IRRIGATION PRINCIPLES ERT 349 SOIL AND WATER ENGINEERING IRRIGATION PRINCIPLES

Surface Irrigation

Surface Irrigation Water flows across the soil surface to the point of infiltration Oldest irrigation method and most widely used world-wide (90%) and in U.S. (60%) Used primarily on agricultural or orchard crops

Types of Systems Water Flooding Basin Relatively flat fields  allow water to find its own way across the surface Minimal preparation and investment Rather inefficient Basin Dikes used to surround an area and allow for water ponding (no runoff) Basins are usually level

Water Flooding

Types of Systems, Contd… Border Strips of land with dikes on the sides Usually graded but with no cross slope Downstream end may be dike Furrow Small channels carry the water (entire surface is not wet) Commonly used on row crops Lateral as well as vertical infiltration Furrows are usually graded

Border Irrigation Border irrigation is the most common surface irrigation method for broadcast seeded crops. Level borders (basins) are used for lowland rice production in Arkansas and California, and throughout Asia.

Furrow Irrigation Graded furrow irrigation is the most common type of surface irrigation in Oklahoma.

Furrow Irrigation Most furrow irrigation in Oklahoma is supplied by ground water through gated pipes.

Water Supply Methods of water supply Head ditch with siphon tubes or side-opening gates Gated pipe (aluminum or plastic pipe with small gates that can be opened and closed) Buried pipeline with periodically spaced valves at the surface

Siphon tubes Siphon tubes supplying water from an open head ditch of the W.C. Austin Irrigation project near Altus, OK. The ditch is higher than the furrows of the field so gravity can carry water over the ditch bank.

Pipe gate The pipe gates can be opened varying amounts to control the size of the furrow stream. Typical operating pressure on gated pipe systems is about 10 psi.

Furrow Irrigation Runoff from furrows (tailwater) is one of the most significant inefficiencies in furrow irrigation. Deep percolation (especially near the head ditch) is the other.

Water Management Runoff recovery systems Drainage ditches for collecting and conveying runoff to the reservoir Reservoir for storing the runoff water Inlet facilities to the reservoir (including desilting basin) Pump and power unit Conveyance system for transporting water (to same or different field)

A tailwater pit A tailwater pit is a storage pond to collect furrow runoff for reapplication. It may be pumped back onto the same field with a low-head pump, or applied to another field.

Uniformity Inherent non-uniformity because recession and advance curves are not parallel Factors affecting Inflow rate Slope Soil infiltration Roughness Channel shape Inflow time Length of run

Improving Irrigation Efficiency Alternate furrow irrigation Increases advance time, but reduces average infiltration depth (twice the width) Cutback irrigation Use large inflow rate during advance, and then reduce the inflow to match the soil's steady-state infiltration rate Intensive management is required

Improving Irrigation Efficiency Cont’d Land smoothing and laser grading Helps to improve uniformity Surge irrigation Alternate on-off periods for applying water Achieve higher efficiencies and uniformities in some soils Lends itself to semi-automation

Land smoothing and grading Laser controlled land forming equipment is used to shape uneven field surfaces into a smooth plane. Slopes of less than 0.1% (0.1 foot per 100 feet) are considered “level”. Slopes of over 0.1% are considered “graded”.

The rotating laser is placed at the field edge and set at the desired gradient and elevation. When the laser strikes the receiver on the earthmover, the machine cuts or fills soil at that location to give the “best fit plane” field surface.

Final smoothing with a land plane is required to achieve a useable field surface. Initially, 4-8 passes over the field will be needed (1-2 passes each in the N-S and E-W directions, and along each diagonal). Where significant fills were made, annual replaning will be needed to resmooth the surface until subsidence is reduced. Thereafter replaning may be needed only every 5-10 years, depending upon tillage methods.

Surge Irrigation Surge irrigation is the application of water to the furrows in short pulses, with periods of no flow in between. The alternate wetting-up and drying-down cycles reduces the infiltration rate more rapidly and increases the advance of water across the field, resulting in a more uniform depth of infiltration from the head end to tail end of the field.