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J. U. Kitheka et al.
rate (3.7% per annum) in the last three to four decades led to an increase in the
clearance of land for agriculture and settlements. More than 80% of the population
in the basin depends on agriculture for its livelihood. Poverty in the basin is among
the highest in Kenya, with 65% of the population in the basin living below the poverty
line (KNBS 2009; ICRAF 2000). The majority of the people living in the basin are
highly dependent on land for their daily sustenance. Therefore, without addressing
poverty in the basin, it is unlikely that the current high rates of soil erosion and land
degradation in the basin can be adequately addressed.
5 Discussion
5.1 Relationship Between Rainfall and Discharge
The influence of rainfall on Nyando River discharge is depicted in flow duration
curves and also in hydrograph patterns. Data analyzed in this study reveal a rainfall
trend with normal inter-annual variability, with no clear long-term trend that can be
attributed to climate change. River discharges show normal year-to-year variation
that is related to rainfall variability in the basin. Although the relationship between
rainfall and river discharge is generally weakly correlated, rainfall variation explains
only 50%–60% of the variation in river discharge. Therefore, there are parameters
other than rainfall in the basin that contribute to streamflow variations. The imperfect
correlation could also be due to the lag between rainfall and river discharge. Peak
flow at the outlet of a river usually occurs some hours after peak rainfall in the
highlands located 170 m away. When storms occur at the extreme end of the basin, it
takes up to a day before this is reflected as peak flow in the lower parts of the basin.
The strong relationship between rainfall and river discharge can also be attributed to
land-use changes, which lead to rapid hydrologic response of the basin due to land
degradation. The data show that Nyando River discharge tends to respond rapidly to
rainfall in the basin, which is typical of degraded basins (cf. Hai et al. 2000). The
occurrence of above-average rainfall in the Nyando River Basin is greater than the
occurrence of below-average rainfall over the same period (2003–2012). However,
the extent to which this can be attributed to climate change needs to be investigated
further.
5.2 Sediment Yield and Rainfall and Discharge
There has been significant change in land use/land cover in the Nyando River Basin
in the last 100 years. While there are year-to-year variations in land use in the basin,
the overall trend is toward increased land degradation which can be attributed to
rapid population growth and expansion (Oruma et al. 2017). There has also been
J. U. Kitheka et al.
rate (3.7% per annum) in the last three to four decades led to an increase in the
clearance of land for agriculture and settlements. More than 80% of the population
in the basin depends on agriculture for its livelihood. Poverty in the basin is among
the highest in Kenya, with 65% of the population in the basin living below the poverty
line (KNBS 2009; ICRAF 2000). The majority of the people living in the basin are
highly dependent on land for their daily sustenance. Therefore, without addressing
poverty in the basin, it is unlikely that the current high rates of soil erosion and land
degradation in the basin can be adequately addressed.
5 Discussion
5.1 Relationship Between Rainfall and Discharge
The influence of rainfall on Nyando River discharge is depicted in flow duration
curves and also in hydrograph patterns. Data analyzed in this study reveal a rainfall
trend with normal inter-annual variability, with no clear long-term trend that can be
attributed to climate change. River discharges show normal year-to-year variation
that is related to rainfall variability in the basin. Although the relationship between
rainfall and river discharge is generally weakly correlated, rainfall variation explains
only 50%–60% of the variation in river discharge. Therefore, there are parameters
other than rainfall in the basin that contribute to streamflow variations. The imperfect
correlation could also be due to the lag between rainfall and river discharge. Peak
flow at the outlet of a river usually occurs some hours after peak rainfall in the
highlands located 170 m away. When storms occur at the extreme end of the basin, it
takes up to a day before this is reflected as peak flow in the lower parts of the basin.
The strong relationship between rainfall and river discharge can also be attributed to
land-use changes, which lead to rapid hydrologic response of the basin due to land
degradation. The data show that Nyando River discharge tends to respond rapidly to
rainfall in the basin, which is typical of degraded basins (cf. Hai et al. 2000). The
occurrence of above-average rainfall in the Nyando River Basin is greater than the
occurrence of below-average rainfall over the same period (2003–2012). However,
the extent to which this can be attributed to climate change needs to be investigated
further.
5.2 Sediment Yield and Rainfall and Discharge
There has been significant change in land use/land cover in the Nyando River Basin
in the last 100 years. While there are year-to-year variations in land use in the basin,
the overall trend is toward increased land degradation which can be attributed to
rapid population growth and expansion (Oruma et al. 2017). There has also been