The following points highlight the five main factors to be taken into consideration before determining the suitability of water for irrigation. The factors are: 1. Chemical Water Composition 2. Crop to be Irrigated 3. Soils to be Irrigated 4. Climate 5. Management of Irrigation and Drainage.
Factor # 1. Chemical Composition of Irrigation Water:
Quality of the water is determined by the total salt content and by its ionic composition. Total salt content (expressed in EC, mg l-1, meq l-1, and ppm) may give a general indication of the water’s quality. Also important is the determination of the main cations and anions, usually, expressed in meq l-1, as several ion ratios influence water suitability. Under certain conditions, the presence of microelements must be taken into account.
Factor # 2. Crops:
Crop is the first and most important factor to be considered. Evaluation of water must be based on the tolerance of a specific crop or crops in the rotation to the total salt content or specific ion concentration. Tolerance of a crop to salinity is that concentration of the soil solution that will give a certain reduction in yield as compared to non-saline conditions.
In the USA, the 50 per cent yield decrement is taken as the tolerance limit for field and forage crops. In Holland and in Algeria, the 25 and 20 per cent decrement is taken. The American 50 per cent decrement is taken for the salinity measured at the bottom of the root zone, whereas in Holland the salinity of the top layer is taken as the criterion. Some crops are specifically sensitive to chloride and sodium ion concentration. The most important of these crops are the deciduous trees, citrus and avocado.
Factor # 3. Soils:
Behaviour of a soil in contact with saline water depends on the initial physical properties and salt content. Clay content of the soil affects the ion adsorption capacity, which in turn influences the hydro-physical properties. Furthermore, presence of an impermeable layer or of a groundwater table affects the salt distribution in the profile.
Initial chemical composition of the soil influences the exchange processes during the water-soil contact. Application of saline water to a salt free soil will salinise the soil, but the use of this same quality of water may reduce the salinity of a saline soil if drainage is adequate. As infiltration and percolation of water may differ greatly for different soils, different degrees of salinisation may be expected with the same quantity and quality of irrigation water.
Factor # 4. Climate:
Evapotranspiration and rainfall are the two main climatic elements to be considered when evaluating suitability of water for irrigation. Water depth to be applied to a crop during a season depends on the evapotranspiration, which, therefore, affects the irrigation regime and consequently the seasonal dynamics of salts in the soil profile.
Amount and distribution of rainfall is the second factor of the climate to be paid attention. A given amount of rainfall distributed uniformly over the growing season will dilute the soil solution, but will not bring about a leaching of the profile, as the same amount of rain falling during a shorter time would do.
Factor # 5. Management of Irrigation and Drainage:
Irrigation method influences the salt accumulation in the soil and in the plant. Application of amounts of water less than the consumptive use will result in accumulation of salts in the main root zone. Increasing the application will leach the salts out of the root zone and equilibrium can be reached between the salt contents of the water and of the soil.
Lack of proper drainage in an area with a high water table will result in capillary rise of groundwater, increasing the soil salinity. Relatively saline water applied in furrows on a permeable soil will have no harmful effect on plant growth, while the same quality of water applied by sprinkling might cause reduced yields.