Surface Irrigation. Basic types flood retreat surface furrow corrugation border strip basin. Advantages and disadvantages Advantages
An example of the effect of exposing sub-soil on the
surface after land levelling on a field of cotton in
Cut areas are where soil has been excavated.
Fill areas are where soil has been deposited during
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
Following figures shows water distribution from smaller closely spaced and larger, widely spaced furrows.
spacing function of the crop and tillage machinery used - for single row crops = 75 to 105 cm
Need not follow slope of land with furrow irrigation - some side-slope is permissible.
Inflow regulated by adjusting spile orifice or removing one or more of a bank of siphons (perhaps 5 to start reduced to 2)
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.
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 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.
For most crops, T should be < 48 hrs to avoid water-logging.
infiltration rate as a function of time,
inflow rate and
Fields too long waterlogging, percolation losses,
rise in water-table, salinity, logistical problems with
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
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
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
design similar to furrows
(also known as “strip checks” in USA)
limitation to non- erosive slopes - cannot put in at angle to main slope as is possible with furrow irrigation;
heavier soils - <0.5% slope
lighter soils - <0.3% slope
top of field should be levelled
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
Optimum basin areas (ha) for different soil type given various inflow rates