Non-conventional water (NCW) is defined here as water from a source not conventionally used for agricultural production, primarily water that is of lower quality. The two major sources are:
- wastewater, following its use for domestic, municipal and industrial purposes;
- saline water from groundwater, drainage and surface sources.
Within these sources there is a spectrum of quality and quantity. The extremes of the quality spectrum for wastewater would be undiluted untreated sewage at the low end and quaternary treatment at the high end.
For saline water the low quality end might be seawater and very brackish groundwater with conductivity > 25 dS m-1, while the high end could be water with conductivity < 4 dS m-1) and desalinated water (which can be used for agricultural production of high value crops or where water is scarce and financial resources are adequate).
The safe use of NCW requires that human health is not impacted and that short and long term environmental quality is maintained at an acceptable level. The economic feasibility of using NCW in agriculture depends on many factors, such as the cost of treatment for safety, pumping costs, distribution costs, and competition from alternative uses such as landscaping and amenity.
In some regions waste water is already a common water source as safer (cleaner) water sources are not available. In other countries NCW is becoming the major source of water for agriculture as conventional sources of good quality water decline or are diverted for other uses. NCW can be used as is or blended with other water to produce the desired quality and quantity. In the case of untreated but usually diluted wastewater, safety measures as outlined in the WHO-FAO-UNEP guidelines have to be adopted.
The target of increasing the use of NCW primarily benefits peri-urban areas and countries where conventional renewable water resources are limited or demand already equals or exceeds supply.
Cities and towns generate a stream of water that has already been used, such as for domestic purposes. This stream of water represents a waste product which must be either disposed of safely or re-used downstream as a resource. Apart from its value as water, it may also contain nutrients which benefit agricultural production. There can however be a mismatch between its rate of production (which may be relatively constant) and the demand by agriculture which varies with the irrigated area and the time of year. Agricultural production close to cities is the most cost effective use. Use of this water requires consideration of:
- future availability;
- re-use safety/guidelines;
- present re-use status;
- governance/policies/legislation/monitoring and compliance;
- storage facilities;
- long-distance transportation networks and pumping requirements.
Production of salt-tolerant crops can generate economic value from saline water where the environmental conditions are conducive. Excess water must be used to prevent salt accumulation in the vadose zone, and for sustainability this excess should not cause environmental or resource degradation.
Whatever the source of NCW, it must be an element in a broader water management approach encompassing demand management and strategies for supply and conservation. The economics of using NCW relative to other sources, and the development of an Implementation strategy, including capacity building, are vital because good management is essential for its safe use in agriculture.