Modeling diffuse water pollution under changing land use.

Abstract

Water is an essential requirement for all human activities and offers life support to many aquatic and non-aquatic ecosystems. However, the water resources are under immense pressure where over 60% of the world population is anticipated to live in fresh water scarce areas by 2030. Water quality impairment results from a combination of point source and non-point sources pollution from the land use and management activities happening within the catchment. Globally, the most prevalent water quality problem is eutrophication, a result of high-nutrient loads (mainly phosphorus and nitrogen). River Kyambura is a tributary to Kazinga channel, Western Uganda. The use of land in the catchment area encourages soil erosion and high nutrient shading from the catchment. This has impaired the useful qualities of water in the river. Furthermore, this continuous sediment and nutrient loading of River Kyambura has increased the trophic state of Kazinga channel and water vegetation has thrived at the mouth of river Kyambura. This study was carried to assess the impact of changing land use and management activities on Non-Point Source pollution within the Kyambura catchment. Land use change assessment for the period 2008 to 2042 was done using landsat imagery classified for 2008, 2015 and 2022 and projected for 2042 indicated land degradation happening with an increased land use for of Built-up Area (6.3%), Barren Land (3.5%), and Farming and Agriculture (3.1%) and a decrease in the percentage coverage of Wetland Areas (8.9%), forested areas (4.2%) and Shrub Vegetation (0.9%). The critical management activities that were identified as sources of pollution were fertilizer application and grazing. A SWAT model was set up for the Kyambura, calibrated for the period 2012-2016 and validated for 2017-2020 using the meteorological, hydrological and limited limnological data. The model was then used to assess the impact of changing land use and management activities on NPS pollution within the catchment. Three scenario runs corresponding to the different LULCs were done to assess the impact of the LULC change on TN and TP load. Sc0(2008) was taken as the base scenario and the results for TN and TP load were 242.7ton/ha/yr and 70.3ton/ha/yr respectively. All scenario runs produced increasing nutrient loads in the river. Sc1(2015) corresponding to the LULC changes between 2008 and 2015 showed a 30.5% and 23.5% increase in TN and TP respectively. Sc2(2022) corresponding to the LULC changes between 2008 and 2022 showed a 35.3% and 50.5% increase in TN and TP respectively. Sc3(2042) corresponding to the LULC changes between 2008 and 2042 showed an 88.2% and 150.2% increase in TN and TP respectively. It can be concluded that land use activities happening in the catchment are typical environmental degradation. There’s a positive correlation between the increase percentage coverage of built-up area, agricultural land and bare land with an increase in the nutrient load from the catchment. It is recommended that soil conservation practices, protection of the riparian buffer zones, wetland restoration and conservation, nutrient management and planning, among others are some of the mitigation measures that can be implemented to curb the water quality problem.