Cheyyar Watershed is situated in Palar Basin, which originates from the Jawad hills. The Cheyyar Watershed covers an area of 12˚13'3" to 12˚38'27" Northern Latitude and 78˚40'04" to 79˚24'24" Eastern Longitude (Figure 1). Cheyyar Watershed covers a geographical area of 2060 sq.km. The reserve forest in Cheyyar Watershed covers approximately 670 Sq.km. The most dominant type of Geology found in the study area is Charnockite. The predominant soil type in the study area is said to be Black and Red loam. The dominant soil texture found in the study area is calcareous clayey soil.

The quality of groundwater that influences agriculture productivity is called irrigation groundwater quality. From the major cation (Na, K, Ca, Mg) and Anion (CO₃, HCO₃, SO₄, Cl, NO₂, F), the parameters that can influence agriculture productivity have been calculated such as Magnesium Ratio (MR), Sodium Adsorption Ratio (SAR), Residual Sodium Carbonate (RSC), Total Hardness (TH), Soluble Sodium Percentage (SSP), Permeability Index (PI), Total Dissolved Solids (TDS), Electrical Conductivity (EC) and USSL classifications to evaluate the irrigation water quality (IWQ) 26, 27, 28, 29, 30.

The level of increased concentration of Mg over the concentration of Ca is known as the Magnesium ratio. Usually, the Mg and Ca in the groundwater has some equilibrium in the proportion of 4:1 to 2:1. In case of an increase in the concentration of Mg due to any geogenic or anthropogenic activities that can significantly affect the suitability of irrigation 31, 28. The MR has been calculated by using the Equation (Equation 1).

Continued use of Na, Ca, Mg and HCO₃ concentration-rich groundwater for irrigation can significantly reduce the permeability of the soil 32, 26. Soil with low permeability can resist water infiltrating and that can affect the growth of the plant. To quantify the permeability, the PI has been calculated by using the equation (Equation 2).

The effect of higher concentrations of CO₃ and HCO₃ in agriculture can be quantified by using RSC (Equation 3). The higher amount of RSC can effectively affect the agricultural yield 33, 34. The RSC can be calculated by using the Equation (Equation 3).

The Sodium Adsorption Ratio is the measure of relative concentration of Na with respect to Ca and Mg. SAR is the important measure to examine the groundwater quality for irrigation since it represents the sodium hazard of the study area 33, 30. The following equation can be used to examine the SAR (Equation 4).

The higher concentration of sodium can reduce the permeability and arrest the growth of the plants. All dissolved cations were used to determine the value of SSP using the equation. Higher SSP values represent low irrigation suitability and vice- versa (Equation 5)

Total Hardness (TH) is used to quality the hardness of water with using the concentration of multivalent cations (i.e., calcium and magnesium) in a given water sample 26, 35. TH is examined with using the following equation (Equation 6).

The USSL diagram (USSL, 1954) has been prepared by using the Aqua Chem 4.0. USSL diagram used to examine the sodium and salinity hazard of the study area and it is inevitable for the assessment of irrigation suitability. The spatial distribution maps have been prepared in Arc GIS platform using IDW interpolation method. The concentration and level of each factors have their own influence on agriculture. Based on the influence of each parameter, the weightage (R_i) has been assigned (Table 1) from 2 to 10 for low suitability to high suitability respectively. Finally, the IWQ has been examined by integrating the weights of every parameter 36.

For assessing groundwater quality for irrigation purposes, the USSL classification is a significant factor. It will help to understand the distribution of SAR and EC over the study region. The sodium hazard is classified as low to very high from S1 to S4 (Figure 2) and the salinity hazard is also classified from low to very high from C1 to C4. The result infers that the salinity hazard is more vulnerable than the sodium hazard. During the study period (2000 – 2019), the overall salinity hazard shifted from medium to high, high to very high. Some samples showed medium to high sodium hazard in the case of sodium hazard. These shifts happened may be due to anthropogenic effluents.

The concentration of Calcium and Magnesium is lower than Sodium concentration, and Sodium pollution is very high 36. The USSL limits have a standard measurement that divides the SAR level into two categories like Excellent (<5epm) and very good (5-10epm). The results (Figure 3) show that the excellent SAR class is dominant over the study area in the studied years, whereas the very good class are scattered in minute portions along the different years.

The RSC values are divided into three classes Good (<1.25 epm), Harmful (1.25 – 2.5 epm) and Serious (>2.5 epm) recommended by USSL (1954). The result of RSC shows that the dominant portion of the watershed falls under good RSC class in all the years and during both seasons. Further, the harmful class was high in the year 2005 (Figure 3) and gradually decreased over the years in 2019. In 2015 pre-monsoon disturbance of RSC was a little high and spread over the region and in post-monsoon was highly harmful.

The EC values of the study area are categorised into four classes excellent, good, poor, and very poor (Figure 4). The result shows that the good EC class covers most of the region in all the years and following this excellent class covers the most. Then the poor class was distributed in minor portions in the years 2005 and 2010. In contrast, in the years 2015 and 2019, the poor EC class have been increased and primarily concentrated in the western portion of the watershed and especially had high distribution in the pre-monsoon season of 2015 and post-monsoon season of 2019. 

The TDS values of the study area are divided into excellent, good and poor. The excellent class is the dominant distribution in all the years during both seasons (Figure 4). The good TDS class are present in some minor portions of the study region during 2010 and 2015. But their distribution is high in the pre-monsoon season of 2005 in the eastern portions and both the seasons of 2019 in the western portions. The poor class of TDS are only found in trace amount during the analysed years.

An increase in the magnesium ratio can highly cause the soil and reduce the primary activities severely 31. A magnesium ratio below 35 ppm is considered to be low and above 350 ppm is considered very high (Figure 5). There were four classes divided Suitable, Unsuitable, Permissible, and Doubtful. In which the doubtful and unsuitable classes were dominant all the years. Notably, in the post-monsoon season of the year 2005 and the pre-monsoon season of the year 2019, almost the entire portion of the study region falls under the unsuitable class of MR class. In pre-monsoon from 2015 to 2019, there was a continuous increase in unsuitable groundwater. But in contrast, during the post-monsoon season, the unsuitable class is gradually decreased and converted to doubtful and suitable water.

The PI classes were categorised into 2 Suitable (>75%) and moderately suitable (25-75%) (Figure 5). The Cheyyar watershed the PI falls under the suitable class in all the years (2005, 2010, 2015 & 2019) during both seasons. There are no significant changes in PI values in the analysed years.

Total hardness is the quality of the mineral content in the water that is irreversible by boiling. The total hardness is equal to the total sodium and magnesium. TH results (Figure 6) are classified as excellent, good, poor, very poor and unsuitable. The excellent class covers almost 70 % of the area in all the years in both seasons. Then the good quality was distributed in the northeast and southwestern parts from 2005 to 2015 and increased to a larger extent during the pre-monsoon season of 2019. Further, the poor and very poor classes are seen only in central and southern part of Cheyyar watershed throughout the analysed years. 

The SSP is usually classified into three categories Good (<40), Permissible (40-60), and Unsuitable. In the study area, during the analysed year and in both the monsoons, the SSP values of groundwater samples are presiding by unsuitable category in the entire portion of the watershed (Figure 6). This is due to the infiltration of contaminated rainwater into subsurface during monsoonal rainfall. 

The Irrigation suitability of the Cheyyar watershed is derived for four years (2005, 2010, 2015 and 2019) (Figure 7) using the following factors discussed above. All the factors influencing irrigation suitability are analysed and weighted through weighted overlay analysis for computing the irrigation suitability. The result of Irrigation suitability is categorised into three classes as unsuitable (<55), moderately suitable (65-75), and marginally suitable (55-65) (Table 2). It is inferred that most of the portion of the study region falls under marginally suitable class in all the years. Then the unsuitable class are witnessed only in very minor portions (Table 3). Further the moderately suitable class is changing across the years where it is marked only in minor portions during 2005 to 2015 and is increased to large extent during the year 2019 and is mostly concentrated in the northeastern part of the watershed. The results clearly reveals that the marginally suitable class found during the years 2005 to 2015 are considerably reduced and some portions are converted into unsuitable class during the year 2019 which increased the distribution of unsuitable class from 3.84 in 2005 to 12.90 in 2019. If this condition continues over a longer period, the major portion of the area under marginally suitable class will get converted exponentially into unsuitable class in the future.