Water storage and quality regulation in restored and degraded sections of Rufuha Wetland in Ntungamo District, Uganda

Abstract

Wetlands restoration reinstates ecosystem services, reviving their potential as nature-based solutions. While a wealth of literature exists on provisioning services from site-specific wetlands with restored and degraded sections coexisting, specific hydrological benefits remain poorly studied and less documented. This study assessed and quantified water quality regulation and storage in the restored and degraded sections of Rufuha wetland in Ntungamo District, Western Uganda, dominated by papyrus and terrestrial grassland, respectively. Monitoring stations were established at their inlets and outlets, where flow discharges were measured using the velocity-area method, physical parameters were measured in situ, and samples for nutrient analysis in the flow were collected from October 2023 to April 2024 for laboratory analysis. The restored section showed significantly higher nutrient retention and water storage capacity than the degraded section (p < 0.05). It significantly stored larger quantities of water, retaining about 66.8 % of the total water input. The degraded section significantly lost more water by flow through than it retained, exhibiting 0 % storage and a storage deficit of 45 %. Generally, the restored section significantly attenuated the peak flow by 57.6 %, reducing it by 14.3 m³/s, and peak flow attenuation in response to precipitation events showed an insignificant (p > 0.05) 30 % reduction, equivalent to 30 m³/s. The degraded section significantly amplified peak flow by 51% overall and by 114.3% in response to precipitation events. Electrical conductivity (EC) significantly decreased by 40.8 % (from 505 to 299 µS/cm) in the restored section, while the degraded section showed a non-significant increase of 2.9%. Temperature and potential hydrogen (pH) showed slight, non-significant increases downstream, with higher values observed in the degraded section. Mass flux analysis revealed significant reductions in the restored section for dissolved oxygen (DO, 79.7 %), total dissolved solids (TDS, 72.8 %), total suspended solids (TSS, 77.5 %), orthophosphate (PO₄³⁻, 54.6 %), total phosphorus (TP, 51.9 %), ammonium-nitrogen (NH₄-N, 58.0 %), and total nitrogen (TN, 69.5 %). Conversely, the degraded section showed an insignificant reduction in TSS (12.4 %) and an increase in NH₄-N (44.5 %), along with significant increases in DO (57 %), TDS (53.6 %), PO₄³⁻ (51.4 %), TP (51.4 %), and TN (80 %). Retention per unit area was relatively low in both wetland sections. The restored section showed a net loss in yield for DO (3.1e-⁵ g m-2 s-1), TSS (4.8e-⁴ g m-2 s-1), TDS (8.2e-⁴ g m-2 s-1), TN (1.7e-⁷ g m-2 s-1), TP (2.5e-⁷ g m-2 s-1), PO₄³⁻ (2.5e-⁷ g m-2 s-1), and NH₄-N (1.9e-⁸ g m-2 s-1). In contrast, the degraded section exhibited a net loss for TSS (1.68e-⁵ g m-2 s-1) only, with net production for all other parameters, each less than (-1.61e-⁴ g m-2 s-1). Findings emphasize the need to upscale wetland restoration as a nature-based solution to enhance ecosystem services.