Field Irrigation and Water Quality

What is field irrigation?

Field irrigation is the large-scale delivery of water using centre pivot, wheel move systems or flood irrigation to irrigate field crops.

How much water do crops need?

For field crops other than vegetables, the water requirement ranges from 375 to 600 mm (15 to 24 inches). The range is greater for vegetables (75 to 500 mm), with lowest quantities required for short season crops, such as radishes. Crops with longer seasons have similar water uses as conventional crops. This is shown in Table 1.

Table 1: Crop water use for commonly grown irrigated crops in Saskatchewan (Information obtained from Sask Water Irrigation Handi-Facts)

Crop	Water Use
Crop	mm	in.
Wheat - Hard Red Spring and durum	400	16
Wheat - Canada Prairie Spring	450	18
Barley	400	16
Canary Seed	400	16
Canola	450	18
Mustard	400	16
Flax	375	15
Sunflower	500	20
Pea	450	18
Fababean	500	20
Lentil	380	15
Silage Corn	500	20
Alfalfa	600	24
Irrigated Pasture	600	24
Potatoes	500	20
Vegetables Radish Parsnips Onion Cucumbers - short season Pumpkin or squash Sweet Corn Broccoli, kohlrabi, kale Winter cabbage and brussel sprouts	75 to 100 400 to 450 400 300 500 350 to 500 200 500	3 to 4 16 to 18 16 12 20 14 to 20 8 20

Is water quality important to field irrigation?

Yes, poor quality water can severely affect crop yields and damage soils. Several laboratory determinations are required to evaluate common irrigation water quality problems. The most common are listed in Table 2.

Table 2: Laboratory determinations needed to evaluate common irrigation water quality problems (modified from Ayers and Westcot 1985)

Water parameter	Symbol	Unit	Usual range in irrigation water
Salinity
Salt Content
Electrical Conductivity	EC_w	dS/m	0 - 3
(or)
Total Dissolved Solids	TDS	mg/L	0 - 2,000
Cations and Anions
Calcium	Ca²⁺	mg/L	0 -800
Magnesium	Mg²⁺	mg/L	0 -120
Sodium	Na⁺	mg/L	0 -900
Carbonate	CO3^2-	mg/L	0 -6
Bicarbonate	HCO3^-	mg/L	0 -600
Chloride	Cl^-	mg/L	0 - 1,100
Sulphate	SO4^2-	mg/L	0 - 2,000
Nutrients
Nitrate-Nitrogen	NO₃-N	mg/L	0 - 10
Ammonium-Nitrogen	NH₄-N	mg/L	0 - 5
Phosphate-Phosphorus	PO₄-P	mg/L	0 - 2
Potassium	K⁺	mg/L	0 - 2
Miscellaneous
Boron	B	mg/L	0 - 2
Acid/Basicity	pH		6.0 - 8.5
Sodium Adsorption Ratio	SAR		0 -15

There may be a need to analyze levels of trace elements as well, particularly if ground water or sewage effluent is used for irrigation purposes. Levels of elements that can be toxic are shown in Table 3.

Table 3: Recommended maximum concentration of selected trace elements in irrigation water (Modified from Canadian Council of Ministers of the Environment 1987)

Element	Recommended maximum concentration (mg/L)	Remarks
Al (aluminum)	5	Can cause non-productivity in acid soils (pH<5.5), but more alkaline soils at pH >7.0 will precipitate the ion and eliminate any toxicity.
As (arsenic)	0.1	Toxicity to plants varies widely, ranging from 12 mg/L for Sudan grass to less than 0.05 mg/L for rice.
Be (beryllium)	0.1	Toxicity to plants varies widely, ranging form 5 mg/L for kale to 0.5 mg/L for bush beans.
Bo (boron)	0.5	Can cause various deformities of plant leaves.
Cd (cadmium)	0.01	Toxic to beans, beets and turnips at concentrations as low as 0.1 mg/L in nutrient solutions. Conservative limits recommended due to its potential for accumulation in plants and soils to concentrations that may be harmful to humans.
Co (cobalt)	0.05	Toxic to tomato plants at 0.1 mg/L in nutrient solution. Tends to be inactivated by neutral and alkaline soils.
Cr (chromium)	0.1	Not generally recognized as an essential growth element. Conservative limits recommended due to lack of knowledge on its toxicity to plants.
Cu (copper)	0.2	Toxic to a number of plants at 0.1 to 1.0 mg/L in nutrient solutions.
F (fluoride)	1	Inactivated by neutral and alkaline soils.
Fe (iron)	5	Not toxic to plants in aerated soils, but can contribute to soil acidification and loss of availability of essential phosphorus and molybdenum.. Overhead sprinkling may result in unsightly deposits on plants, equipment and buildings.
Li (lithium)	2.5	Tolerated by most crops up to 5 mg/L; mobile in soil. Toxic to citrus at low concentrations (<0.075 mg/L). Acts similarly to boron.
Mn (manganese)	0.2	Toxic to a number of crops at a few-tenths to a few mg/L, but usually only in acid soils.
Mo (molybdenum)	0.01	Not toxic to plants at normal concentrations in soil and water. Can be toxic to livestock if forage is grown in soils with high concentrations of available molybdenum.
Ni (nickel)	0.2	Toxic to a number of plants at 0.5 mg/L to 1.0 mg/L; reduced toxicity at neutral or alkaline pH.
Pb (lead)	0.01	Can inhibit plant cell growth at very high concentrations.
Se (selenium)	0.02	Toxic to plants at concentrations as low as 0.025 mg/L and toxic to livestock if forage is grown in soils with high levels of selenium. An essential element to animals but in very low concentrations.
Ti (titanium)	-----	Effectively excluded by plants; specific tolerance unknown.
V (vanadium)	0.1	Toxic to many plants at relatively low concentrations.
Zn (zinc)	2	Toxic to many plants at widely varying concentrations; reduced toxicity at pH >6.0 and in fine textured or organic soils.

The microbial safety of water is of particular concern to effluent irrigation. It is difficult to determine many of the disease-causing (pathogenic) organisms that may be present and to get a handle on this "indicator' organisms are used instead. The most common indicator organisms are the coliform bacteria (either total coliforms, fecal coliforms or one specific coliform, E. coli). When the coliform bacteria are low or not detected it is assumed that there are no pathogenic organisms present (this assumption is, unfortunately, not always correct). There are no comprehensive guidelines available, but it has been tentatively suggested that total coliforms should not exceed 1,000/100 mL and fecal coliforms should be kept below 1,000, 200, or 100 per 100 mL of water depending on which source is consulted.

The total soluble salt content of the water can also be of great concern. This issue is discussed in the fact sheet, Field Irrigation and Salinity in Water.

Where can I get my water analyzed?

Commercial laboratories that are able to analyze the composition of chemicals in irrigation water include: Envirotest Laboratories (Saskatoon), BDS Laboratories (Qu'Appelle), and Saskatchewan Research Council (Saskatoon). Specialized water analysis concerning the nutrient requirements of specific crops can be obtained from several U.S. Chemistry Laboratories including Scott's Testing Laboratories (Pennsylvania).

The bigger picture

Prairie surface water sources are typically suitable for large scale irrigation. Clogging is unlikely to occur due to the large nozzles used in centre-pivot irrigation systems, even when water with a high fouling potential is used (which could include high iron and/or dissolved organic matter). There are, however, many Saskatchewan groundwater sources that are not suitable for irrigation because the water quality is simply too poor for the plants (usually too saline). Other compounds can also present problems when ground water is used for irrigation, such as boron.

In order for wastewater irrigation to be used, it must be beneficial to agriculture and beneficial to the environment. When dealing with irrigation water of questionable water quality, it is essential to take into consideration climate, crops, irrigation practices and internal drainage. An irrigation specialist should be consulted to help producers evaluate the suitability of irrigation water and tailor it to a specific situation.

Acknowledgements

Strategic support and funding for this publication has been provided by the Canada-Saskatchewan Agri-Food Innovation Fund (AFIF) and WateResearch Corp.

This Fact Sheet is a part of the publication Water Quality Requirements for Saskatchewan's Agri-Food Industry, identifying key water quality needs for each sector. These guidelines were compiled from a variety of references, primarily including extension publications, reports, books and internet sources. Water quality is a vast and complex subject and readers are encouraged to consult with experts and refer to the scientific literature for a greater understanding into specific water quality needs.


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