Agriculture

Irrigation Water Treatment

Water use for irrigation

Agriculture is by far the largest water use at global level. Irrigation of agricultural lands accounted for 70% of the water used worldwide. In several developing countries, irrigation represents up to 95% of all water uses, and plays a major role in food production and food security. Future agricultural development strategies of most of these countries depend on the possibility to maintain, improve and expand irrigated agriculture

On the other hand, the increasing pressure on water resources by agriculture faces competition from other water use sectors and represents a threat to the environment.

Water is a resource that may create tensions among countries down and upstream. Irrigated agriculture is driving much of the competition since it accounts for 70-90% of water use in may of these regions.

Source: Turkmenistan and Uzbekistan figures from David R. Smith “Climate Change, Water Supply, and Conflict in the Aral Sea Basin”, paper presented at the “Pri-Aral Workshop 1994″, San Diego State University, March 1994: Others from Peter H. Gleick, Water in Crisis (NY, Oxford University Press, 1993)

Within the European Union (EU) agriculture represents around 30% of total water abstraction. The intensity of irrigation in different countries obviously varies depending on the climate, the crops cultivated and the farming methods. For example, the role of irrigation is completely different in Southern European countries, where irrigation is essential for agricultural production, compared to Central and Western Europe.

In fact the major part of irrigated land in Europe is located in the South with Spain, Italy, France, Greece and Portugal accounting for 85% of the total irrigated area in the EU. For example, in Spain irrigated agriculture accounts for 56% of total agricultural production, occupying only 18% of the total agricultural surface.

Water resources for irrigation

Water used for agriculture comes from natural or other alternative sources.

Natural sources includes rainwater and surface water (lakes and rivers). These resources must be used in a sustainable way.

Rain water resources rely on the atmospheric conditions of the area. Surface water is a limited resource and normally requires the construction of dams and reservoirs with a significant environmental impact.

Alternative sources of irrigation water are the reuse of municipal wastewater and drainage water.

However the use of recycled water for irrigation may have some adverse impacts on the public health and the environment. This will depend on the recycled water application, soil characteristics, climate conditions and agronomic practices. Therefore it is important that all these factors are taken into account in the management of recycled water.

Lets study this a little bit further.

Reuse of water for irrigation

Water reuse for irrigation is a normal practice worldwide. In Europe, for example there is a large project in Clermont-Ferrand, France since 1997 where more than 10.000m3/day of tertiary treated urban wastewater are reused for irrigation of 700Ha of maize. In Italy more than 4000 Ha of various crops are irrigated with recycled water. Spain also counts with several similar projects.

The water quality used for irrigation is essential for the yield and quantity of crops, maintenance of soil productivity, and protection of the environment. For example, the physical and mechanical properties of the soil, ex. soil structure (stability of aggregates) and permeability, are very sensitive to the type of exchangeable ions present in irrigation

Salinity Hazard

Salt content in Irrigation Water – Can Be Remove Through Reverse Osmosis Plant

The excess of salts content is one of the major concerns with water used for irrigation. A high salt concentration present in the water and soil will negatively affect the crop yields, degrade the land and pollute groundwater.

The suitability of water reuse for irrigation with high salt content depends on the following factors:

– Salt tolerance of the type of crop

– Characteristics of the soil under irrigation

– Climate conditions. The quality of the irrigation water plays an essential role in arid areas affected by high evaporation rates and cause high concentrations of salt accumulating in the soil.

– Soil and water management practices

In general water reuse for irrigation purposes must have a low to medium salinity level (i.e. electrical conductivity of 0.6 to 1.7dS/m). (See table below).

Special account should be taken to coastal areas where the infiltration of sea water poses a high risk of salinity in the water that is then pump from wells to be used in irrigation. For example in Spain the overexploitation of groundwater resources for agriculture lowered the water table and as a consequence caused the seawater intrusion in the coastline.

Hazard TDS (ppm or mg/L) dS/m or mmhos/cm

None <500 <0.75

Slight 500-1000 0.75-1.5

Moderate 1000-2000 1.5-3.00

Severe >2000 >3.0

Salinity with moderate content of salts can be used if moderate leaching occurs.

Water with high saline (ECi>1.5) and sodium (SAR>6) should not be used for water irrigation. Nevertheless, in some places with water shortage, water with high salinity concentration is used as a supplement for other sources and therefore a good management and control is essential and the salt tolerance of the plants must be considered.

If water with a very high salinity is used (extreme water shortage circumstances) the soil must be permeable, drainage must be adequate, water must be applied in excess to provide considerable leaching and salt-tolerance crops should be selected.

Real hazard!! A percentage of 21% of total irrigated land is estimated to be damaged by salt (see table below).

Salinization of soils on Irrigated Lands

Source: Adapted from F. G hassemi,  A.J.Jakeman, and H.A. Nix, salinization of Land and Water Resources (Sydney: University of New South Wales Press, 1995)

If a farmer annually applies 10,000 tons of irrigation water to a Ha of crops, which is typical, between 2 and 5 tons of salt will be added to that land every year. Unless these salts are flushed out, enormous quantities can build up over the course of years or decades.

Measurement units of salinity

Salt concentration is taken by the total dissolved solids (TDS) expressed in mg of salt per liter of water (mg/L) or grams of salt per cubic meter of water (g/m3)

(i.e. mg/L= gr/m3 = ppm).

Salt concentration can also be measure by the electrical conductivity of irrigation water (ECi).

Electrical conductivity is normally expressed in millimhos per centimeter (mmhos/cm) or deci Siemens per meter (dS/m) or micro Siemens per centimeter (1.e. 1dS/m = 1000μS/cm).

The relationship between salt concentration (C) and electrical

conductivity (EC) is approximately C = 640 EC.

Another way of estimating salt concentration is by measuring the electrical conductivity of water extracted from a saturated soil sample (ECe).

The approximate relationship between the electrical conductivity of irrigation water (ECi) and soil salinity is ECe = 1.5 ECi, if about 15 percent of the applied water is draining from the crop root zone.

Salt tolerance of different crops

The yield of different crops in relation with the salinity content of the water used for irrigation, depends on the type of crop, soil and environmental conditions.

The most distinct signs of injury from salinity is reduced crop growth and loss of yield. Crops can tolerate salinity up to certain levels without a measurable loss in yield (salinity threshold). When the salinity level is bigger than the threshold, the crop yield reduces linearly as salinity increases.

C

Management Practices for Irrigating with Saline or Sodic Water

The following consideration should apply:

– Adequate internal drainage. This measure is intended to avoid free movement of water in the root area.

The appropriate leaching requirement depending on tolerance levels for specific crops should apply to avoid the accumulation of salt. For example if natural drainage is not adequate, a drainage system must be installed.

– Higher water availability in soil. At high salt concentrations plants will not absorb all the normally available water.

– Proper management and control of SAR and salinity controls. Ex. add soluble calcium such as gypsum (calcium sulphate) to decrease the SAR to a safe value.

Monitor of salt and sodium with saline-alkali soil tests every 1 to 2 years

Irrigation water quality

The water quality used for irrigation is essential for the yield and quantity of crops, maintenance of soil productivity, and protection of the environment. For example, the physical and mechanical properties of the soil, ex. soil structure (stability of aggregates) and permeability, are very sensitive to the type of exchangeable ions present in irrigation waters.

Irrigation water quality can best be determined by chemical laboratory analysis. The most important factors to determine the suitability of water use in agriculture are the following:

– PH

– Salinity Hazard

– Sodium Hazard (Sodium Adsorption Ration or SAR)

– Carbonate and bicarbonates in relation with the Ca & Mg content

– Other trace elements

– Toxic anions

– Nutrients

– Free chlorine

Parameters of reuse water with agronomic significance

Parameter Significance for irrigation with recycled water Range in secondary and tertiary effluents Treatment goal in recycled water

Total Suspended Solids Turbidity Measures of particles can be related to microbial pollution; it can interfere with disinfection; clogging of irrigation systems; deposition 5-50 mg/L <5-35TSS/L

1-30 NTU <0.2-35NTU

BOD5COD Organic substrate for microbial growth; can bring bacterial re-growth in distribution systems and microbial fouling. 10-30mg/L <5-45mgBOD/L

50-150mg/L <20-200mgCOD/L

Total coliforms Measure of risk of infection due to potential presence of pathogens; can bring bio-fouling of sprinklers and nozzles in irrigation systems <10-107cfu/100mL <1-200cfu/10mL

Heavy metals Some dissolved minerals salts are identified as nutrients and are beneficial for the plant growth, while others may be phytotoxic or may become so at high concentrations. Specific elements (Cd, Ni, Hg, Zn, etc) are toxic to plants, and maximum concentration limits exist for irrigation < 0.001mgHg/L<0.01mgCd/L

<0.02-0.1mgNi/L

Inorganic High salinity and boron are harmful for irrigation of some sensitive crops <450-4000mgTDS/L<1mgB/L

Chlorine residual Recommended to prevent bacterial re-growth; excessive amount of free Chlorine (>0.05mg/L) can damage some sensitive crops 0.5->5mgCl/L

Nitrogen Fertilizer for irrigation; can contribute to algal growth and eutrophication storage reservoirs, corrosion (N-NH4), or scale formation (P) 10-30mgN/L <10-15mgN/L

Phosphorus 0.1-30mgP/L <0.1-2mgP/L

Source of information: Valentina Lazarova Akiçca Bahri; Water Reuse for irrigation: agriculture, landscapes, and turf grass; CRC Press.

Menu of Options for Improving Irrigation Water productivity

Category Option or Measure

Technical – Land leveling to apply water more uniformly- Surge irrigation to improve water distribution

– Efficient sprinklers to apply water more uniformly

– Low energy precision application sprinklers to cut evaporation and wind drift losses

– Furrow diking to promote soil infiltration and reduce runoff

– Drip irrigation to cut evaporation and other water losses and to increase crop yields (see table below)

Managerial – Better irrigation scheduling- Improving canal operation for timely deliveries

– Applying water when most crucial to a crop’s yield

– Water-conserving tillage and field preparation methods

– Better maintenance of canals and equipment

– Recycling drainage and tail water

Institutional – Establishing water user organizations for better involvement of farmers and collection of fees- Reducing irrigation subsidies and /or introducing conservation -oriented pricing

– Establishing legal framework for efficient and equitable water markets

– Fostering rural infrastructure for private-sector dissemination of efficient technologies

– Better training and extension efforts

Agronomic – Selecting crop varieties with high yields per Liter of transpired water- Intercropping to maximize use of soil moisture

– Better matching crops to climate conditions and the quality of water available

– Sequencing crops to maximize output under conditions of soil and water salinity

– Selecting drought-tolerant crops where water is scarce or unreliable

– Breeding water-efficient crop varieties

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Sources: Amy L. Vickers, Handbook of Water Use and Conservation (Boca Raton, FL: Lewis Publishers, in press); J.S. Wallace and C.H. Batchelor, “Managing Water Resources for Crop Production”, “Philosophical Transactions of the Royal Society of London: Biological Science, vol. 352, pp.937-47 (1997)