Marginal water resources

Water is a fundamental and irreplaceable resource for all life on earth. Growing world populations have led to growing water needs resulting in water supplies in many regions unable to meet demand - the result: water scarcity. In the face of growing water scarcity, the management of alternative water sources for agricultural and domestic use has become a necessity. The treatment and reuse of marginal water resources (sewage water, contaminated water, saline water and seawater) are now understood to be essential for meeting the growing water needs of the world's population.

Within an agricultural region there are likely to be several sources that may be suitable for agricultural use. The most common sources of marginal quality water are:

(1) Agricultural drainage water

(2) Municipal and industrial waste water

(3) Shallow groundwater

With careful planning and management these sources can be entirely suitable for agricultural purposes.(read more...)

One of the most common water quality concerns for irrigated agriculture is salinity. Recommendations for effective management of irrigation water salinity depend upon local soil properties, climate, and water quality; options of crops and rotations; and irrigation and farm management capabilities.
All major irrigation water sources contain dissolved salts. These salts include a variety of natural occurring dissolved minerals, which can vary with location, time, and water source. Many of these mineral salts are micronutrients, having beneficial effects. However, excessive total salt concentration or excessive levels of some potentially toxic elements can have detrimental effects on plant health and/or soil conditions.

Groundwaters with low salinity level and surface waters can be an additional source of irrigation water and used for irrigation directly, or after mixing them with fresh water or after desalinization.

Saline groundwater is widespread everywhere. On the plains it usually occurs like continuous mirror and only in some places desalination spots are formed under big sand dunes at the expense of infiltration of precipitation or temporary surface water created by rain and which originates on mountain slopes or in loamy basins (claypans) on the plains.
Potential usable groundwater resources in desert and semidesert areas of Central Asia and Kazakhstan in the area of 3.6 mln square km are estimated approximately 5,000 square metres /s. Of them fresh water is 2,100, water with low salinity level (with salinity level 1-3 g/l) is 700, brackish water (3-10 g/l) is 1,000 and saline water (10-35 g/l) is 1,200 square metres/s (Nikitin and others., 1978). Thus, saline water resources are 2,200 square metres/s, or 70 square km/year, which in terms of water content exceeded the discharge of such a big river as Amudarya. (read more... )
Resources of temporary surface water, which accumulate on loamy basins – claypans are also of practical interest. In years with average water availability, they are 704 mln square metres, including in Turkmenistan - 332, in Uzbekistan - 99 and in Kazakhstan - 273 mln square metres (Leshchinskiy, 1974).
In deserts of Central Asia and Kazakhstan, resources of temporary surface water, which originates in foothills and on loamy basins (claypans) in plains is quite considerable. Water coming from foothills in years with average water availability in Turkmenistan alone is estimated as 542 mln square metres/year (Leshchinskiy, Kirsta, 1967). Water coming from foothills is already used for water supply in pastures by building dams and ponds, but so far not in sufficient scales. (read more... )

In accordance with the existing practice of water use, almost all drainage water is returned to rivers and reservoirs. In deserts and semideserts, where irrigated farming is developed, huge amount of drainage water, coming from irrigated fields, is discharged.
In 1975 the volume of waste water in the world reached 1,200 square km/year, of which drainage water in agriculture was about 500 square km/year. Since one third of irrigated lands in the world are in desert areas, then arid zones account for about 170 square km/year of this amount of drainage water. If overall volume of waste water increased from 1,200 up to 3,000 square km/year, then the amount of drainage water in desert areas may increase from 170 up to 300 square km/year. (read more... )

An important source of saline water is drainage effluent (including perched groundwater) from irrigated areas. Drainage water, once thought of as wastewater, is now used in many countries for irrigation. The salinity levels vary, but often the salt levels are higher than those of conventional primary irrigation water sources. Reuse of drainage effluent is important when the supply of good quality irrigation water is limited, and it is also an efficient means of reducing water pollution.

The total water mineralization is defined as the sum of mass concentrations of solid inorganic substances dissolved in water, electrolytes (cations and anions) and non-electrolytes. The term salinity relates to the total concentration of the main dissolved inorganic ions, i.e. Na+, Ca2+, Mg2+, K+, HCO3 –, SO42– and Cl– in groundwater, channel waters and drainage waters. The particular concentrations of these cations and anions can be expressed by means of chemical equivalents (mmolc/l) or on a mass basis (mg/l). The total concentration of salts (i.e. the salinity) is then expressed as the sum of the individual cations and anions in mmolc/l, or in mg/l.

For reasons of analytical simplification the real salinity indicator is the electrical conductivity of water (EC) expressed in dS/m, or in mS/m or μS/cm. Electrical conductivity is always expressed at a standard temperature of 25°C in order to allow for the comparison of electrical conductivity in various climatic conditions.
The adverse effect of irrigation water quality on the physical properties of soil is associated with the accumulation of sodium ion on the soil exchange complex which imparts instability to the soil aggregates and whose disruption is followed by dispersion of clay particles resulting in clogging of soil pores.
It is necessary to express the total concentration of dissolved salts in irrigation waters in terms of the electrical conductivity of water for diagnostic and classification purposes. Based on electrical conductivity, waters are classified into four classes according to Richards (1954). The boundary values between the individual classes are 250, 750 and 2250 μS/cm. These limit values were determined on the basis of the relationship between the electrical conductivity of waters and the electrical conductivity of saturated soil extracts.
Low-salinity water (EC < 250 μS/cm) can be used for irrigation of most crops on most soils with little probability that soil salinity will develop.
Medium-salinity water (250 < EC < 750 μS/cm) can be used if the soil is washed with a moderate amount of water. In most cases crops with a moderate salt tolerance can grow without applying special methods of salinity regulation.
High-salinity water (750 < EC < 2250 μS/cm) cannot be used on soils with restricted drainage. Special management of salinity regulation and choice of crops with good salt tolerance are required even for soils with adequate drainage.
Very high-salinity water (EC > 2250 μS/cm) is not suitable for irrigation under ordinary conditions, but it can be used under very special circumstances. The soils must be permeable, drainage must be adequate, water must be applied in excessive amounts in order to ensure high soil leaching to be able to grow crops that are very salt tolerant.

The classification of water based on the level of salinity (concentration of salts) is given in the below table. This indicator is a limiting factor when saline water is used for irrigation.

Table 1. Classification of water based on salinity level
(www.fao.org/soils-portal, 2014)

Only very salt tolerant plants can be successfully grown if EC of irrigation water exceeds 10 dS/m. As a rule water with electrical conductivity less than 2 dS/m is used. Many drainage waters, including high grounwater in irrigated lands have electrical conductivity of 2-10 dS/m. Such waters can be used to grow certain crops, although in practice they are not used in agricultural production, and are only discharged into basins and water courses. Used drainage water can also sometimes be reused for irrigation, especially in order to reduce the volume of drainage water. Such waters, as a rule, have electrical conductivity of 10-25 dS/m. Ordinary crops cannot be grown using such waters. The experience of using them in crop production is very inconsiderable. Highly saline waters (25-45 dS/m) and salt brines (>45 dS/m) are not used either in agricultural production.

Irrigators in many watersheds and irrigation districts have had to face and respond with changing practices to increasing scrutiny about how to best manage available water resources. Along with that has come growing attention to the quality of water available for irrigation. In fact, growing competition for use of limited water supplies has been the impetus for some irrigators to consider using water previously thought to be of only marginal quality and suitability for irrigation.
Typically, qualities of irrigation water which deserve consideration include the salt content, the sodium concentration, the presence and abundance of macro- and micro-nutrients and trace elements, the alkalinity, acidity, and hardness of the water. Under some circumstances, the suspended sediment concentration, bacterial content, and temperature of irrigation water may also deserve attention.
Salinity – the amount of dissolved salt in water. All water used for irrigation contains some salt! This salt generally comes from weathering of soil, leaching of salts dissolved from geologic marine sediments into the soil solution or groundwater, and flushing of salts off of roads, landscapes and stream banks during and following precipitation events. Typically, groundwater contains more salt than surface water. Additionally, the amount of salt found in irrigation water generally is greater in arid and semi-arid areas than in humid and sub-humid areas.
Salinity directly affects plant growth, generally has an adverse effect on agricultural crop performance, and can also affect soil properties. Consequently, without knowledge of both soil and water salinity and correspondingly appropriate management, long-term irrigated crop productivity can decrease.
Salt is defined as a water-soluble compound resulting from the combination of an acid and a metal. Generally, we associate the terms saline, salt, and salinity with sodium chloride (NaCl), otherwise known as common table salt. However, salinity of irrigation water generally is a combination of numerous salts. The cations and anions most frequently found in irrigation water are: sodium (Na+), calcium (Ca2+), magnesium (Mg2+), chloride (Cl-), sulfate (SO42-), and bicarbonate (HCO3-). These salts are most often thought of by their common names (Table 2).

Table 2. Some salts commonly contributing to the salinity of water.

When drainage water is used for irrigation, there arises a need for grouping of soil cross section based on salinity level and also degree of drainage in order to calculate to the full permeability of soil and to avoid negative consequences of using saline water for irrigation. In this regard, the classification of soils, which is given in table 3 is of huge interest.

Table 3. Soil properties to choose land for irrigation