Contributions of Human Activities and Climatic Variability to Changes in River Rwizi Flows in Uganda, East Africa

Abstract

This study employed Soil and Water Assessment Tool (SWAT) to analyze the impacts of climate variability and human activities on River Rwizi flows. Changes in land use and land cover (LULC) types from 1997 to 2019 were characterized using remotely sensed images retrieved from Landsat ETM/TM satellites. SWAT was calibrated and validated over the periods 2002–2008 and 2009–2013, respectively. Correlation between rainfall and river flow was analyzed. By keeping the optimal values of model parameters fixed while varying the LULC maps, differences in the modeled flows were taken to reflect the impacts of LULC changes on rainfall–runoff generation. Impacts due to human activities included contributions from changes in LULC types and the rates of water abstracted from the river as a percentage of the observed flow. Climate variability was considered in terms of changes in climatic variables such as rainfall and evapotranspiration, among others. Variability of rainfall was analyzed with respect to changes in large-scale ocean-atmosphere conditions. From 2000 to 2014, the portion of River Rwizi catchment area covered by cropland increased from 23.0% to 51.6%, grassland reduced from 63.3% to 37.8%, and wetland decreased from 8.1% to 4.7%. Nash–Sutcliffe Efficiency values for calibration and validation were 0.60 and 0.71, respectively. Contributions of human activities to monthly river flow changes varied from 2.3% to 23.5%. Impacts of human activities on the river flow were on average found to be larger during the dry (14.7%) than wet (5.8%) season. Using rainfall, 20.9% of the total river flow variance was explained. However, climate variability contributed 73% of the river flow changes. Rainfall was positively and negatively correlated with Indian Ocean Dipole (IOD) and Niño 3, respectively. The largest percentages of the total rainfall variance explained by IOD and Niño 3 were 12.7% and 9.8%, respectively. The magnitude of the correlation between rainfall and IOD decreased with increasing lag in time. These findings are relevant for developing River Rwizi catchment management plans.

Modelling Land Use/Land Cover Change of River Rwizi Catchment, South-Western Uganda Using GIS and Markov Chain Model

“…These research findings conform to results from previous studies; [52] [53] [54] [55] [56], all in the upper Rwizi catchment, where cropland, built up area and forestland increased overall while grazing land and wetland area decreased. Further investigations by [57] [58], in the Mt.…”

“…Further investigations by [57] [58], in the Mt. Elgon catchment, [59], in somodo watershed south western, Ethiopia, [60], in Tana River Basin, Kenya, [44], in Kilombero catchment in Tanzania, [61], in the Gaborone dam catchment, Botswana , [56] [62], in the uMngeni river catchment, South Africa and [63], in the Andassa watershed in the Upper Blue Nile basin of Ethiopia, all reported a positive change in cropland and built up area while grazing land registered a negative change, forest cover increased in some and decreased in others in their respective analysis periods. The observed trends point to a high degree of universality in both proximate and underlying drivers of land cover changes across different spatial distributions in Africa.…”

Enrichment, Bioaccumulation and Health Risks of Trace Metals in Soils and Leafy Vegetables Grown on the Banks of the Ugandan Lifeline River, River Rwizi

“…It has various tributaries emanating from the hilly regions of the Nkore area, covering a total distance of approximately 55 km with a catchment area of 2521 km 2 [22]. The river flows eastward for about 57 km until the gauge at Mbarara water works before it joins the Kagera transboundary river, which then pours into L. Victoria and progresses through the White Nile until the Mediterranean Sea in Egypt [30].…”

“…It has various tributaries emanating from the hilly regions of the Nkore area, covering a total distance of approximately 55 km with a catchment area of 2521 km 2 [22]. The river flows eastward for about 57 km until the gauge at Mbarara water works before it joins the Kagera transboundary river, which then pours into L. Victoria and progresses through the White Nile until the Mediterranean Sea in Egypt [30]. Also known as the "Life-line River" [20,30], River Rwizi is the main river that traverses Mbarara District and is thus the principal source of water for millions of people and livestock in Mbarara City boroughs.…”

“…The river flows eastward for about 57 km until the gauge at Mbarara water works before it joins the Kagera transboundary river, which then pours into L. Victoria and progresses through the White Nile until the Mediterranean Sea in Egypt [30]. Also known as the "Life-line River" [20,30], River Rwizi is the main river that traverses Mbarara District and is thus the principal source of water for millions of people and livestock in Mbarara City boroughs. It is the biggest and longest river in the Ankole sub-region, serving 14 districts (Buhweju, Bushenyi, Isingiro, Kiruhura, Kyotera, Lyantonde, Rakai, Sheema, Mbarara, Rubirizi, Rwampara, Sembabule, and Ntungamo).…”