232
J. U. Kitheka et al.
The influence of rainfall on TSSC in the basin is complex, with a correlation
coefficient r of 0.40. This means that in addition to river discharge, rainfall variability influences variations in TSSC in the Nyando River. The TSSC, and therefore
water turbidity, increases as rainfall increases. The relationship between rainfall and
TSSC is complicated by the availability of sediment within a given stage of the
river during the rainy season. During early stages of the rainy season, low vegetation
cover means that more loose sediment is available for transport by runoff into the
river channel. In later stages, improvement of vegetation cover reduces sediment
availability, leading to relatively low TSSC (and low turbidity), even when river
discharge may be increasing. TSSC usually increases with river discharge because
the increased flow generally results from direct surface runoff, which is associated
with soil erosion. The supply of suspended sediments usually limits the variation of
TSSC in the river.
The relationship between sediment load of the river and river discharge is strong,
with a correlation coefficient r value of 0.99 and coefficient of determination R
2 of
0.98. This indicates a strong correlation between sediment load and river discharge.
Increase in river discharge results in an increase in the total suspended sediment load.
The computation of sediment load of the Nyando River is based on the mean and
maximum river discharge of 20 m
3 /s and 89 m
3 /s, respectively. We used the mean
TSSC of 0.4 g/l, which seems to be the long-term mean for the river. The results show
that the mean sediment load is 691 tons/day, which translates to 252,288 tons/year.
But the maximum sediment load is 3.07 × 10
3 tons/day, which translates to 1.12
× 10
6 tons/year. The sediment load of the Nyando River can therefore be assumed
to vary from 0.25 × 10
6 to 1.12 × 10
6 tons/year. It should, however, be noted that
during periods of extreme flood flows (> 400 m
3 /s), the sediment load of the river can
be 100 times greater than average. During flood flows, the river discharges 22,809
tons of sediment per day, which for a period of one month is equivalent to 684,288
tons. Therefore, during periods of flood flows, the river can discharge 50% of the
total annual sediment load in just one month. Based on the above data, we compute
the sediment production rate in the basin to be in the range of 72–318 tons/km
2 /year.
This is comparable to those reported for the Upper Tana Basin in the Central Kenya
Highlands (Njogu et al. 2018; Mwendwa et al. 2019; Kitheka et al. 2019).
4.7 Analysis of Land-Use Change
There has been significant change in land use in the Nyando River Basin from the
pre-colonial to post-colonial periods (Table 1). In the past 100 years, there have been
major changes in land use and vegetation cover in the basin (see Swallow et al. 2002;
Onyango 2003). The analysis of Landsat satellite images and aerial photographs for
the period between 2003 and 2012 show that forest cover increased by 8.32% in
this period due to reforestation activities in the basin. There was, however, in the
same period a 100% increase in the area under human settlement. The wetlands area
decreased by 82% in the lower parts of the basin in the same period due to conversion
J. U. Kitheka et al.
The influence of rainfall on TSSC in the basin is complex, with a correlation
coefficient r of 0.40. This means that in addition to river discharge, rainfall variability influences variations in TSSC in the Nyando River. The TSSC, and therefore
water turbidity, increases as rainfall increases. The relationship between rainfall and
TSSC is complicated by the availability of sediment within a given stage of the
river during the rainy season. During early stages of the rainy season, low vegetation
cover means that more loose sediment is available for transport by runoff into the
river channel. In later stages, improvement of vegetation cover reduces sediment
availability, leading to relatively low TSSC (and low turbidity), even when river
discharge may be increasing. TSSC usually increases with river discharge because
the increased flow generally results from direct surface runoff, which is associated
with soil erosion. The supply of suspended sediments usually limits the variation of
TSSC in the river.
The relationship between sediment load of the river and river discharge is strong,
with a correlation coefficient r value of 0.99 and coefficient of determination R
2 of
0.98. This indicates a strong correlation between sediment load and river discharge.
Increase in river discharge results in an increase in the total suspended sediment load.
The computation of sediment load of the Nyando River is based on the mean and
maximum river discharge of 20 m
3 /s and 89 m
3 /s, respectively. We used the mean
TSSC of 0.4 g/l, which seems to be the long-term mean for the river. The results show
that the mean sediment load is 691 tons/day, which translates to 252,288 tons/year.
But the maximum sediment load is 3.07 × 10
3 tons/day, which translates to 1.12
× 10
6 tons/year. The sediment load of the Nyando River can therefore be assumed
to vary from 0.25 × 10
6 to 1.12 × 10
6 tons/year. It should, however, be noted that
during periods of extreme flood flows (> 400 m
3 /s), the sediment load of the river can
be 100 times greater than average. During flood flows, the river discharges 22,809
tons of sediment per day, which for a period of one month is equivalent to 684,288
tons. Therefore, during periods of flood flows, the river can discharge 50% of the
total annual sediment load in just one month. Based on the above data, we compute
the sediment production rate in the basin to be in the range of 72–318 tons/km
2 /year.
This is comparable to those reported for the Upper Tana Basin in the Central Kenya
Highlands (Njogu et al. 2018; Mwendwa et al. 2019; Kitheka et al. 2019).
4.7 Analysis of Land-Use Change
There has been significant change in land use in the Nyando River Basin from the
pre-colonial to post-colonial periods (Table 1). In the past 100 years, there have been
major changes in land use and vegetation cover in the basin (see Swallow et al. 2002;
Onyango 2003). The analysis of Landsat satellite images and aerial photographs for
the period between 2003 and 2012 show that forest cover increased by 8.32% in
this period due to reforestation activities in the basin. There was, however, in the
same period a 100% increase in the area under human settlement. The wetlands area
decreased by 82% in the lower parts of the basin in the same period due to conversion