Prof. Dr. Ragab Ragab, President of the International Commission on Irrigation & Drainage, ICID (www.icid.org)

Non-conventional water resources combine all types of water, other than fresh water. Examples are drainage water, brackish groundwater, and wastewater (treated, untreated, domestic, industrial, mining water, breweries, food packaging industry, sugar industry, fish farming wastewater, etc.). The non-conventional waters are needed to meet our water and food requirements as the population is increasing exponentially while the amount of freshwater remains the same. Moreover, there is a need to double food production by 2050 and to narrow the gap between water supply and demand, which is currently around 30% globally.  

Worldwide, there is not enough fresh water. It is less than 1% of the world’s water reserve. The rest is saline. However, saline water use for irrigation requires certain management techniques to safeguard the environment and produce a good amount of food. Management of saline water follows guidelines that differ from other waters, e.g., treated wastewater has restrictions related to the number of microbes that can be present (WHO, 2006). In the following section, the focus will only be on saline water management in relation to the most suitable irrigation system, the leaching requirement, the salinity measurement, modelling salinity, using two different water types fresh and saline, the crop irrigation requirement, and land management to minimize the saline water impact.

1. Using saline water requires a suitable irrigation system. Low nozzle sprayers/sprinklers below the canopy / close to the ground and sub-surface drip irrigation are suitable. However, Nano-Drip subsurface irrigation using Ultra Low Drip irrigation systems (flow 0.1 to 0.3 L/h) would be a good option and saves 30% of irrigation water.
2. Leaching should only be considered when the salt concentration exceeds the plant tolerance limit. Leaching can be carried out by the unavoidable irrigation inefficiency, occasional rain, and seasonal application of fresh water. Excessive or routine leaching after each irrigation is not recommended as leaching can also leach nutrients, wastes water, and adds extra salt if leaching water is saline.
3. When two sources of water, e.g. fresh and saline water, are available alternating use of the fresh water at the beginning of the growing season, as the young crop is sensitive to salinity, followed by irrigating with the saline water at a later stage, when the crop less sensitive, is a better management practice than irrigating with the mix of the two water resources for the whole season.
4. Calibrated and validated models (e.g., SALTMED) [1] can be used as good management tools to predict the long-term salinity impact on soil, plant, groundwater, and leaching requirement without the need to conduct field experiments. They can also be used in a non-conventional way to: predict missing parameters and “difficult-to-measure” parameters (i.e., Pi 50, Kcb, Kc, photosynthesis efficiency, etc.); to predict climate change impact (CO2, radiation, rainfall, temperature, etc.); produce an experimental design such as the best crop rotation, tillage level, fertilizer management, and scheduling. They can also be used to estimate the crop water requirement and time to irrigate (scheduling) while also utilized to design a program for data collection.
   
5. Using the actual evapotranspiration, ET, measured or calculated from equations based on validation against measurements is recommended above the commonly used equations (e.g. Modified Penman-Monteith) as they produce potential ET representing the atmospheric demand, not the crop demand for water by using only meteorological data with no plant representation in the equation. The potential ET is higher than the actual ET and will lead to excessive unnecessary waste of water. Accurate estimation of irrigation water requirement is important since irrigating with excessive saline water may result in adding more salts, leaching nutrients and fertilizers, decreasing soil and groundwater qualities, decreasing water productivity and water use efficiency, and irrigating less area.
   
6. Land management is also crucial when using saline water for irrigation. Land preparation is therefore important to ensure uniform distribution of irrigation water, infiltration, and better salinity control. Subsoiling, chiselling, and ploughing breaks up compaction and improve water infiltration and leaching. Special treatments such as deep ploughing, adding and mixing sand with the soil layer, and the addition of organic matter, gypsum, or green manure improve soil permeability. Conservative tillage, zero or minimum tillage also has several advantages as it reduces soil evaporation, increases water availability, reduces surface salinity, increases organic matter, reduces soil erosion, increases nutrient availability, and reduces agrochemical use, labor and machinery.
   
7. The spatial variability and soil heterogeneity make area-based measurements more representative. In situ continuous measurements of both soil moisture and salinity at the same time is more accurate than laboratory methods. The salinity relation with yield or other crop parameters is better described using scaled relations, e.g., relative yield vs salinity rather than absolute yield vs salinity.