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## Surface Irrigation

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- flood retreat
- surface
- furrow
- corrugation
- border strip
- basin

- Advantages
- the large expenditure on storage, headworks, and distribution canals is usually paid for by the Government, and the capital cost on the farm is low - this is an advantage (for the farmer) in the case of peasant agriculture:
- surface irrigation may be more likely to be traditionally understood,
- surface irrigation is more suitable for some crops, such as rice and forage crops;
- leaching is easier and cheaper if it is needed
- surface methods can use large flows available for short periods, and so allows canalised water supply to be easily shared by several farmers

- difficult to get even distribution of water on light permeable soils which have high infiltration rates;
- surface irrigation is not suitable for crops which need frequent light watering (shallow rooted and/or drought susceptible crops);
- efficient surface irrigation usually requires smooth land and if the land is not naturally smooth, it may be expensive to level it and the topsoil and fertility can be disturbed;
- field layout for surface irrigation may restrict mechanisation unless special measures are adopted

Land levelling and its effects

- Methods
- grid surveys;
- deep ripping;
- land planes;
- use of lasers

Some problems associated with land levelling

An example of the effect of exposing sub-soil on the

surface after land levelling on a field of cotton in

Iran.

Cut areas are where soil has been excavated.

Fill areas are where soil has been deposited during

land levelling

Not really a method of irrigation but many people depend on flooding for crop production.

Problem often caused by dam construction such as in Kenya (Turkwell dam) - see New Sci. 18/6/87; p35

- Constructed with reversed mouldboard ploughs or specially designed ridgers

- must be sufficiently high to take discharge without overtopping
- depth typically 15 to 20 cm
- top width typically 25 to 30 cm for most situations but wide, flat ridges needed for some (clay) soils or crops

Following figures shows water distribution from smaller closely spaced and larger, widely spaced furrows.

- Closely spaced furrows
- entire root zone is wetted before the wetting front reaches the moist soil below, and deep percolation losses are minimised.
- Distance between furrows is more than twice the distance to the moist soil,
- water penetrates to moist soil below and is lost by deep percolation while the crop roots continue to be dry.

spacing function of the crop and tillage machinery used - for single row crops = 75 to 105 cm

- plant on side near top to avoid salinity (if planted at top of ridge) and waterlogging (if planted at bottom of furrow)

- discharge is function of S1/2
- upper limit is about 2% slope but if there is high intensity rainfall and erodible soils, upper limit may be as little as 0.3%

Need not follow slope of land with furrow irrigation - some side-slope is permissible.

Distribution problem and control of inflow

- Advance & recession wave
- need water to infiltrate to bottom of root zone at end of field (this changes as the crop is established)
- Apply maximum possible inflow to get wetting front to the bottom of the field then cut back to reduce wastage - ideally water should continue to reach bottom of field but only a small amount should drain off

Inflow regulated by adjusting spile orifice or removing one or more of a bank of siphons (perhaps 5 to start reduced to 2)

- Using more small siphons -> better control but more expensive
- Infiltration rate onto a steep field will be less than onto a lower slopes

If the water is turned before wetting fron reaches wet soil below, there will be negligible loss through deep percolation because the excess water that drains out of the large pores of the soil behind the wetting front is utilised in wetting the dry soil beneath.

- methods of getting water onto the field varies:
- breach bank - poor control - large openings can get larger through erosion
- siphons
- spiles - usually of aluminium
- Withers and Vipond quote 45/s l/min where s is the % slope (or 0.6/s in litres/sec) but this is only the median value - try values either side in field trials - will depend on soil type, slope, & dimensions of furrow - OR calculate from Mannings equation

Flow through an orifice is given in British units (cusecs and ft) as :

However some trials I did indicated that flow in siphons was proportional to d 2.5 (where d is diameter) - may be because the flow at the smaller diameters were more turbulent

Flow must also depend on the length and material of the siphon - use published graphs rather than equations

Stream is typically 3 l/sec on relatively flat land

initially high then reduce to reduce runoff

If stream is too fast, there will be erosion in the furrows

If stream is too slow, irrigation will take too long

Following table shows the relation of maximum non-erosive flow rates to critical slopes in furrows based on the equation Qm = C/S seen in some books

However, the erosive slope will depend on the type of soil and also the velocity down a slope is inversely proportional to the square root of the slope not the reciprocal of the slope.

Gated pipes attached to pump- water pressure varied. Advance faster, leaching less.

Evaluation of required duration of irrigation, T is required to be calculated unless tensiometers or neutron probes are used to indicate when sufficient water has been applied

Phillips equation is not easy to solve for t. For this situation, the use Kostiakov's equation is suggested:

I = Kta

log I = log K + a log t

Plot log I against log t to obtain K (from the intercept) and a (from the slope).

where Ir is the required depth of irrigation to be applied. T should be taken as the uptake opportunity at the end of the field.

If the time to reach the bottom of the field is > 25% of T then field size needs to be changed.

We will come back to this shortly

The following figure shows an example scheme for moving siphons in a furrow irrigation scheme.

The amount of water flowing out of the head ditch is constant but the block of furrows being irrigated is changing as time progresses.

The numbers refer to the number of siphons in each block of 100 furrows.

Thus, a value of 200 means there are 2 siphons per furrow in order to “push” the wetting front down to the far end of the field.

Thereafter the number is reduced.

The shaded squares shows the full picture at time = 16 hours and for furrow block D throughout its irrigation cycle.

Shorter fields needed for soils with high percolation losses

Percolation losses at the top of the field occur due to extra time for advance wave to reach the bottom of the field. Rule of thumb is that time to reach end of field < 1/4 of time required to apply adequate water.

Empirically I found that

L = L1t0.7

where L1 is the distance advanced in the first hour

where T is the time for the replenishment of deficit.

Thus:

For most crops, T should be < 48 hrs to avoid water-logging.

infiltration rate as a function of time,

inflow rate and

furrow dimensions.

Fields too long waterlogging, percolation losses,

rise in water-table, salinity, logistical problems with

equipment.

If fields short very expensive, wastage on roads,

headlands, field channels & surface drains.

Another equation that will help to determine the correct inflow rate is:

L1 = C Q0.5

where Q is the inflow rate.

i.e. to double the L1 quadruple the inflow rate and

vice versa.

Note - increasing the inflow rate too much will cause

erosion within the furrow.

Percentage of infiltrated water that is lost below root zone for various fractions of total irrigation time (t) that it takes for the water to reach end of furrow, & for several values of exponent a in Kostiakov’s equation

As already pointed out, the velocity of the water will depend on the inflow rate and the furrow dimensions - the table is very simplistic

Furrow irrigation in layered soils

Water entering dry loam soil underlain with coarse

dry sand. Lines indicate positions of wetting

front at different time intervals.

Water from overlying loam draining from isolated place(s) [fingering] leaving the rest of the sand dry.

This image shows that when the wetting front reaches the clay, it enters immediately; but the rate of advance is retarded because of the slower hydraulic conductivity

- Constructed with roller
- Cannot run at angle to slope but otherwise field

design similar to furrows

(also known as “strip checks” in USA)

- use scraper
- water runs directly down-slope - therefore

limitation to non- erosive slopes - cannot put in at angle to main slope as is possible with furrow irrigation;

- problem with furrow down border, use side bunds

- Width
- 6 to 30 m between borders depending on soil type and slope (see table for suggested designs);
- multiple of tractor widths
- reduce for longer fields

heavier soils - <0.5% slope

lighter soils - <0.3% slope

Top

top of field should be levelled

Length

design of field lengths similar to furrow irrigation

See handout for suggested dimensions for various soil types

During trials, if not uniform across strip, try:

- increasing inflow

- reducing strip width

- increasing no. of inlet points

- In small scale irrigation, irrigators often use small square basins to control the flow. Basins are from 5 m to 20 m sides.
- Minimum of land levelling
- Difference is that water is prevented from running off until irrigator decides by breaking bank or through spillway arrangement.
- Sometimes used in large-scale projects with dimensions as shown in following table.

Optimum basin areas (ha) for different soil type given various inflow rates

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