Module 5 - UNIT 2 PDF
Module 5 - UNIT 2 PDF
Module 5 - UNIT 2 PDF
ENGAGE
Farmers are far more experts than engineers in terms of knowing how much and how
often farms need watering. State what you think about how a farmer, compared to an
engineer, would estimate crop water requirement.
EXPLORE
Read: Module on crop water requirement
EXPLAIN
The water requirement of crops refers to the water they need in order to thrive. In the
previous unit, it was stated that most plants only utilize up to 5% of water that they
absorb and the 95% is transpired. Therefore, the rate of evapotranspiration becomes a
very conservative estimate of a crop’s water needs.
For each crop, we need to be able to measure its evapotranspiration rate in any of the
methods that were discussed in hydrology such as field measurements (lysimeters,
evaporation pans) and empirical formulas (Blanney-Criddle, Penman-Monteith
methods).
Since the same crop grown in different climatic variations have different water needs, it
has been accepted to evaluate the evapotranspiration rate for a standard or
reference crop and find out that of all other crops in terms of this reference. Grass has
been chosen as standard reference for this purpose. The evapotranspiration rate of this
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standard grass is, therefore, called the reference crop evapotranspiration and is
denoted as ETO, which is a function of climatic variables.
ETO represents the maximum evapotranspiration rate that can occur however for each
crop, it may vary due to growth stage, leaf coverage, duration of crop period, etc.
which is why this is converted to ETC by multiplying ETO with crop coefficient KC.
Example. If we want to compute for the water requirement of potato during its initial
stage of growth given KC=0.45 and an ETO = 9mm/day.
If the actual field area was, then we can determine the volume of water required for a
command area over the growth stage.
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There is also a need to regulate how often to water crops so that it does not need to be
done every day. This is what we refer to as watering interval and is quite simple to do.
100 𝑥 𝑑
The gross depth of irrigation is computed by: 𝑑
𝑒
Field Application Efficiency, 𝐞𝐚
The gross irrigation depth (d gross), in mm, takes into account the water loss
during the irrigation application
3. Calculate the irrigation water need (crop water requirement) in mm over the
total growing season.
4. Calculate the number of irrigation applications over the total growing season by
dividing the crop water requirement (for the total growth season) by the net
irrigation depth:
𝑐𝑟𝑜𝑝 𝑤𝑎𝑡𝑒𝑟 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑚𝑒𝑛𝑡
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑖𝑟𝑟𝑖𝑔𝑎𝑡𝑖𝑜𝑛 𝑎𝑝𝑝𝑙𝑖𝑐𝑎𝑡𝑖𝑜𝑛𝑠
𝑑
5. Calculate the irrigation interval in days by dividing the total number of growth
season in days by the answer in step 4.
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Example. The table below gives the details on the growth period of tomatoes, grown on
loamy soil, and utilizes surface irrigation.
Estimate the monthly crop water requirement and use the Simple Calculation Method
to determine the water interval required for tomatoes.
3. To estimate crop water requirement, we first tabulate the given data to visualize
𝐸𝑇 against growing period against crop coefficient. Take note that there is a
monthly reading of 𝐸𝑇 as these are regularly monitored hydrologic data similar
to rainfall readings, wind speeds, etc. in this manner we are able to see that
water requirement vary with growth stage and that certain days overlap the
next month where 𝐸𝑇 has changed. In order to use the formula for 𝐸𝑇 , the crop
coefficient 𝐾 should correspond to the monthly 𝐸𝑇 .
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15 15 season whereas two growth seasons fall
APR 𝐾 0.75 1.15 0.95
30 30 on the other months.
MAY 𝐾 1.15* **Rounding off of the adjusted 𝐾 are
upon your discretion but for uniformity,
we will retain unitl 2 decimal places.
5 25 **Notice also that in the calculations, it
JUNE 𝐾 1.15 0.80 0.86
30 30 is assumed that each month of the year
is of 30 days.
In this new tabulation, we can apply the formula 𝐸𝑇𝐶 𝐾𝐶 𝐸𝑇𝑂 to estimate
the daily water requirement and then multiply this by 30 days to compute the
monthly water requirement. The sum of the monthly water 𝐸𝑇𝐶 is referred to as
the total water requirement over the crop period.
4. Number of applications
789 𝑚𝑚
𝑁 19.725 20 𝑎𝑝𝑝𝑙𝑖𝑐𝑎𝑡𝑖𝑜𝑛𝑠
40 𝑚𝑚
5. So for the computed the number of applications, it means that in the 150-
day crop period, we should irrigate every
150 𝑑𝑎𝑦𝑠
7.5 𝑑𝑎𝑦𝑠
20 𝑎𝑝𝑝𝑙𝑖𝑐𝑎𝑡𝑖𝑜𝑛𝑠
To be on the safe side, the interval days are usually rounded down.
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Other general irrigation equations that are used are:
𝑪𝑰𝑹 𝑼𝑪 𝑷𝒆𝒇𝒇
Crop Irrigation Requirement, 𝑪𝑰𝑹
Portion of the Consumptive Use Where: 𝑈 = consumptive use in m/yr
that must be supplied by irrigation 𝑃 = effective precipitation data (i.e. AAR)
𝑼𝑪 𝑷𝒆𝒇𝒇 𝑪𝑰𝑹
𝒒𝒇
Farm Delivery Requirements, 𝒒𝒇 𝟏 𝑳𝒇 𝟏 𝑳𝒇
water required for irrigation in Where: 𝐿 = Farm Losses due to:
m/yr 1. deep seepage (less than 5%)
2. surface runoff (should not exceed 5%)
*percolation (15 to 50% of applied water)
𝑪𝑰𝑹
𝑪𝑰𝑹
Farm Irrigation Efficiency, 𝒒𝒇
𝒒𝒇
Average efficiency ranges from 40 to 60%
Under favorable conditions, above 80% is possible.
𝒒𝒇
𝑸𝑫 𝒒𝒔 + 𝒒𝒇
𝟏 𝑳𝑪
Diversion Requirement, 𝑸𝑫 Where:𝑞 = flow to supplement conveyance losses
Total flow requirement 𝑞 = farm delivery requirement
𝐿 = conveyance losses in decimal percentage
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A typical irrigation layout
Headworks
Main
Channel
Branch
Channels
Major
DIstributaries
Minor
Distributaries
• Major and minor distributaries/ tertiary channels Carry 0.25 to 5 cumecs, feeds
minor distributaries or water courses
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Further, the permissible minimum radius of curvature for a channel curve is
shorter for lined canals than unlined ones and is shorter for small cross sections
than for large cross sections of canals.
The alignment should be such that the cutting and filling of earth or rock should
be balanced, as far as possible.
The alignment should be such that the canal crosses the natural stream at its
narrowest point in the vicinity.
In order to finalize the layout of canal network for an irrigation project, the alignment of
channels should be marked on topo-sheets, until an optimum is reached. This alignment
is then transferred to the field by fixing marking posts along the centerline of the canal.
1. Earth Linings
• Stabilized earth linings: Here, the sub-grade is stabilized using either clay
for granular sub-grade or by adding chemicals that compact the soil.
• Loose earth blankets: Fine grained soil is laid on the sub-grade and evenly
spread. However, this type of lining is prone to erosion, and requires a
flatter side slopes of canal.
• Compacted earth linings: Here the graded soil containing about 15
percent clay is spread over the sub-grade and compacted.
• Buried bentonite membranes: Bentonite is a special type of clay soil,
found naturally, which swell considerably when wetted. Buried bentonite
linings for canals are constructed by spreading soil-bentonite mixtures
over the sub-grade and covering it with gravel or compacted earth.
• Soil-cement lining: Here, cement and sandy soil are mixed and then
compacted at optimum moisture content or cement and soil is machine
mixed with water and then laid.
2. Concrete lining
• gives very satisfactory service, frequently high in
their initial cost, long life and minimum
maintenance make them economical.
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lining and cause undue uplift pressure, drains are laid below the lining to release
the water and relieve the pressure
3. Shotcrete lining
• Shotcrete, that is, cement mortar in the ratio of
1 cement to 4 sand proportions through a
pump-pipe-nozzle system on the surface of the
channel.
• Equipment units used for shotcrete construction are relatively small and easily
moved.
• convenient for lining small sections, for repair of old linings, and for placing linings
around curves or structures.
• a layer of tiles is laid over a layer of brick masonry. The top layer
is generally laid in 1:3 cement mortal over 15mm thick layer of
plaster in 1:3 cement plaster.
• The size of tiles is generally restricted to 30mm x 150mm x 53m. This type of lining is
stable even if there is settlement of foundation, since the mortar joint between
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• bricks or tiles provides for numerous cracks so fine that seepage is insignificant.
5. Boulder Lining
• Also called dry stone lining or stone pitching, is used
for lining the earthen canal cross section, by proper
placement and packing of stones, either after
laying a filter layer over the soil surface or without
any such filter, depending upon the site
requirement.
• This type of lining is of course suitable where stones of required specification are
available in abundance locally.
6. Others
Geoweb lining
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Normal cultivation operations, such as tilling, ploughing, etc. cannot be easily carried
out in wet soils. In extreme cases, the free water may rise above the ground level
making agricultural operations impossible. This is called water logging Certain water
loving plants like grasses, weeds, etc. grow profusely and luxuriantly in water-logged
lands, thus affecting and interfering with the growth of the crops.
Water logging leads to a condition called salinity, which is caused when the capillary
fringe of the elevated water table rises within the root zone of plants. Since the roots of
the plants continuously draw water from this zone, there is a steady upward movement
of water which causes rise of salts, especially alkali salts, to come up to the ground
surface.
1. Select a suitable slope for the channel banks. These should be nearly equal to the
angle of repose of the natural soil in the subgrade so that no earth pressure is exerted
from behind on the lining.
2. Decide on the freeboard, which is the depth allowance by which the banks are
raised above the full supply level (FSL) of a canal. For channels of different discharge
carrying capacities, the values recommended for freeboard are given in the following
table:
3. Berms or horizontal strips of land provided at canal banks in deep cutting, have to be
incorporated in the section. The berms serve as a road for inspection vehicles and also
help to absorb any soil or
rock that may drop from the
cut-face of soil or rock of
the excavations. Berm width
may be kept at least 2m. If
vehicles are required to
move, then a width of at
least 5m may be provided.
4. For canal sections in filling, banks on either side have to be provided with sufficient
top width for movement of men or vehicles.
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5. Assume a safe limiting velocity of flow, depending on the type of lining, as given
below:
• Cement concrete lining: 2.7 m/s
• Brick tile lining or burnt tile lining: 1.8 m/s
• Boulder lining: 1.5 m/s
6. Assume the appropriate values of flow friction coefficients
7. The longitudinal slope (S) of the canal may vary from reach to reach, depending
upon the alignment. The slope of each reach has to be evaluated from the alignment
of the canal drawn on the map of the region.
8. For the given discharge Q, permissible velocity V, longitudinal slope S, given side
slope , and Manning’s roughness coefficient, n, find out the cross section parameters of
the canal.
• Continuity equation: Q = A * V
• Dynamic equation: V = 1/n A R 2/3 S 1/2
ELABORATE
Try to work on the following problems:
1. Look up the data for onions in the table given above. Assume that onions
will be planted in sandy soil and water is applied by drip irrigation.
Determine the watering interval required for (a) a short duration crop
period and (b) a long duration crop period.
EVALUATE
After completing your problem set, exchange solutions with a classmate or two. Assess
each other’s work starting with the presentation. Comment on handwriting, format,
organization of your output and the basic quality of the output.
Is it complete? Did your classmate follow instructions? Next, go into the details of the
content. Was the problem copied completely? Is the sequence of the solution correct
or logical? Were the formulas effectively used in the solution?
Give your classmate a score out of 50. Make this activity constructive by giving your
classmate an honest assessment, and in the same way, respect their comments on your
work. Be kind…
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