Deliverable 16. Training course: Irrigation and saline water management - Handbook to be implemented in regional extension program (AAUG)

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Water Quality Evaluation for Irrigation Purposes

Factors govern water use for agricultural purposes:

¨                     Quantity and availability of water.

¨                     Economics of water.

¨                     Soil type (textures and structure).

¨                     Crop patterns.

¨                     Irrigation Techniques.

¨                     Quality of Water

The following parameters should be considered to evaluate the suitability of water for irrigation purposes:

       1.         Salinity

The quantity and kind of salts present in the groundwater is probably the most important single parameter for evaluating the suitability of water for irrigation. Salinity of irrigation water is usually determined by measuring its electrical conductivity and is the most important parameter in determining the suitability of water for irrigation. The electrical conductivity is expressed as mmho/cm or decisiemens per meter (dS/m). Salinity is expressed also into (TDS) which is the concentration of soluble salts in the water sample in mg/l , where:

EC * 640 = ppm (part per million) or mg/l

EC * 0.36 = Osmotic pressure

EC * 10 = meq/ L of total cations or anions.

% Salts * 10,000 = ppm

Potential problems are related to the total salt content, to the type of salt or to excessive concentration of one or more elements. In a long term strategy, the total quantity of salt applied in the soil with the irrigation effluent (salt added) and the rate at which salt is removed by leaching and crop uptake (salt removed) should be approximately the same.

 The presence of salts affects plant growth in three ways:

¨                     Osmotic effects which caused by disolved salts concentration in the soil water.

¨                     Specific ion toxicity, caused by the concentration of an individual ion ,e.g. boron, sodium, and chloride.

¨                     Soil particles dispersion caused by high sodium and low salinity.

In irrigated areas, salts originate from the local ground water or from salts in the applied water. Under such conditions, good drainage is essential to allow movement of water and salt below the root zone. Salinity control is of particular concern in areas where annual rainfall is not adequate to maintain an acceptable salt balance. This concern is acute in arid and semi-arid regions throughout the world, and should be accounted for planning purposes particularly the increment of salts in the domestic water after using it in the houses as shown below in table 1.

Table 1. Typical minerals increase from domestic water use.

 Source: Metcalf & Eddy, 1991