Effects of agricultural land use on sediment and nutrient retention in valley-bottom wetlands of Migina catchment, southern Rwanda

Abias Uwimana; Anne A van Dam; Gretchen M Gettel; Kenneth Irvine

doi:10.1016/j.jenvman.2018.04.094

Effects of agricultural land use on sediment and nutrient retention in valley-bottom wetlands of Migina catchment, southern Rwanda

J Environ Manage. 2018 Aug 1:219:103-114. doi: 10.1016/j.jenvman.2018.04.094. Epub 2018 May 4.

Authors

Abias Uwimana¹, Anne A van Dam², Gretchen M Gettel³, Kenneth Irvine⁴

Affiliations

¹ IHE Delft Institute for Water Education, Aquatic Ecosystems Group, P.O. Box 3015, 2601 DA Delft, The Netherlands; University of Rwanda, College of Science and Technology, Rwanda.
² IHE Delft Institute for Water Education, Aquatic Ecosystems Group, P.O. Box 3015, 2601 DA Delft, The Netherlands. Electronic address: a.vandam@un-ihe.org.
³ IHE Delft Institute for Water Education, Aquatic Ecosystems Group, P.O. Box 3015, 2601 DA Delft, The Netherlands.
⁴ IHE Delft Institute for Water Education, Aquatic Ecosystems Group, P.O. Box 3015, 2601 DA Delft, The Netherlands; Aquatic Ecology and Water Quality Management, Wageningen University, P.O. Box 47, 6700 AA, Wageningen, The Netherlands.

PMID: 29734014
DOI: 10.1016/j.jenvman.2018.04.094

Abstract

Factors affecting the retention and export of water, sediments (TSS), nitrogen (TN) and phosphorus (TP) were examined in the Migina river catchment, southern Rwanda from May 2012 to May 2013. Landscape characteristics and seasonal changes in land use and land cover (LULC), rainfall, discharge, and area-specific net stream yields of TSS, TP and TN were measured monthly in 16 reaches of the Munyazi sub-catchment with five valley bottom LULC categories (grass/forest, ponds/reservoirs, ploughed, rice, and vegetables). Valley bottoms dominated by grass/forest and ponds/reservoir types were generally associated with positive net yields of nutrients and sediments, while those with agricultural land covers had a net negative yields, resulting in net export. Water was retained only in reaches with ponds/reservoirs. Seasonally, there was a strong relationship between net yield and discharge, with 93%, 60% and 67% of the annual TSS, TP and TN yields, respectively, transported during 115 days with rain. During low flow periods, all LULC types had positive net yields of TSS, TP and TN (suggesting retention), but during high flow periods had negative net yields (suggesting export). Significant effects of hillside land use on sediment and nutrient yields were also found. Stream and river water quality in Rwandan valley bottoms are at risk of further deterioration due to declining natural ecosystems (grassland and forest) and increasing agricultural and urban development. It is important to adopt appropriate land management practices (minimal tillage, optimization of water use, strategic implementation of retention ponds and vegetation buffer zones) to intercept TSS, TP and TN in runoff from storm water and agricultural areas. Special attention is needed for critical periods of the year when farming activities (e.g. land preparation, fertilizer application) coincide with high flow events.

Keywords: Agriculture; Catchment management; Land use; Nutrient retention; Sediment retention; Water quality; Wetlands.

MeSH terms

Agriculture*
Environmental Monitoring
Nitrogen
Phosphorus
Rivers
Rwanda
Water Movements*
Water Pollutants, Chemical*
Wetlands*

Substances

Water Pollutants, Chemical
Phosphorus
Nitrogen