QUALITY OF IRRIGATION WATER

The quality of irrigation water depends upon the kind and amount of salts is dissolved in it

By Dr. S.M. ALAM
Dec 20 - 26, 2004

While ample irrigation water is necessary for a productive crop, poor quality water can cause soil-related problems that negatively impact growth. Some of the predominant soil-related problems that affect growth include salinity (high soluble salts), zinc deficiency, phosphorus deficiency and excessive sodium, which causes poor physical soil conditions.

The quality of irrigation water depends upon the kind and amount of salts is dissolved in it. A chemical analysis will show what salts are present and in what amount?

Salts in irrigation water: Water, as it comes in contact with rocks and soils, dissolves salts that occur in these materials. These salts include such well-known substances as gypsum (calcium sulphate CaSO4.2H2O), espon salt (MgSO4.2H2O), glauber's salt (sodium sulphate, Na2SO4.10H2O), baking salt (sodium bicarbonate, NaHCO3) and table salt (sodium chloride, NaCl). Salts lose their identity when dissolved in water by separating into ions. Thus, when sodium chloride dissolves in water it breaks up into sodium and chloride ions. The principal ions in irrigation water are: calcium (Ca++), magnesium (Mg++), sodium (Na+), potassium (K+), carbonate (CO3--), bicarbonate (HCO3-), sulphate (SO4--), chloride (Cl-), nitrate (NO3-). The first four are cations or basic ions and the last five are anions or acidic ions. Potassium, carbonate and nitrate usually occur in low concentrations. Boron is present in all irrigation water but in most cases, it occurs only in very low concentrations. Three problems may develop as a result of the use of salty irrigation water: a salinity problem, a sodium problem and a boron problem.

Salinity results from adding salt to soils, usually in irrigation water, faster than it is removed by natural processes, such as surface runoff and downward percolation. Irrigation water is the major contributor of soluble salts in Pakistan but excessive nutrient additions from fertilizers, manures of waste materials may also contribute to the accumulation of salts.

Correct diagnosis of problems concerning irrigation water quality are critical for efective management. Water quality testing is an important step in diagnosing existing problems and identifying potential problems. Several values are helpful in evaluating the quality of a particular water source. These include calcium conetnration, electrical conductivity (EC) and sodium adsorption ration (SAR). Water quality variable level considered to cause concern:

Bicarbonate (HCO3)

> 60 ppm (> 3 meq/L)

> 305 ppm (> 5 meq/L)

Chloride (CI) > 100 ppm (>3 meq/L) contributes to measured EC level. (High CI along may pose a problem for soybean rotation. Sodium adsorption ratio (SAR) > 10 causes sodic soil which has poor physical condition. Lower levels can cause injury in some cases.

The calcium and bicarbonate concentrations provide an estimate of the amount of lime that will be deposited and predictions can be made concerning the change in soil pH with long-term use. Electrical conductivity is a measure of the total salts that are dissolved in the water, which allows an estimate of the potential for salinity injury to rice with use of the water. Chloride concentration is important because of the potential for chloride toxicity to soybeans and because it often is the major contributor to high electrical conductivity. The SAR is a ratio of sodium to calcium and magnesium. This number provides an estimate of how much sodium is in the water relative to calcium and magnesium. The SAR allows the prediction of whether sodic (high sodium) soils are likely to develop with long-term use of the water.

CLASSIFICATION OF WATER

C1 Low salinity water can be used for irrigation with most crops on most soils, with little likelihood that a salinity problem will develop. Some leaching is required, but this occurs under normal irrigation practices except in soils of extremely low permeability [0 to 250 Electrical conductivity (Micromhos per centimeter at 25oC)].

C2 Medium salinity water can be used if a moderate amount of leaching occurs. Plants with moderate salt tolerance can be grown in most instances without special practices for salinity control [250 to 750 Electrical conductivity (Micromhos per centimeter at 25oC)].

C3 High salinity water cannot be used on soils with restricted drainage. Even with adequate drainage, special management of salinity control may be required, and plants with good salt tolerance should be selected [750 to 2,250 Electrical conductivity (Micromhos per centimeter at 25oC)].

C4 Very High salinity water is not suitable for irrigation under ordinary conditions but may be used occasionally under very special circumstances. The soils must be permeable, drainage must be adequate, irrigation water must be applied in excess to provide considerable leaching, and very salt - tolerant crops should be selected [2,250 to 5,000 Electrical conductivity (Micromhos per centimeter at 25oC)].

SODIUM CLASSIFICATION

Low sodium water (S1) can be used for irrigation on almost all soils, with little danger of the development of a sodium problem. However, sodium sensitive crops, such as stone fruit trees and avocados, may accumulate injurious amounts of sodium in the leaves.

Medium sodium water (S2) may present a moderate sodium problem in fine-textured (clay) soils unless there is gypsum in the soil. This water can be used on coarse-textured (sandy) or organic soils that take water well.

High - sodium water (S3) may produce troublesome sodium problems in most soils and will required special management - good drainage, high leaching, and additions of organic matter. If there is plenty of gypsum in the soil, a serious problem may not develop for some time. If gypsum is not present, it or some similar material may have to be added.

Very High sodium water (S4) is generally unsatisfactory for irrigation except at low or medium salinity levels where the use of gypsum or some other amendment makes it possible to use such water.

Salinity and water logging of arable lands is a problem of agriculture worldwide. In Pakistan, a total of 6.3 million hectares of land is estimated to be affected by salinity to varying degrees. The groundwater in salt affected area is also brackish and thus unfit for crop production. the permanent solution to the problems requires a comprehensive drainage system to control the rising water table and leaching of salts with good quality water. This drainage leaching combination, being energy intensive and expensive, cannot be applied on larger areas and other alternate options need to be explored. Different research organizations in the country have been pursuing research to find alternative biological methods to deal with the salinity problem.

The emphasis has been to economically utilize the waste saline lands and brackish water for growing salt tolerant plants rather than reclaiming the soils to grow conventional crops. The biological approach involves screening and selection of highly salt tolerant plant species/varieties from the natural existing germplasm or from the development through breeding, wide hybridization and other modern biotechnological techniques and then introducing the selected plants for increased establishment and productivity in saline areas. Many plant species/varieties have been studied for their relative salt tolerant using gravel culture technique. About a dozen of selected species include grasses like kallar grass, sporobolus, bushes such as Atriplex species, kodia/suaeda and tree species of genera Acacia, Eucalyptus/Causarina and Prosopis. Among the crop plants, barley, brassica and spinach have also shown good tolerance to salinity. These selected salt tolerant plants have been successfully grown on saline land using saline water for irrigation. The biomass obtained by cultivation of selected plant species can be used as fodder, green manure, and feed stock for pulp and paper, compost fertilizer biogas and alcohol.

Scarcity of water is worldwide problem, the gravity of which can be assessed from the fact that the next world war may be fought between nations on distribution of water. This is situation of the planet earth, which is 70% water, but it should also be kept in mind that less than 3 % of this water if fresh. Most of this water is either in the form of rice and snow or in deep ground water aquifers. Less than 1% of this water, which comes to about 0.01% of the earth water, is considered available for human needs. More than 1 billion of this earth's people do not have access to safe drinking water, about 2.4 billion people lack adequate sanitation and some 3.4 million die each year from water related diseases. In Pakistan, like in many other countries, precipitation exceeds rainfall and availability of water from the mountain tops and the reservoirs is depleting day by day. The situation, however, is not as bleak as it seems as with proper management of the available water, we can still manage to cope with the water shortage. This has been manifested in the past couple of years when inspite of sizeable reduction in availability of irrigation water, the yield of the crops did not reduce proportionally.