Investigating supplementary semi saline irrigation of taro and yam in Tonga
Abstract
Water security is an issue which plagues Pacific Island Nations and Territories (PICTs) despite being surrounded by water. Due to the genesis of coral atoll islands, such as Tonga �tapu, Kingdom of Tonga (Tonga) groundwater storage is minimal, and infrastructure is insufficient to capture enough rainfall for fresh water to support the island for more than 30 days. When it does not rain, crops growing under rainfed systems suffer from reduced inputs, increasing the negative effect of a drought, where fresh water must be flown in through foreign aid. Breakthroughs in thermodiffusive desalination technology presenting cheaper, more efficient and lower maintenance options for irrigation may provide relief from the consequential impacts of these droughts. Whilst the technology is vastly improved from preceding options, the product is brackish water which is mildly saline � unsuitable for human consumption. It is little understood how the environment and common crops grown in Tonga will react to supplementary saline irrigation. To evaluate the potential of this technology, the study conducted two field trials to assess the salinity tolerance of taro and yam to semi saline irrigation. Four treatments for taro, including pumped groundwater, rainfall only, 30mmol/L NaCl and 60mmol/L NaCl were applied at select rates based on actual rainfall. For yam, pumped groundwater and 60mmol/L NaCl was applied periodically to yams which were planted under rainout shelters to prevent rainfall interference. Plant growth parameters and physiological measurements were observed and analysed using statistical packages available in RStudio. The study found that under the treatments, taro exhibited no effects up to 30mmol/L NaCl and minor decrease in corm yield at 60mmol/L NaCl. For yam, no yield effect was identified up to 60mmol/L NaCl treatment. The soil analysis indicated that due to the management of saline application, soil salinity was temporarily raised and would likely return to baseline salinity levels if given appropriate periods of rest. Further study is required to assess the long-term impacts and best practice for supplementary saline irrigation. However, these results indicate that supplementary saline irrigation at rates up to 60mmol/L NaCl may provide a sustainable solution to supporting Tonga�s water security issues, however further studies will need to be conducted to assess the impact of semi saline solution to Tongan soils over time and multiple cropping cycles. Furthermore, the taro trial would benefit from being repeated in low rain conditions.