10 Hydrology and Climate Impacts on Streamflow and Sediment …
235
a massive increase in human settlement and cultivation on steep slopes without
application of appropriate soil conservation measures, which accounts for the high
rates of soil erosion in the basin (see also Walsh et al. 2004). Soil erosion in the
basin is influenced by several factors, including topography, rainfall pattern, landuse type, degree of vegetation cover, and soil erodibility (cf. ICRAF 2000 and 2002;
Swallow et al. 2002; Walsh et al. 2004). Soil erosion and resulting land degradation
have cumulatively altered the hydrologic response of the basin such that streamflow
responds quite rapidly to rainfall storms. The basin is characterized by relatively high
rates of sediment load which are estimated to range between 252,288 tons/year and
1.12 × 10
6 tons/year (see also ICRAF (2000 and 2002). These results are consistent
with those reported from other degraded river basins in the Kenya highlands (Oruma
et al. 2017; Njogu et al. 2018; Mwendwa et al. 2019; Kitheka et al. 2019).
During flood periods, the sediment load of the Nyando River increases more than
100-fold, although this occurs for a relatively short period of less than one month. This
study shows that during flood flows, the river discharges 684,288 tons of sediment
into Lake Victoria, which means that during flood conditions, the river discharges
almost 50% of its total annual sediment load in just one month. This indicates that the
Nyando River discharges a high volume of sediment to Lake Victoria on an annual
basis. There is a need to implement a long-term sediment load monitoring program
in order to establish the significance of discharge contributions from other rivers to
total river sediment load discharged into Lake Victoria (cf. Oruma et al. 2017). The
entire basin is characterized by severe soil erosion and land degradation (ICRAF
2002).
There is a relationship between sediment transport and river discharge. An increase
in river discharge results in an increase in TSSC and consequently an increased
suspended sediment load. The TSSC usually increases with river discharge because
the increased flow generally originates from direct surface runoff on degraded lands
(cf. Hai et al. 2000). Because of significant inter-annual variations in rainfall that
subsequently induce inter-annual variations in streamflow, it is expected that sediment yield will also exhibit significant inter-annual variation. There appears to be an
increase in the degree of variability, including an increase in frequency and magnitude
of extreme rainfall events and high streamflow in the basin. It is therefore expected
that the variability of sediment yield will also increase. These variations may be a
result of climate change and/or of normal climatic variation. Previous studies have
shown that within the basin, most of the surface runoff is generated from degraded
lands which are also the source areas of sediment transported by the river. Therefore,
any serious soil conservation program must focus on reducing erosion of soil from
degraded areas (cf. Swallow et al. 2002).
5.3 Flooding in the Lower Nyando Basin
Flooding in the Nyando River Basin is mainly limited to the lower parts of the Kano
plains. Most floods occur during the long rainy season between March and May.
235
a massive increase in human settlement and cultivation on steep slopes without
application of appropriate soil conservation measures, which accounts for the high
rates of soil erosion in the basin (see also Walsh et al. 2004). Soil erosion in the
basin is influenced by several factors, including topography, rainfall pattern, landuse type, degree of vegetation cover, and soil erodibility (cf. ICRAF 2000 and 2002;
Swallow et al. 2002; Walsh et al. 2004). Soil erosion and resulting land degradation
have cumulatively altered the hydrologic response of the basin such that streamflow
responds quite rapidly to rainfall storms. The basin is characterized by relatively high
rates of sediment load which are estimated to range between 252,288 tons/year and
1.12 × 10
6 tons/year (see also ICRAF (2000 and 2002). These results are consistent
with those reported from other degraded river basins in the Kenya highlands (Oruma
et al. 2017; Njogu et al. 2018; Mwendwa et al. 2019; Kitheka et al. 2019).
During flood periods, the sediment load of the Nyando River increases more than
100-fold, although this occurs for a relatively short period of less than one month. This
study shows that during flood flows, the river discharges 684,288 tons of sediment
into Lake Victoria, which means that during flood conditions, the river discharges
almost 50% of its total annual sediment load in just one month. This indicates that the
Nyando River discharges a high volume of sediment to Lake Victoria on an annual
basis. There is a need to implement a long-term sediment load monitoring program
in order to establish the significance of discharge contributions from other rivers to
total river sediment load discharged into Lake Victoria (cf. Oruma et al. 2017). The
entire basin is characterized by severe soil erosion and land degradation (ICRAF
2002).
There is a relationship between sediment transport and river discharge. An increase
in river discharge results in an increase in TSSC and consequently an increased
suspended sediment load. The TSSC usually increases with river discharge because
the increased flow generally originates from direct surface runoff on degraded lands
(cf. Hai et al. 2000). Because of significant inter-annual variations in rainfall that
subsequently induce inter-annual variations in streamflow, it is expected that sediment yield will also exhibit significant inter-annual variation. There appears to be an
increase in the degree of variability, including an increase in frequency and magnitude
of extreme rainfall events and high streamflow in the basin. It is therefore expected
that the variability of sediment yield will also increase. These variations may be a
result of climate change and/or of normal climatic variation. Previous studies have
shown that within the basin, most of the surface runoff is generated from degraded
lands which are also the source areas of sediment transported by the river. Therefore,
any serious soil conservation program must focus on reducing erosion of soil from
degraded areas (cf. Swallow et al. 2002).
5.3 Flooding in the Lower Nyando Basin
Flooding in the Nyando River Basin is mainly limited to the lower parts of the Kano
plains. Most floods occur during the long rainy season between March and May.