Basinwide analysis of water resources and pollute transport using a distributed parameter model

Abstract

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 fertilizers used in the upstream areas remain unresolved. This study incorporated the Water and Energy Transfer Processes (WEP) model to assess the water resources and pollutant transport of the catchment concerning the present status and six possible future scenarios. The required data for the model runs including meteorological, geographical, hydrological, and data related to water quality and anthropogenic activities, were collected and processed identifying the suitable model parameter values. The amounts of N and P in fertilizers applied in this catchment exceeded the actual plant requirement. In both wet and dry seasons, the differences between the measured water quality parameters in upstream and downstream were not statistically significant. The model results of the hydrological component showed that the catchment response to the rainfall was highly regulated due to reservoir storage effect. The model results of the material transport component showed that, 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 PO43- were within the ranges of the previously published results. Scenario analysis found almost all water quality parameters reduced with the reduction of fertilizer input (maximum 30.64% reduction) and with the increase in temperature (maximum 2.27% reduction), but they increased with the increase in rainfall (maximum 13.49% increase). The findings will be useful in identifying best water resources management practices and coping with the residual N and P in streams and water bodies in a more pragmatic manner.