Soil erosion and sediment transport in Tanzania: Part I – sediment source tracing in three neighbouring river catchments

Maarten Wynants,Linus Munishi,Kelvin Mtei,Alex Taylor,Aloyce Patrick,Samuel Bodé,David Gilvear,Patrick Ndakidemi,William H Blake,Pascal Boeckx

doi:10.1002/esp.5217

Abstract

AbstractWater bodies in Tanzania are experiencing increased siltation, which is threatening water quality, ecosystem health, and livelihood security in the region. This phenomenon is caused by increasing rates of upstream soil erosion and downstream sediment transport. However, a lack of knowledge on the contributions from different catchment zones, land‐use types, and dominant erosion processes, to the transported sediment is undermining the mitigation of soil degradation at the source of the problem. In this context, complementary sediment source tracing techniques were applied in three Tanzanian river systems to further the understanding of the complex dynamics of soil erosion and sediment transport in the region. Analysis of the geochemical and biochemical fingerprints revealed a highly complex and variable soil system that could be grouped in distinct classes. These soil classes were unmixed against riverine sediment fingerprints using the Bayesian MixSIAR model, yielding proportionate source contributions for each catchment. This sediment source tracing indicated that hillslope erosion on the open rangelands and maize croplands in the mid‐zone contributed over 75% of the transported sediment load in all three river systems during the sampling time‐period. By integrating geochemical and biochemical fingerprints in sediment source tracing techniques, this study demonstrated links between land use, soil erosion and downstream sediment transport in Tanzania. This evidence can guide land managers in designing targeted interventions that safeguard both soil health and water quality.

Highlights

River catchments in Tanzania have some of the highest sediment yields of sub-Saharan Africa, linked in part to a distinct topography and the semi-arid climate (Vanmaercke et al, 2014), and to the effects of increasing land-use pressures (Borrelli et al, 2017; Wynants et al, 2019)
By using complementary sediment source tracing techniques, this study aims to assess the dominant sources of transported sediment in Tanzanian river systems distinguishing between catchment zones, dominant land-use types and erosional processes
The Makuyuni system is spatially and hydrologically complex (Figure 1 and Supporting Information Figure S1), wherein its northern tributaries drain the Monduli, Lesimingore and Lepurko volcanic highlands, dominated by Andosols and Leptosols (Nachtergaele et al, 2008). They subsequently flow to the middle elevation zone, dominated by Chernozems, and converge with each other and with the southern tributaries further down in the drier Maasai steppe, from which the main river flows towards Lake Manyara

Summary

Introduction

River catchments in Tanzania have some of the highest sediment yields of sub-Saharan Africa, linked in part to a distinct topography and the semi-arid climate (Vanmaercke et al, 2014), and to the effects of increasing land-use pressures (Borrelli et al, 2017; Wynants et al, 2019). The loss of permanent vegetation through deforestation, agricultural expansion and overgrazing is driving accelerating rates of erosion, which is causing a rapid depletion of soil resources, threatening food, water and livelihood security in the region (Fenta et al, 2020; Maitima et al, 2009). These processes are potentially amplified by natural rainfall variations (Ngecu & Mathu, 1999; Wynants et al, 2020) and projected increases in extreme climatic events (Borrelli et al, 2020).

Objectives

Methods

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Conclusion