Chemical Composition of Irrigation Water:
1. Sources of irrigation water:
a. Rain water: Lowest salt content of all types of water.
2. Surface water:
a. River and canles, (flowing water)
b. Tank (Stagnant water)
c. Ground Water:
The salt water content of ground water is dependent on the source of water and on the course overs which it flows.
d. Sea water:
Sea water after its suitable dilution should be used for irrigation on sands to raise salt to tolerant crops.
2. The nature and amount of salts present in natural waters are effected by the following factors.
a. Physio-chemical characteristics of the rocks with which water is coming in contact.
b. Geological history of the area.
c. Climate of the area including duration and frequency of rainfall, humidity, intensity and duration of sunshine wind velocity, etc.
e. Chemical, physical and mineralogical characteristics of the soil with which water comes into contact when it percolates into the underground reservoirs.
f. Topography of the area
g. Plant and vegetation cover
h. Ground water mixing.
i. Human interference in brining about micro changes in the hydrological cycles, by discharging industrial effluents, use of fertilizers and chemicals etc.
Collection of Irrigation Water Samples:
The minimum quantity of water required for ordinary chemically analysis is about 2L. care should be taken to obtain a representative sample. Satisfactory water samples can be obtained by mixing several portions collected at different times. Samples from wells should be collected after the pump has been running for some time and sample from stream should be taken from the running water.
In general shorter the elapsed time between collection and analysis of a sample, the more reliable will be the analytical data.
Description of location, village, taluka, district, etc will be noted while collecting the water sample.
Water Quality Criteria for Irrigation:
The quantity of irrigation, water is judged by the following characteristics.
A. Total salt concentration in terms of EC or salinity.
The salinity of water refers to the concentration of calcium ( Ca2+) , Magnesium ( mg) , sodium ( Na) and potassium ( K) salts of chloride (Cl) , Sulphate (SO4) , Bicarbonate ( HCO3) , carbonates ( CO3) and nitrates ( NO3). These are the major salts in surface and groundwater. It is expressed as dSm-1 or millimphos /cm.
B. Relative proportions of cations or anions:
a. Sodium Adsorption Ratio (SAR):
A high sodium in irrigation water leads to development of sodic soils. A simple method of evaluating danger of high sodium water is the sodium adsorption ratio. An increase in SAR of irrigation water increases the SAR of the soil solution and thus ultimately increases the Exchangeable Sodium Percentage ( ESP ) of the soil.
The concentration are expressed in me/L.
b. Calcium / Magnesium Ratio:
In SAR equation, calcium and magnesium have been treated equal. Recent studies have shown that at the same level of salinity and SAR water with higher Mg/Ca ratio ( greater than one) cause more deteriorate in physical properties and induces more ESI in irrigated soils.
c. Residual Sodium Carbonate:
The presence of carbonates and bi-carbonates in irrigation water decreases the concentration of calcium and magnesium by precipitating them and thus ultimately concentration of the sodium increases in irrigation water.
It is calculated as: RSC( me/L) = CO3 Square + HCO3-) – (Ca2+ Mg2+)
Concentration in me/L.
C. Concentration of Certain Specific Elements:
Elements such as boron etc toxic to the plant growth beyond a concentration of few mg/L. Boron above 3 mg/ L and nitrate above 10 mg/L are harmful.