Irrigation water quality index based suitability assessment of Mamunugama Tank cascade system

Abstract

Tank cascade systems are considered as one of the traditional land water management systems that have been accepted as globally important agricultural heritage system. This study was conducted to assess the suitability of water for irrigation purposes in the Mamunugama Tank Cascade System (TCS), located in the Kurunegala district, specifically in terms of water quality parameters that affect irrigation. This TCS consists of six interconnected tanks: Ulpath Tank, Athaudagama Tank, Ihala Thimbiriyawa Tank, Mamunugama Tank, Kandubodagama Tank, and Weera Tank. Three water samples were collected from each tank on a monthly basis over a period of seven months, from August 2023 to February 2024. Water samples were analyzed for selected water quality parameters. The metals (Ca, Mg, Na, K) required for calculating the irrigation indices were analyzed using a Flame Atomic Absorption Spectrometer, while EC was measured using a conductivity meter. The HCO3- was determined using a titrimetric method. Irrigation suitability was assessed using sodium percentage (Na%), sodium adsorption ratio, permeability index (PI), magnesium hazard, Kelly’s Ratio, and salinity hazard. Sodium percentage ranged from 20.6 ± 3.5 % to 30.8 ± 4.8% during dry months, graded all tanks as good. In wet months, Na% ranged from 13.2 ± 4.4 to 36.6 ± 51, graded as excellent to good. The possible sources of sodium in the water could include natural weathering of minerals in the soil and agricultural runoff containing sodium-based fertilizers. The Sodium adsorption ratio ranged from 0.25 ± 0.09 to 0.78 ± 0.10. It was graded as excellent across all tanks in both dry and wet months. Athaudagama Tank (89.78 ± 51.50) and Wera Tank (99.52 ± 22.43) have suitable water quality during dry months, whereas the other tanks have moderately suitable water based on PI. During wet months, the PI ranged from 111.69 ± 23.01 to 192.97 ± 38.22, indicating that all tank water was suitable for irrigation. According to the Magnesium Hazard assessment, Mamunugama and Wera Tanks were unsuitable for irrigation in both dry and wet months, while the remaining tanks were suitable. Kelly’s Ratio was well below the threshold value in all the tanks during both dry and wet months. There was a notable increase in salinity hazard during dry months compared to wet months. The Irrigation Water Quality Index, based on the parameters sodium percentage, sodium adsorption ratio, permeability index, magnesium hazard, Kelly’s ratio and salinity hazard, rated the water in all tanks as good to excellent for irrigation. This indicates a stable water quality profile, with occasional shifts in certain parameters during different seasons.