Basic types • flood retreat • surface • furrow • corrugation • border strip • basin
Advantages and disadvantages • 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
Disadvantages • 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
Construction • Constructed with reversed mouldboard ploughs or specially designed ridgers
Shape and size of furrows • 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)
Furrow slopes • 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
Another consideration …. 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.
Furrow stream • 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.
Surge Irrigation Gated pipes attached to pump- water pressure varied. Advance faster, leaching less.
Duration of irrigation 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.