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Irrigation Water Supply and Quality 
There is approximately 97.3 % of the total global water supply which is made up of oceans, seas and other saline inland water bodies, yet these are not available for human consumption because of high salt content. Fresh water accounts for only 2.7 % of the total estimated global water supply and 75% of this percentage is frozen in polar ice caps and glaciers. Some fresh water is blocked in inaccessible areas under the ground. Thus the fraction of fresh water available for humans is estimated at less than 0.003% of the total global water availability. Conservation of water and non pollution of fresh water sources is necessary even though water is a renewable source.
There are three main sources of available fresh water. The first one is rainwater (or precipitation).
The second source is surface water; which includes ponds lakes and streams.
The third is ground water or underground aquifers which is the water that percolates down the surface soil into pore spaces of rocks. The total volume of ground water found in the aquifer is estimated to be 42.3 x 1010 m³.
There are other alternative sources of fresh water.  
One is the use of air wells. An air well or aerial well is a structure or device that collects water by promoting the condensation of moisture from air.
High-mass air well of Belgian engineer
All air well designs incorporate a substrate with a temperature sufficiently low so that dew forms. Dew is a form of precipitation that occurs naturally when atmospheric water vapor condenses onto a substrate. It is distinct from fog, in that fog is made of droplets of water that condense around particles in the air. Condensation releases latent heat which must be dissipated in order for water collection to continue.
On the other hand, soil moisture for plants and supplements for streams and lakes is provided by ground water. It is a reserve supply of water that the agricultural sector and urban water supply sectors are tapping increasingly. The water table indicates the level at which ground water is found, and rises and falls based on the amount of water that percolates down to this level during the rains and the amount that is pulled out from it.
There are various measures of water quality such as taste and odor, microbial content and dissolved concentrations of naturally occurring and manufactured chemical constituents which define the suitability of water for different uses.
To define irrigation water quality, there are numerous parameters used, to assess salinity hazards, and to determine the appropriate management strategies. A complete water quality analysis will include the determination of:
1. the total concentration of soluble salts,
2. the relative proportion of sodium to the other cations
3. the bicarbonate concentration as related to the concentration of calcium and magnesium, and
4. the concentrations of specific elements and compounds
In irrigation water, salts are present in relatively small but significant amount. They originate from dissolution or weathering of the rocks and soil, including dissolution of lime, gypsum and other slowly dissolved soil minerals. These salts are carried with the water to wherever it is used. In the case of irrigation, the salts are applied with the water and remain behind in the soil as water evaporates or is used by the crop. The suitability of water for irrigation is determined not only by the total amount of salt present but also by the kind of salt. There are two types of salt problems which are very different, those associated with the total salinity and those associated with sodium.
Sediments
Sediments are solid fragments of inorganic or organic material that come from the weathering of rock and are carried and deposited by wind, water, or ice. They originate from bed load transport, beach and bank erosion, and land runoff. They are naturally sorted by size through prevalent hydrodynamic conditions. Sediments range in particle distribution from micron-sized clay particles through silt, sand, gravel, rock, and boulders.
The environmental impacts of sedimentation include the following: loss of important or sensitive aquatic habitat, decrease in fishery resources, loss of coral reef communities, human health concerns, changes in fish migration, increases in erosion, loss of wetlands, nutrient balance changes, and increases in turbidity, loss of submerged vegetation.
Salinity
Water with high salinity is toxic to plants and poses a salinity hazard. Soils with high levels of total salinity are call saline soils. High concentrations of salt in the soil can result in a "physiological" drought condition. A salinity problem exists if salt accumulates in the crop root zone to a concentration that causes a loss in yield. The plant symptoms are similar in appearance to those of drought, such as wilting, or a darker, bluish-green color and sometimes thicker, waxier leaves.
Corn plant damaged by saline sprinkler water
Water salinity is usually measured by the TDS (total dissolved solids) or the EC (electric conductivity). TDS is sometimes referred to as the total salinity and is measured or expressed in parts per million (ppm) or in the equivalent units of milligrams per liter (mg/l).
Table 3 can be used for conversion factors available in calculating for other water EC levels.
Electric conductivity measures salinity from all the ions dissolved in a sample. This includes negatively charged ions (e.g., Cl¯, NO¯3) and positively charged ions (e.g., Ca++, Na+). Another common source of confusion is the variety of unit systems used with ECw. The preferred unit is deciSiemens per meter (dS/m), however millimhos per centimeter (mmho/cm) and micromhos per centimeter (µmho/cm) are still frequently used. Conversions to help you change between unit systems are provided in Table 3.
Sodium
Irrigation water containing large amounts of sodium are of special concern due to sodium's effects on the soil and poses a sodium hazard. Sodium hazard is usually expressed in terms of SAR or the sodium adsorption ratio. SAR is calculated from the ratio of sodium to calcium and magnesium. The latter two ions are important since they tend to counter the effects of sodium.
Continued use of water having a high SAR leads to a breakdown in the physical structure of the soil. Sodium is adsorbed and becomes attached to soil particles. The soil then becomes hard and compact when dry and increasingly impervious to water penetration. Fine textured soils, especially those high in clay, are most subject to this action. Certain amendments may be required to maintain soils under high SAR's. Calcium and magnesium, if present in the soil in large enough quantities, will counter the effects of the sodium and help maintain good soil properties.
If the sodium percentage in the soil is increased to 10% or more , the aggregation of soil grains breaks down and the soil becomes less permeable, crusts and dry, and its pH increases towards that of alkaline soils. Since calcium and magnesium will replace sodium more readily than vice versa, irrigation water with a low sodium adsorption ratio (SAR) is desirable
General classifications of irrigation water based upon SAR values are presented in Table 4.
meq/L = mg/L divided by atomic weight of ion divided by ionic charge (Na+=23.0 mg/meq, Ca++=20.0 mg/meq, Mg++=12.15 mg/meq)
Example. 
Find the sodium adsorption ratio of a water with the following characteristics; sodium 250mg/L, calcium 110mg/L, and magnesium 48mg/L. if the conductivity of this water is 80 micromhos/cm at 25˚C, is this water suitable for agriculture?
Solution:
Referring to Table 4, the water has got low sodium content based on its SAR values but the use of crops with sodium sensitivity must be given attention.
Sodium in irrigation water can also cause toxicity problems for some crops, specially when sprinkler is applied. Toxicity problems occur if certain constituents (ions) in the soil or water are taken up by the plant and accumulate to concentrations high enough to cause crop damage or reduced yields. The degree of damage depends on the uptake and the crop sensitivity.
The ions of primary concern are chloride, sodium and boron. Although toxicity problems may occur even when these ions are in low concentrations, toxicity often accompanies and complicates a salinity or water infiltration problem. Damage results when the potentially toxic ions are absorbed in significant amounts with the water taken up by the roots. The absorbed ions are transported to the leaves where they accumulate during transpiration. The ions accumulate to the greatest extent in the areas where the water loss is greatest, usually the leaf tips and leaf edges. Accumulation to toxic concentrations takes time and visual damage is often slow to be noticed. The degree of damage depends upon the duration of exposure, concentration by the toxic ion, crop sensitivity, and the volume of water transpired by the crop. In a hot climate or hot part of the year, accumulation is more rapid than if the same crop were grown in a cooler climate or cooler season when it might show little or no damage.
Restriction on Use: The “Restriction on Use” shown in Table 1 is divided into three degree of severity: none, slight to moderate, and severe. The divisions are somewhat arbitrary since change occurs gradually and there is no clearcut breaking point. A change of 10 to 20 percent above or below a guideline value has little significance if considered in proper perspective with other factors affecting yield. Field studies, research trials and observations have led to these divisions, but management skill of the water user can alter them. Values shown are applicable under normal field conditions prevailing in most irrigated areas in the arid and semi-arid regions of the world.