: Field Capacity = 28%; Permanent wilting point = 14%
  • i.e. Available moisture = 28 - 14 = 14% by weight i.e. Pm
  • Bulk density (Db) = 1.2 g/cm3
  • Root Zone depth (D) = 1 m = 1000 mm
  • Equivalent depth of available water (d) = Pm . Db . D
  • = 0.14 x 1.20 x 1000 mm = 168 mm
  • This is the net depth of irrigation.
solution to example 1 contd
Solution to Example 1 contd.
  • Gross Water Application is equal to:
  • Net Irrigation/Efficiency = 84/0.7 = 120 mm

Note: This is the actual water needed to be pumped for irrigation.

It is equivalent to:

120 /1000 mm x 10,000 m2 = 1200 m 3 per hectare.

2 8 2 irrigation interval ii
2.8.2 Irrigation Interval (II):
  • This is the time between successive irrigations.
  • Irrigation interval is equal to:
  • Readily Available Moisture or Net Irrigation divided by Evapotranspiration, ET
  • The shortest irrigation interval is normally use in design. The irrigation interval varies with ET.
  • It is equivalent to Readily Available Water divided by the Peak ET
example 2
Example 2
  • For the Last Example. the Peak ET is 7.5 mm/day, Determine the Shortest Irrigation Interval.
  • Solution: From Example 1, Readily Available Moisture (RAM) = 84 mm
  • i.e. Shortest irrigation interval = RAM/ Peak ET = 84/7.5 = 11 days.
irrigation period ip
Irrigation Period (IP)
  • This is the number of days allowed to complete one irrigation cycle in a given area.
irrigation period contd
Irrigation Period Contd.











Assuming water is applied in a border in a day,

the total period of irrigation is then 11 days.

irrigation interval and period
Irrigation Interval and Period
  • In irrigation scheduling, the irrigation period should be less that the irrigation interval. This is because if the period is not smaller, before the latter parts of the area are to be irrigated, the earlier irrigated areas will need fresh irrigation.
  • At peak evapotranspiration (used in design), irrigation interval should be equal to irrigation period. i.e. Generally IP < II
2 8 4 desired irrigation design capacity qc
2.8.4 Desired Irrigation Design Capacity (Qc)
  • This is the flow rate determined by the water requirement, irrigation time, irrigation period and the irrigation application efficiency.
  • It is the flow rate of flow of the water supply source e.g. pumps from a reservoir, or a borehole required to irrigate a given area.
desired irrigation design capacity qc contd
Desired Irrigation Design Capacity (Qc) Contd.
  • Where:
  • Qc is the Desired Design Capacity;
  • d is the Net Irrigation Depth = Readily Available Moisture;
  • F is the number of Days to complete the Irrigation (Irrigation Period);
  • H is the number of Hours the System is perated (hrs/day) and
  • Ea is the Irrigation Efficiency
example 3
Example 3
  • A 12-hectare field is to be irrigated with a sprinkler system. The root zone depth is 0.9 m and the field capacity of the soil is 28% while the permanent wilting point is 17% by weight. The soil bulk density is 1.36 g/cm and the water application efficiency is 70%. The soil is to be irrigated when 50% of the available water has depleted. The peak evapotranspiration is 5.0 mm/day and the system is to be run for 10 hours in a day.
  • Determine: (i) The net irrigation depth
  • (ii) Gross irrigation ie. the depth of water to be pumped
  • (iii) Irrigation period
  • (iv) Area to be irrigated per day and (v)
  • the system capacity.
solution to example 3
Solution to Example 3
  • Solution: Field Capacity = 28%; Permanent Wilting Point = 17%
  • ie. Available Moisture = 28 - 17 = 11% , which is Pm
  • Root zone depth = 0.9 m;
  • Bulk density = 1.36 g/cm3
  • Depth of Available Moisture, = Pm . Db. D
  • = 0.11 x 1.36 x 900 = 135 mm
  • Allowing for 50 % depletion of Available Moisture before Irrigation, Depth of Readily Available Moisture = 0.5 x 135 mm = 67.5 mm
solution of example 3 contd
Solution of Example 3 Contd.
  • i) Net irrigation depth = Depth of the Readily Available Moisture = 67.5 mm
  • ii) Gross Irrigation = Net irrigation

Application efficiency

  • = 67.5/0.7 = 96.4 mm
  • iii) Irrigation interval = Net irrigation or RAM

Peak ET

  • = 67.5/5 = 13.5 days
  • = 13.5 days = 13 days (more critical)
  • In design, irrigation interval = irrigation period
  • ie. irrigation period is 13 days
solution of example 3 contd43
Solution of Example 3 Contd.
  • iv) Total area to be irrigated = 12 hectares
  • Area to be irrigated per day = Total area / irrigation period = 12 ha/ 13 days
  • = 1 ha/day
  • v) System Capacity, Qc = A. d m3 /s
  • F. H. Ea
  • Area, A = 12 ha = 12 x 10000 m2 = 120,000 m2
  • Net irrigation depth, d = 67.5 mm = 0.0675 m
  • Irrigation period , F = 13 days
  • Number of hours of operation, H = 10 hrs/day
  • Irrigation efficiency, Ea = 0.78
solution of example 3 concluded
Solution of Example 3 Concluded
  • System capacity, Qc = 120,000 m2 x 0.0675 m 13 days x 10 hrs/day x 0.7
  •   = 89.01 m 3/hr
  • Recall: 1 m 3 = 1000 L and 1 hr = 3600 s
  • ie. 89.01 m3 /hr = {89.01 x 10 3 L}/3600 secs
  • = 24.73 = 25 L/s
  • The pump to be purchased for sprinkler irrigation must have capacity equal to or greater than 25 L/s.
  • Alternatively, more than one pump can be purchased.
2 9 irrigation efficiencies
  • These irrigation efficiencies are brought about by the desire not to waste irrigation water, no matter how cheap or abundant it is.
  • The objective of irrigation efficiency concept is to determine whether improvements can be made in both the irrigation system and the management of the operation programmes, which will lead to an efficient irrigation water use.
2 9 1 application efficiency
2.9.1 Application Efficiency

Ea is inadequate in describing the overall quantity of water

since it does not indicate the actual uniformity of irrigation,

the amount of deep percolation or the magnitude of

under-irrigation. See diagrams in text.

example 4
Example 4
  • Delivery of 10 m3/s to a 32 ha farm is continued for 4 hours. The tail water is 0.27 m3/s. Soil probing after irrigation indicates that 30 cm of water has been stored in the root zone. Compute the Application Efficiency.
  • Solution: Total volume of water applied
  • = 10 m3/s x 4 hrs x 3600s/hr = 144,000 m3
  • Total tail water = 0.27 x 4 x 3600 = 3888 m3
  • Total water in root zone = 30 cm = 0.3 m x 32 ha x 10,000 m2/ha = 96,000 m3
solution to example 4 contd
Solution to Example 4 Contd.
  • = 96,000/144,000 = 66.7%.
2 9 2 water conveyance efficiency
2.9.2 Water Conveyance Efficiency



Water lost by evap

And seepage



example 5
Example 5
  • 45 m3 of water was pumped into a farm distribution system. 38 m3 of water is delivered to a turn out (at head ditch) which is 2 km from the well. Compute the Conveyance Efficiency.


= 38/45 = 84%

2 9 3 christiansen uniformity coefficient c u
2.9.3. Christiansen Uniformity Coefficient (Cu)

This measures the uniformity of irrigation

W here: is the summation of deviations from the mean depth infiltered

m is the mean depth unfiltered and

n is the number of observations.

example 6
Example 6
  • A Uniformity Check is taken by probing many stations down the border. The depths of penetration (cm) recorded were: 6.4, 6.5, 6.5, 6.3, 6.2, 6.0, 6.4, 6.0, 5.8, 5.7, 5.5, 4.5, 4.9. Compute the Uniformity Coefficient.
  • Solution: Total depth of water infiltered = 76.7 cm
  • Mean depth = 76.7/13 = 5.9 cm
example 6 concluded
Example 6 Concluded
  • This is a good Efficiency. 80% Efficiency is acceptable.

= 6.2

m = 5.9 cm; n = 13

= 92%

2 9 4 water storage efficiency e s
2.9.4 Water Storage Efficiency (Es)
  • 2.9.5 Irrigation Efficiency

ET is Evapotranspiration;

Wl is Leaching Requirement;

Re is Effective Precipitation;

is change in storage;

Wi is water diverted, stored or pumped for irrigation.

2 10 irrigation scheduling
  • This means Predicting when to Irrigate and how much to Irrigate
  • For efficient water use on the farm, the farmer needs to be able to predict when his crops need irrigation. This can be done by:
  • Observing the plants;
  • Keeping a Water Balance Sheet
  • By Measuring the Soil Moisture Content or
  • Computer Software 
2 10 1 observing the plants
2.10.1 Observing the Plants:
  • This is a direct way of knowing when the crops need water.
  • The farmer observes the plants for any signs of wilting or change in leaf colour or growth rate.
  • The method is simple but its major disadvantage is that the signs of shortage appear after the optimum allowable depletion has already been exceeded.
2 10 2 keeping a water balance sheet
2.10.2. Keeping a Water Balance Sheet
  • This approach works on the principle that the change in water content of the soil is represented by the difference between water added by irrigation(or rainfall) and the amount lost by evapotranspiration.
  • The records are kept for each farm and crops as shown in Table 2.4 below.
  • The method requires no equipment and is easy to operate.
  • It can be operated on a daily or weekly or 10 day basis.
table 2 3 example of a water balance sheet
Table 2.3: Example of a Water Balance Sheet

Irrigation Plan: Apply 30 mm of water at 30 mm deficit.

2 10 3 measuring soil moisture
2.10.3 Measuring Soil Moisture
  • This is the best scheduling and the most widely used. Soil moisture can be indirectly measured using devices and instruments eg. tensiometers, resistance blocks or neutron probes.
  • Direct measurement of soil moisture can be by weighing or the gravimetric method.
  • These methods are either too expensive or complicated.
  • The simplest and most practical method is to estimate the moisture content by the 'feel and appearance' of the soil.
  • Soil is collected at the root zone and checked to guess the right time to irrigate.
2 11 irrigation water sources quality measurement
  • 2.11.1 Sources of Irrigation Water Supply
  • i) Rainfall or Precipitation: This is a practical and dominant factor.
  • The supply varies with time and place e.g. while Grenada receives 2,100 mm annual rainfall, Antigua receives only 1,100 mm. Trinidad receives 1, 950 mm (Data supplied by Gumbs, 1987).
  • To be of greatest benefit for crop production, the rainfall amount should be enough to replace water in the root zone on a regular basis.
sources of irrigation water contd
Sources of Irrigation Water Contd.
  • ii) Underground water sources: This can be shallow or bore holes.
  • iii) Surface Sources: Streams, rivers, lakes, farm ponds etc.
  • Streams should be gauged to ensure that there is enough water for irrigation.
  • Rivers or streams can also be dammed to raise the height of flow and make more water available for irrigation.
  • Farm ponds can also be dug to store water from rivers or channels (e.g. field station) or to collect water from rainfall 
sources of irrigation water contd66
Sources of Irrigation Water Contd.
  • iv) Springs and waste water e.g. industrial water and sewage: Determine quality before use.
  • (For details of harnessing water for irrigation in the Caribbean, see Gumb's Soil & Water Conservation Methods, Chapter 7).
2 11 2 irrigation water quality
2.11.2 Irrigation Water Quality:
  • Irrigation water quality depends on
  • i) Amount of suspended sediment eg. silt content
  • ii) The chemical constituents of water
i amount of suspended sediment
i) Amount of Suspended Sediment:
  • The effect of sediment may depend upon the nature of the sediment and the characteristics and soil conditions of the irrigated area.
  • Silt content in irrigation may be beneficial if it improves the texture and fertility of say sandy soil.
  • It can also be detrimental if it is derived from a sterile sub-soil, and applied to a fertile soil.
  • Silt accumulation can cause aggradation in canals or distribution systems. In sprinkler systems, silt can cause abrasion.
ii the chemical constituents of water
ii) The Chemical Constituents of Water:
  • There are three main elements or compounds that can cause hazards in irrigation water. They include:
  • Sodium,
  • Boron and
  • Salts.
a salinity hazards
a) Salinity Hazards:
  • The units of salt concentration in irrigation water can be parts per million (p.p.m), milli equivalents/litre(ME/litre) or electrical conductivity.
  • On the basis of salinity, irrigation water can be classified as C1 to C4(see chart).
  • They refer to low, medium, high and very high salinity levels respectively.
  • While C1 water can easily be used for irrigation without need for leaching requirement,
  • C4 water is not useable, except in permeable soils where adequate leaching and drainage is possible and for highly tolerant crops.
b sodium hazard
b) Sodium Hazard:
  • It is Measured in Sodium Absorption Ratio (S.A.R).
  • SAR is defined as the proportion of sodium relative to other cations.

Parameters are measured in ME/litre.

sodium hazard contd
Sodium Hazard Contd.
  • Irrigation Water is also divided into S1 to S4 in terms of Sodium (SAR) Content.
  • S1 Water can be used readily
  • S2 and S3 can be used with adequate Leaching and Drainage and addition of Organic and Chemical amendment.
  • S4 Water has very high Sodium Content and is unsuitable for irrigation except where calcium, gypsum or other chemical amendments are possible.
  • (See water quality chart).
  • See Table 2.4 in Note Book for Permissible limit of Boron for several classes of irrigation water
2 11 3 measurement of irrigation water
2.11.3 Measurement of Irrigation Water
  • Water is the most valuable asset of irrigated agriculture and can be detrimental if used improperly.
  • An accurate measurement permits an intelligent use.
  • The methods to use for measurement should depend on the flow, environmental conditions and the degree of accuracy required.
methods of measuring irrigation water
Methods of Measuring Irrigation Water
  • a) Direct method: Collect water in a contained of known volume e.g. bucket. Measure the time required for water from an irrigation source e.g. siphon to fill the bucket.
  • Flow rate = Volume/time m3/hr or L/s etc.
  • b) Weirs: Weirs are regular notches over which water flows.
  • They are used to regulate floods through rivers, overflow dams and open channels.
  • Weirs can be sharp or broad crested; made from concrete timber, or metal and can be of cross-section rectangular, trapezoidal or triangular.
  • Sharp crested rectangular or triangular sections are commonly used on the farm.
weirs contd
Weirs Contd.
  • The discharge through a weir is usually expressed as:
  • Q = C L Hm
  • where Q is the discharge;
  • C is the coefficient dependent on the nature of weir crest and approach conditions;
  • L is the length of crest;
  • H is the head on the crest and
  • m is an exponent depending on weir opening.
  • Weirs should be calibrated to determine these parameters before use eg. for trapezoidal weirs(Cipoletti weir),
  • Q = 0.0186 L H1.5
  • Q is discharge in L/s;
  • L, H are in cm.
methods of measuring irrigation water contd
Methods of Measuring Irrigation Water Contd.
  • c) Orifices: An orifice is an opening in the wall of a tank containing water.
  • The orifice can be circular, rectangular, triangular or any other shape.
  • The discharge through an orifice is given by: Q = C A 2 g h
  • Where Q is the discharge rate;
  • C is the coefficient of discharge (0.6 - 0.8);
  • A is the area of the orifice;
  • g is the acceleration due to gravity and
  • h is the head of water over an orifice.
methods of measuring irrigation water contd79
Methods of Measuring Irrigation Water Contd.
  • d) Flumes: Hydraulic flumes are artificial open channels or sections of natural channels.
  • Two major types of hydraulic flumes are Parshall or Trapezoidal ones.
  • Flumes need to be calibrated after construction before use.
  • See Chapter 6 for further information.
  • e) For streams, use gauging. A current meter is used to measure velocity at 0.2 and 0.8 Depth or at only 0.6 depth.
  • Measure areas of all sections using trapezoidal areas.
  • Q = ai vi
methods of measuring irrigation water contd80
Methods of Measuring Irrigation Water Contd.
  • Using Floats: A floating object is put in water and observe the time it takes the float e.g. a cork to go from one marked area to another.
  • Assuming the float travels D meters in t secs
  • Velocity of water at surface = ( D/t ) m/s
  • Average velocity of flow = 0.8 (D/t)
  • Flow rate, Q = Cross sectional area of flow x velocity.