Distributed modelling of water resources and pollute transport in Malwathu Oya Basin, Sri Lanka

The Nachchaduwa sub-catchment (598.74 km²) of the Malwathu Oya basin is seasonally stressed in the dry periods and its downstream parts undergo intermittent floods during monsoon seasons while the fate and behaviour of excess nitrogen (N) and phosphorus (P) added to the waterways due to agricultural fertilisers used in the upstream areas remain unresolved. This study incorporated the Water and Energy Transfer Processes (WEP) model to assess the present status of the catchment concerning water resources and pollutant transport. Results showed that the catchment response to the rainfall is highly regulated due to reservoir storage effect (ungauged basin with regulated flows). The amounts of N and P in fertilisers applied in this catchment exceeded the actual plant requirement. In both wet [Maha cultivation season (October to March)] and dry [Yala cultivation season (April to September)] seasons, the differences between the upstream and downstream values of measured water quality parameters (N and P components - total, dissolved and particulate) in waterways were not statistically significant. On average, the wet season had about 5~7 times the dry season value of the total suspended solids (TSS) in the streams, and in both seasons, the modelled TSS, NO₃^- and PO₄^3- were within the ranges of the previously published results. This study will be continued in the future to analyse possible hydrological and material transport related scenarios to identify best water resources management practices and to pragmatically cope with the excess fertiliser usage, an issue commonly found in most of the similar catchments.