10. Design & Operation of Systems.
The main objective of irrigation schemes or systems is to produce a particular desired pattern of plant growth. Maximum vegetative growth does not necessarily correspond to maximum yield of the part of the plant desired e.g. fruit, nuts, or roots. In addition, achieving maximum yield may require inefficient use of available resources, whether it is land, water, equipment, or labour. 'Optimum yield' is usually the desired objective. This has been defined as the yield at which the benefit/cost ratio is at maximum, although even this may be hard to achieve if any of the resources required for the irrigation system e.g. land, water, or equipment is limited. Therefore, it is important to clearly define the purpose or desired outcome of an irrigation system
IRRIGATION: THE WIDER VIEW
Irrigation can be used to produce outcomes in terms of crop production. Irrigation schemes may also have other outcomes. Large-scale schemes may have an impact on human settlement patterns, causing local migrations of labour to or from irrigated areas with subsequent changes in the availability and cost of housing and services.
Land prices may also be severely affected as land use patterns change. Impoundment of rivers and streams into dams and lakes can result in major environmental changes (such as changes to river characteristics and to flora and fauna populations) in areas both adjacent and far removed from those features. Small scale, even down to individual garden size systems, can still cause far-reaching changes. For example, the base flow component (year-round flow) of many urban streams derives almost entirely from home irrigation of gardens. If this was to stop these streams would cease flowing for much of the year. In addition, much of the excess water from irrigation systems that flow to streams, rivers and lakes is high in chemicals leached out of gardens or other crop areas. These chemicals, particularly nitrogen and phosphorous from fertilisers and pesticides, can cause major changes in vegetation and animal populations in and adjacent to water bodies.
SOURCES OF WATER
Water for irrigation is generally obtained from one or more of the following sources:
· On-site storage such as dams or tanks where runoff is collected and stored for later use
· Bore water or underground wells
· Irrigation channels where water is distributed from storage facilities often large distances away
· From streams, lakes or rivers
· From mains or town water systems (usually carried by pipes and/or aqueducts or channels).
FEASIBILITY OF IRRIGATION
Before undertaking or designing a large-scale irrigation project, a feasibility study should be undertaken to show whether it is desirable to commit resources such as capital, labour, time, land, etc. to such a project. Specialist irrigation consultants are used for large projects, as many complex and far-reaching factors such as topographic features, national and international agricultural and financial markets, transport and communications networks, state and federal policies need to be considered.
The economics of small irrigation systems, including systems for nurseries, farms and even home gardens, also need to be evaluated. Factors to consider include the availability of materials, the expertise and cost required to install and operate the system, and the outcomes and profitability of the system.
