Irrigation Systems

CHRISMAR PUNZAL
NOVEMBER 12, 2022
Irrigation
❑ Irrigation is the application of water to soil to supplement
deficient rainfall to provide moisture for plant growth.
❑ Water resources engineering, irrigation and civilization (9000-
10000 years ago)
❑ Egyptian, Mesopotamian and Hohokam Indians irrigation
systems
❑ Early 1900s, irrigation expanded worldwide (from 8 to 40
million hectares)
❑ Drastic slowed rate of expansion from 1980s and beyond
❑ Worldwide (260 million hectares), some areas are
withdrawn, while others are added
❑ Salinity problems have reduced the area
irrigated worldwide
Irrigation
❑ Today, one third of the global harvest
comes from 16% of the world’s
croplands that is irrigated.
❑ Many countries rely on irrigated lands
for more than half their domestic food
production.

World irrigated area

per capita
Irrigation Development
❑ The first step in planning an irrigation project is to establish the capability of the land to produce
crops that provide adequate returns on the investment in irrigation works.
❑ USBR classification system terms:
a) Arable Land
b) Irrigable Land
c) Productive Land
d) Full irrigation service land
e) Supplementary irrigation service land
❑ Suitability for irrigation farming: high water holding capacity; readily penetrable by water.
a) Low infiltration rate
b) Soil is deep enough
c) Free of salt ions
d) Adequate nutrient supply
e) Mild to moderate slopes
f) Ideal land location
g) Adaptable to multiple crops
Irrigation Development
Irrigation Development
Soils have been classified for agricultural purposes by the
U.S. Department of agriculture according to their
relative proportion of the basic constituents of soils
(sand, silt, and clay).
Irrigation Development
❑ The storage and movement of soil water are
important factors in irrigation planning.
❑ Water in zone of aeration:
a) In the large soil pores under gravity
b) In small pore space under capillarity
c) With soil particles by molecular attraction
❑ A curve of moisture content versus negative
pressures applied is a smooth curve.
❑ Critical points on the curve include the field
capacity and permanent wilting point.
❑ An efficient irrigation procedure should be based
on these points.
Irrigation Development
The general equation of moisture movement (rate of flow per unit of cross-sectional are:
𝑄 𝜕𝐻
𝑞= = −𝑘
𝐴 𝜕𝑠

❑ H is the algebraic sum of the capillary, gravitational, and vapor-pressure heads.

❑ k depends on the size of the soil pores, but it also decreases as the moisture content of the soil decreases. H and k are difficult to
measure.
❑ When rain or irrigation water is applied to a soil surface, both gravity and capillary potential tend to cause its downward
movement by infiltration.
Irrigation Development
❑ Infiltration rates into irrigated soils are commonly measured with a
ring infiltrometer.
❑ Water is applied to this tube and the rate of disappearance is
measured to provide an indication of infiltration rate.

❑ A single determination of infiltration rate may be misleading because

if local variation in soil characteristics.
❑ The oldest method of measuring soil moisture is to obtain a
sample of soil and determine its loss in weight when oven-dried.
Irrigation Development
❑ In-situ measurement of soil moisture can be made by electrical resistance methods.
❑ When the element is buried in the soil, it maintains a moisture equilibrium with the surrounding soil and the
resistance between the electrodes varies with the moisture content of the material composing the block.

www.google/images/
❑ Another method of determining moisture content is the use of tensiometers. A calibration curve can be
generated by determining the relation between soil moisture tension and moisture content of the soil.
❑ A neutron-scattering device is used to estimate soil moisture by measuring the count of slow moving neutrons
as a result of collision with hydrogen atoms. The higher the count, the higher the moisture content
Irrigation Development
❑ The total water requirement for an
irrigation project, usually referred to as the
diversion requirement qd, consists of the
water needed by the crop plus the losses
associated with the application and
delivery of water.
❑ If no direct determination of total water
requirement is possible, an estimate can
be made by estimating the crop water
requirement.
❑ The overall consumptive use for large areas
may be estimated by calculating the
hydrologic balance for the area.
❑ In the absence of data, maximum
evapotranspiration at full crop yield is used.
Irrigation Development
Crop Water Requirements and Crop Evapotranspiration
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❑ Many tests indicate the
existence of an optimum
consumptive use that
produces a maximum crop
yield.
❑ The cost of water and other
fixed charges on the farm,
such as cost of investment,
labor, fertilizer, taxes and
insurance, enter into the
determination of the most
economic use of water.
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❑ The crop-irrigation requirement is that portion of the
consumptive use that must be supplied by irrigation.
❑ Only storage in the root zone, which usually extends to
a depth of about 1.2m, should be considered.
❑ Precipitation during the growing season is effective
only when it remains in the soil and is available to the
plants.
❑The average annual effective precipitation for the
period of record is subtracted from the estimated
annual consumptive use to determine the annual crop
irrigation requirement.
Irrigation Development
❑ Losses at the farm during irrigation include deep seepage and surface runoff.
❑ Surface runoff should not exceed about 5% of the applied water with proper irrigation methods.
❑ The amount of water qf expressed as a depth per year that must be delivered to the farm is
❑ The ratio of the irrigation water consumed (Uc – Peff) to qf is called the farm-irrigation

efficiency.

❑ Diversion requirement accounts for the water lost in delivery to the farm. This loss consists of evaporation from the canal,
transpiration by vegetation along the canal bank, seepage from the canal and operational waste.
❑ The farm delivery and project diversion requirements expressed in terms of volume by multiplying qf and qd by the
respective net areas to be irrigated.
Irrigation Development
❑ Unsatisfactory water may contain (1) chemicals toxic to plants or to persons using the
plants as food, (2) chemicals that react with the soil to produce unsatisfactory moisture
characteristics, and (3) bacteria injurious to persons or animals eating irrigated with the
water.
❑ It is the concentration of a compound in the soil solution that determines the hazard.

❑ Free drainage of soil allows the downward movement of salts and helps prevent serious
accumulations.
❑ High salt concentrations may sometimes be avoided by mixing the salty water with better-
quality water from another source so that the final concentration is within safe limits.
❑ The theoretical depth of water qa of salinity C required to maintain the soil solution at
concentration Cs is:
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The total water requirement or
diversion requirement for an irrigation
system includes the water needed by
the crop in addition to the losses
associated with the application and
delivery of water.
Irrigation Development
❑ Water needs at various growth stages

❑ Too little water will result in moisture stress in the rice crop, reducing yield per hectare.
❑ Most critical stage is from 60 to 30 days before expected maturity; next is the tillering
stage, and third is the transplanting stage.
Irrigation Development
❑ Submergence damage at various growth stage
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Venues of the major water losses occurring in an irrigation system
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Loss coefficients
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Effective rainfall
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Effective rainfall
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Sample lateral layout
Water requirement
❑ In estimating irrigation diversion
requirement of a certain area, data on:
a) Evapotranspiration
b) Rainfall for the month
c) Conveyance and farm ditch and
seepage losses
d) Dimensions of the lateral and farm
ditch
e) Total area to be irrigated
Evapotranspiration
❑ It is a combination of two separate processes whereby water is lost on the one hand from the
soil surface by evaporation and on the other hand from the crop by transpiration (ET).
❑ Evaporation is the process whereby liquid water is converted to water vapor and removed
from the evaporating surface. Energy is required to change the state of molecules of water from
liquid to vapor.
❑Direct solar radiation
❑Ambient temperature of air
❑Transpiration consists of the vaporization of the liquid water contained in plant tissues and the
vapor removal to the atmosphere. Crops lose their water through the stomata.
❑Energy supply
❑Vapor pressure gradient
❑Wind
❑Radiation, air temperature, air humidity
Factors affecting Evapotranspiration
❑ Weather parameters – radiation, air temperature, humidity and wind speed
The evaporation power of the atmosphere is expressed by the reference crop evapotranspiration 𝐸𝑇𝑜 . The
reference crop evapotranspiration represents the evapotranspiration from a standardized vegetated surface.
❑ Crop factors – crop type, variety and development stage
Crop evapotranspiration under standard conditions 𝐸𝑇𝑐 refers to the evaporating demand form crops that
are grown in large fields under optimum soil water, excellent water management, and environmental
conditions.
❑ Management and environmental conditions – soil salinity, poor land fertility, limited application of
fertilizers and presence of hard soil horizons, and pests and poor soil management.
FAO-Penman-Monteith Equation
FAO-Penman-Monteith Equation
FAO-Penman-Monteith Equation
FAO-Penman-Monteith Equation
FAO-Penman-Monteith Equation
FAO-Penman-Monteith Equation
FAO-Penman-Monteith Equation
FAO-Penman-Monteith Equation
Calculation of Eto with mean monthly
data
Given the monthly average climatic data of Aporl of Bangkok located at 13-44N, and at an
elevation of 2m, calculate the reference crop evapotranspiration.
Monthly Values:
Tmax = 34.8C
Tmin = 25.6C
Vapor Pressure = 2.85 kPa
Windspeed = 2 m/s
Crop factors
Crop factors
Rainfall
Effective Rainfall
Peak demand
Groundwater considerations
Soil water contribution
Rice irrigation requirement
Salinity Issues
Salinity Issues
Homework
Calculate the net irrigation requirement for the following crop:
Area: 31d31’N, 34d30E, 10m elev
Crop: Citrus: Ini=60, Dev =90, Mid=120, Late=95

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Eto 2.1 2.5 3.2 4.1 4.4 4.9 5.1 4.9 4.4 3.8 2.9 4.2
(mm)
Rain 97 59 32 5 0 0 0 0 0 11 46 80
(mm)
Irrigation Development
❑ On a project level, the infrastructure includes
structures such as reservoirs, well fields, river
diversion, canals, high-pressure pipelines, low-head
pipelines, semi-closed pipelines, and various hydraulic
structures such as inlet structures, drop structures,
check structures, diversion boxes and measurement
structures.
❑ Water supply infrastructure for irrigation systems
requires an institutional framework such as irrigation
districts, mutual companies, or commercial
companies.
Irrigation Development
❑ On a project level, the infrastructure includes
structures such as reservoirs, well fields, river
diversion, canals, high-pressure pipelines, low-head
pipelines, semi-closed pipelines, and various hydraulic
structures such as inlet structures, drop structures,
check structures, diversion boxes and measurement
structures.
❑ Water supply infrastructure for irrigation systems
requires an institutional framework such as irrigation
districts, mutual companies, or commercial
companies.
Irrigation Development
The three basic types of irrigation methods are surface
irrigation, sprinkler irrigation and micro-irrigation.

Water supplies for irrigation depend on: (1) managing

precipitation from streams or from reservoir storages, (2)
existing groundwater sources or (3) reclaimed water
from municipalities either directly or as recharge to
groundwater.
Irrigation Development
The three basic types of irrigation methods are surface
irrigation, sprinkler irrigation and micro-irrigation.

Water supplies for irrigation depend on: (1) managing

precipitation from streams or from reservoir storages, (2)
existing groundwater sources or (3) reclaimed water
from municipalities either directly or as recharge to
groundwater.
Irrigation Development
The three basic types of irrigation methods are surface
irrigation, sprinkler irrigation and micro-irrigation.

Water supplies for irrigation depend on: (1) managing

precipitation from streams or from reservoir storages, (2)
existing groundwater sources or (3) reclaimed water
from municipalities either directly or as recharge to
groundwater.
Irrigation Development
Surface irrigation includes the following types:
1. Continuous-flood or paddy irrigation
2. Basin irrigation
3. Furrow irrigation
4. Level basin irrigation
5. Border-strip irrigation
6. Surge-flow irrigation
7. Reuse irrigation
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Furrow Irrigation
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❑ Sprinkler irrigation includes the following types.

a) Permanent, solid-set sprinklers

b) Hand-move sprinklers
c) Continuous move sprinkler systems
d) Center-pivot irrigation systems
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Moving sprinkler irrigation system (center pivot)
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❑ Micro-irrigation includes the following types.
a) Drip irrigation
b) Trickle irrigation
c) Subsurface irrigation
d) Bubbler irrigation
e) Moving LEPA
Farm Infrastructures
Control structures are required in open canals (ditch delivery system) to regulate velocity, head,
and the quantity of water released into distribution laterals, basins, borders, and furrows.

Division boxes: (1) fixed proportional flow divider (2) Weir-type overflow outlets
Farm Infrastructures
Commonly used drop structures in open-channel
delivery systems
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Impacts of Development
Impacts on water quality
Irrigation Planning
❑ Purpose: Augment rainfall with water needed for growing of crops at all stages of farming such crops

a) Soil preparation

b) Planting
c) Early growth

d) Tillering

e) Flowering and harvesting

❑ Main Components of Irrigation System Development

a) Supply Development
b) Delivery System

c) Service Area Development

d) Software

e) Economic Factors
 Irrigation Planning
❑ Land Suitability
a) Productive Capacity-climatic conditions, soil characteristics, topography and relief, water supply and drainage
b) Cost of Production
-labor, equipment, soil amendments, water
-land development: clearing, grading, construction of ditches and drains, land conditioning, farm pumps, sprinklers

b) Physical Factors
◦ Water Quality
a) Sodium, Calcium, Magnesium
b) Metals
c) Chlorides and nitrates
◦ Soil
a) High water-holding capacity
b) Infiltration rates
c) Depth of root development
◦ Topography
a) Slope
b) Relief
c) Position
◦ Drainage Requirements
Irrigation Planning
c) Project Development Considerations
1. Water rights
2. Access roads and other auxiliaries
3. Financial aspects
Irrigation Development
❑Supplemental irrigation must be viewed as a long-term
investment in insurance against serious drought.
❑It is important that irrigated lands be properly drained
to prevent the land from becoming waterlogged.
❑A typical irrigation system include structures and
devices such as dams, spillways, diversion works, canals,
ditches, wells, pumps, and pipelines.
❑Farm ditches must be at a sufficient elevation to
permit gravity flow to the field.
❑Stilling basins are used for proportional flow division
from underground pipe.
❑Concrete pipe is widely used for permanent
underground installation with riser pipes at intervals to
bring the water to the surface.
Irrigation Development
❑It is important to get a good estimate of Manning’s n.
❑Because of the irregularities in the alignment and cross-sections, uniform flow assumption is
not justifiable.
❑Engineering works on canals serve a number of purposes: increasing the flow capacity,
stabilizing the alignment, deepening the channel, and preventing bank erosion and sloughing.
❑Bank stabilization is achieved in a variety of ways including the use of riprap, sand bags,
mattresses and gabions.
❑The exact route of a canal is determined by the slopes that can be tolerated.
❑Fine-grained soils generally scour at a lower velocity than coarse grained soils.
❑Compare boundary shear stress with the permissible unit tractive force.
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❑ Design velocities should be slightly less than the maximum permissible if topography
permits.
❑ Earth canals are generally trapezoidal, with side slopes determined by the stability of the
bank material.
❑ Freeboard must be provided above the design water level as a precaution.
❑ Cut and fill are balanced for economy.
❑ Lining are provided to reduce seepage.
❑ Seepage rates are measured.
❑ Lining types include clay, asphalt, plastic membranes, cement mortar, gunite, and reinforced
concrete.
❑ Seepage loss from properly lined canals may be as low as 0.015 m/d.
Irrigation Development
Numerous structures are necessary for the proper
operation of canals, which may include, diversion
structures, intakes, settling basins, chutes or drops,
flumes or siphons, culverts or tunnels and wasteways,
among others .
Irrigation Development
Irrigation Canal Design
❑ The design of a channel involves the selection of channel alignment, shape, size, and bottom
slope and deciding whether the channel should be lined to prevent the erosion of the
channel sides and bottom and reduce seepage.
❑ Procedures are not directly available for selecting optimum channel parameters directly.
❑ Artificial channels must behave in a stable, predictable manner to ensure that a known flow
capacity will be available for a design discharge.

❑ Channel Linings: (1) Rigid linings; (2) Flexible linings

Irrigation Canal Design
❑ In the design of a rigid-boundary channel, the channel cross section and size are selected such
that the required discharge is carried through the channel for the available head with a
suitable amount of freeboard.
❑ The channel alignment, bottom slope, channel side slopes, shape and dimensions are selected
based on requirements.
❑ The flow velocity is selected such that the lining is not eroded and any sediment carried in
the flow is not deposited.
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The values listed in this table are for a straight
channel having a flow depth of about 1 m. As a
rough estimate, Lane [1955] suggested
reducing these values by 5 per cent for slightly
sinuous channels, 13 per cent for moderately
sinuous channels, and 22 per cent for very
sinuous channels. For other flow depths, these
velocities may be multiplied by a correction
factor, k, to determine the permissible flow
velocity [Mehrotra, 1983].
For very wide channels, k = y1/6
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❑ Angles of repose for non-cohesive material
(After U.S. Bureau of Reclamation).
Irrigation Canal Design
Permissible shear stress for non-cohesive
materials (After U.S. Bureau of Reclamation).

These values are for straight channels. Lane (1955)

recommended reducing these values by 10 per cent for
slightly sinuous channels, 25 per cent for moderately
sinuous channels, and 40 per cent for very sinuous
channels.
Irrigation Canal Design
❑ Permissible shear stress for cohesive
materials (After U.S. Bureau of Reclamation).

❑ These values are for straight channels. Lane

(1955) recommended reducing these values
by 10 per cent for slightly sinuous channels,
25 per cent for moderately sinuous channels,
and 40 per cent for very sinuous channels.
Irrigation Canal Design
❑ The procedure for designing a channel by the tractive force approach involves the selection of a cross section such
that the unit tractive force acting on the channel sides is equal to the permissible shear stress for the channel material.
Check the unit tractive force on the channel bottom is less than the permissible stress also.

1. For the channel material, select a side slope, the angle of repose and the critical shear stress for non-cohesive and
cohesive materials. Determine the permissible shear stress by taking into consideration whether the channel is
straight or not.
2. For the non-cohesive material, compute the reduction factor, K, and then determine the permissible shear stress
for the sides by multiplying by K the permissible stress determined in step 1.
3. Equate the permissible stress for the sides determined in step 2 to 0.76γySo and determine y from the resulting
equation.
4. For y determined in step 3 and for the selected values of the Manning n and the side slope, s, compute the bottom
width, Bo, from Manning equation for the design discharge.
5. Check that the shear stress on the bottom, γySo, is less than the permissible shear stress of step 1.
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Permissible shear stress for lining
materials
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Classification of vegetal covers by degree of
retardance
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