Water Quality Criteria For Irrigation Biology Essay

Water quality for Irrigation refers to its suitability for agricultural use. The concentration and

composition of dissolved constituents in water can be determined to know its quality for

irrigation use. Quality of water is an important consideration in any appraisal of salinity or

alkalinity conditions in an irrigated area. Good quality of water (good soil and water

management practices) can promote maximum crop yield. The most important chemical and

physical characteristics of water (groundwater & surface water) for irrigation purpose are as

follows:

Classification of salinity (EC) in groundwater

EC is an assessment of all soluble salts in samples. The most influential water quality guideline

on crop productivity is the water salinity hazard and is a measure of electrical conductivity (EC).

The higher the EC, the lesser suitable water is available to plants, because plants can only

transpire "pure" water, useable plant water in the soil solution decreases dramatically as EC increases. The amount of water transpired through a crop is directly related to yield; therefore

irrigation water with high EC reduces yield potential.

The Electrical conductivity (EC) of the groundwater in study area varies from 204-1946

µs

-1

/cm and 548 - 2472 µs

-1

/cm in pre-and post-monsoons respectively (Table 5a). Based on the

EC the groundwater of Chinnaeru river basin has been classified into 4 classes after Handa, 1969

(Table 3a). Majority of groundwater samples from both pre- and post- monsoon periods have

medium to high salinity and hence the groundwater is not good for irrigation.

The surface water of the Chinnaeru river basin is comparatively lower EC (Table 5b) than

that of groundwater (Table 5a). Many surface water samples both from pre- to post- monsoons

have low to medium salinity and they are good for irrigation.

Hydrochemical facies

The hydrochemical evolution of groundwater can be understood by the plotting the major

cations and anions in piper trilinear diagram (piper, 1944). This diagram reveals similarities and

dissimilarities among groundwater samples because those with similar qualities will tend to plot

together as groups (Todd, 2001). This diagram is very useful in bringing out chemical

relationships among groundwater in more definite terms (Walton, 1970). The geochemical

evolution can be understood from the piper plot, which has been divided into six sub categories

Wilcox Model

The total concentration of soluble salts in irrigation water can be expressed as low (EC =<250 µS

cm-1

), medium (250-750 µS cm-1

), high (750-2250 µS cm-1

) and very high (>2250 µS cm-1

); and

classified as C-1, C-2, C-3 and C-4 salinity zones respectively (Richards, 1954, Abhay Kumar

Singh et al.,2011). While a high salt concentration (high EC) in water leads to formation of

saline soil and a high sodium concentration leads to development of an alkaline soil. Salinization

is one of the most prolific adverse environmental impacts associated with irrigation. Saline

condition severely limits the choice of crop, adversely affect crop germination and yields and can

make soils difficult to work. Excessive solutes in irrigation water are a common problem in

semiarid areas where water loss through evaporation is maximum. Salinity problem encountered

in irrigated agriculture are most likely to arise where drainage is poor. This allows the water

table to rise close to the root zone of plants, causing the accumulation of sodium salts in the soil

solution through capillary rise following surface evaporation of water.

The sodium or alkali hazard in the use of water for irrigation is determined by the

absolute and relative concentration of cations and is expressed in terms of sodium adsorption

ratio (SAR). Irrigation water is classified into four categories on the basis of sodium adsorption

ratio (SAR) as: S-1(<10), S-2(10-18), S-3(18-26) and S-4 (>26). There is a significant

relationship between SAR values of irrigation water and the extent to which sodium is adsorbed

by the soil. High sodium and low calcium in water rises the cation-exchange between water and

soil and is responsible for saturated sodium in an irrigated area. This can destroy the soil

structure due to dispersion of Na+ in the clay particles.

The calculated values of SAR for groundwater range from 1.32 to 9.70 in pre-monsoon

and from 1.02–12.10 in post-monsoon periods. The EC ranges for groundwater from 204 to 1946

in pre-monsoon and from 548 to 2472 in post-monsoon periods.

In Wilcox diagram (Fig.11a), The EC is taken as salinity hazard and SAR as alkalinity

hazard, shows low alkalinity hazard (S1) and high salinity hazard (C3) for majority of

groundwater samples from both seasons. However, one sample CR-43 fall in S1-C1 class in pre

monsoon, while few samples from post-monsoon represent medium to high alkalinity hazard (S2-S3) and high to very high salinity (C3-C4). It seems that there is a gradual increase in both

alkalinity and salinity characters from the groundwater samples during pre-to post-monsoon

periods due to long term precipitation and water–rock interaction in space and time.

The calculated values of SAR for surface water range from 1.12 to 8.00 in pre-monsoon

and from 0.72 to 8.40 in post-monsoon. The EC ranges from 190 to 1709 in pre-monsoon and

from 330-2460 in post-monsoon peroids. The surface water samples show a range from S1-S2

alkalinity hazard and C1-C3 range of salinity hazard in pre-monsoon periods, whereas in post

monsoon, the surface water samples show similar alkalinity hazard (S1-S2) but different salinity

hazard (C2-C4) when compared to pre-monsoon period