Groundwater is one of the most important natural resources in the northern coastal belt of Sri Lanka, as there are no major water supply schemes or perennial rivers. Overexploitation, seawater intrusion and persistent pollution of this vital resource are threatening human health as well as ecosystems in the Jaffna Peninsula. Therefore, the main intent of the present paper is to apply geospatial techniques to assess the spatial variation of groundwater quality and availability for the sustainable management of groundwater in the coastal areas. The electrical conductivity (EC) and depth to water (DTW) of 41 wells were measured during the period from March to June 2014, which represents the dry period of the study area. Surface interpolation, gradient analysis, a local indicators of spatial autocorrelations (LISA) and statistical analysis were used to assess the quality and availability of groundwater. The results revealed that the drinking and irrigation water quality in the study area were poor and further deteriorated with the progression of the dry season. Good quality and availability of groundwater were observed in the western zone compared to other zones of the study area. A negative correlation was identified between depth to water and electrical conductivity in the western zone. Hence, relatively deep wells in the western zone of the study area can be used to utilize the groundwater for drinking, domestic and agricultural purposes. The outcomes of this study can be used to formulate policy decisions for sustainable management of groundwater resources in Jaffna Peninsula.

The tank cascade system (TCS) has been used for over 2000 years for water management in Sri Lanka. Since surface water is limited in the dry zone of Sri Lanka, agricultural production, especially of upland crops, relies on groundwater for irrigation. We sampled 29 wells in the Ulagalla cascade, a prominent TCS near Anuradhapura city in the dry zone of Sri Lanka, in Yala (dry) and Maha (wet) seasons, the two main cropping seasons in Sri Lanka. We evaluated the suitability of groundwater for irrigation using the analytic hierarchy process and geographical information system. Water quality did not vary notably between seasons. However, it deteriorated with the onset of high intensity heavy rain, especially during the Maha season. A water quality zoning map indicated that groundwater in 4% and 96% of the study area is suitable and moderately suitable for irrigation, respectively. Irrigation water quality in tank cascade landscapes and similar environments can be assessed using this methodology and our results.

With increasing demand for food and energy, there is a great need for improving sugarcane productivity. New cultivars and management strategies can be assessed using process-based crop models. Information on cultivars needs to be updated frequently, but it is still limited in most crop models. Therefore, it is important to identify possible candidates for varietal parameterization and calibration. Because sensitivity analysis is computationally expensive, we used a less expensive emulator-based approach to conduct a global sensitivity analysis using the apsimr package and GEM-SA software. We studied the sensitivity of four yield outputs of the APSIM-Sugar model to 13 parameters in rainfed and irrigated conditions in Japan and Sri Lanka. Unlike previous studies, our aim was to give comprehensive insights into the variation in sensitivity due to variation in climate. The results confirmed distinct variation of parameter influence between climates and between management conditions. We identify possible candidates for parameterization and calibration of new cultivars for APSIM-Sugar under different environments, and show the effect of variation in climate on variation in parameter influence under different management conditions. It was confirmed that both radiation use efficiency and transpiration efficiency were sensitive and have to be examined to use new cultivars, though these are not listed as cultivar parameters.

As measurements are expensive and laborious, the estimation of soil hydraulic properties using pedotransfer functions (PTFs) has become popular worldwide. However, the estimation of soil hydraulic properties is not the final aim but an essential input value for other calculations and simulations, mostly in environmental and crop models. This modeling approach is a popular way to assess agricultural and environmental processes. However, it is rarely used in Sri Lanka because soil hydraulic data are rare. We evaluated the functionality of PTFs (developed to estimate field capacity (FC) and the permanent wilting point (PWP) of Sri Lankan soils) for process-based crop models. We used the Agricultural Production Systems sIMulator (APSIM) as the test model. Initially, we confirmed the importance of PWP (LL15) and FC (DUL) by assessing the sensitivity of the soil input parameters on the growth and yield of rice under rainfed conditions. We simulated the growth and yield of rice and the four selected outputs related to the APSIM soil module using the measured and estimated values of FC and PWP. These simulations were conducted for ten years in 16 locations of Sri Lanka, representing wet, intermediate, and dry zones. The simulated total aboveground dry matter and weight of the rough rice, using both input conditions (the measured and PTF-estimated soil hydraulic properties), showed good agreement, with no significant differences between each other. Outputs related to the soil module also showed good agreement, as no significant differences were found between the two input conditions (measured and PTF-estimated soil hydraulic properties). Although the DUL and LL15 are the most influential parameters for the selected outputs of APSIM–Oryza, the estimated FC and PWP values did not change the predictive ability of APSIM. In this way, the functionality of PTFs for APSIM crop modeling is confirmed.

Agro-wells (shallow agricultural wells) serve as the main source of water for irrigation of agricultural crops in dry zone of Sri Lanka. Agro-wells are extensively used by the farmers in Malwathu Oya Cascade-I, located in dry zone without proper assessment of its suitability for irrigation. Hence, this study was conducted to evaluate the quality of groundwater for irrigation in the Malwathu Oya cascade-I. Some selected vital water quality parameters (electrical conductivity, pH, concentration of sodium, potassium, magnesium, calcium, carbonate, bicarbonate, arsenic, lead, cadmium, nitrate nitrogen, ammonium and nitrogen) and depth to groundwater were measured once a month from June 2012 to May 2013 to represent, both pre and post monsoons in 20 selected agro-wells. Salinity, residual sodium carbonate, sodium adsorption ratio and sodium percentage were calculated using measured parameters. Point based inverse distance weighted interpolation method available in GIS was used to produce maps of spatial distribution of water quality parameters. Based on the height of water column at the end of the dry season, 20%, 25%, 30% and 25% of the wells had low, moderate, high and very high groundwater potential for agriculture, respectively. It was observed that almost all the chemical parameters monitored tend to concentrate towards the lower part of the cascade during the pre-monsoon period. As per the US salinity hazard diagram, 5% of the wells had low salinity and low sodium (C1S1) while, 35% of the wells had medium salinity and low sodium hazard (C2S1). These sources can be used for irrigation with most crops on most soils with little danger of exchangeable sodium. Sixty percent (60%) of the water samples had high salinity, low sodium hazard (C3S1) and can be used with adequate drainage and special measures for salinity control.

Malwathu Oya cascade is one of the largest and intensively cultivated small tank cascade systems in Anuradhapura district of Sri Lanka. Agro-wells are the most valuable resource for many farmers in this cascade area as these shallow dug wells have a good potential to sustain crop production during the dry season. However, deterioration of groundwater quality during the dry period has been reported in many parts of the dry zone. Therefore, this study focused on the characterization of agro-well water for its suitability for irrigation in the Malwathu Oya cascade based on its quality and availability. Groundwater samples were collected during March to September 2012 to cover the dry period during which the water quality deterioration has been reported in the dry zone. Electrical Conductivity (EC), pH, concentrations of sodium, potassium, magnesium and calcium, and depth of groundwater were measured from 20 selected agro-wells. The residual sodium carbonate (RSC), sodium adsorption ratio (SAR) and sodium percentage (Na %) were calculated using the measured parameters. Based on the depth of groundwater available at the end of dry season, 35%, 25%, 25% and 15% of the wells recorded low, moderate, high and very high groundwater potential, respectively. The EC data showed that 35%, 55% and 10% of the wells had good, doubtful and unsuitable quality of irrigation water, respectively. According to SAR and RSC, all the wells had water good for irrigation. Based on the percentage sodium, 10% of the wells had excellent, 60% had good, 25% had permissible and 5% had doubtful quality irrigation water. According to the US salinity hazard diagram 40% of wells had medium salinity and low sodium hazard. Fifty percent of the samples had high salinity and low sodium hazard while 10 % of the samples had very high salinity and medium sodium hazard. Based on the water quality parameters measured, 25%, 15%, 50% and 10% of the wells in Malwathu Oya cascade fall into good, permissible, doubtful and unsuitable category for irrigation, respectively.