121
3 Results and Discussion
The land cover assessment showed insignificant change between 1990 and 2010;
this means that a fraction of area used for rainfed herbaceous crops in 1990 was also
used for rainfed herbaceous crops in 2000 and in 2010 (Table 3). The same was true
for built-up urban areas and cultivated wetland (dambo
4
), as indicated in the output
of Table 3. This somewhat contradicts the expected high land use change, resulting
in high discharge. This would seem to suggest that the high discharge rates (Fig. 4)
were caused by the increased urbanization that took place between 2011 and 2018.
These findings are consistent with other findings (FAO 2013; Kambalame 2017)
that show increased urbanization results in increased surface runoff (Fig. 2).
Even though the scope of the land cover analysis was limited to the year 2010,
satellite images taken between 2011 and 2018 indicate a significant shift in land use
in the wetland catchment. Land that was covered by rainfed herbaceous crops and the
wetland is significantly replaced with built-up areas as shown in Fig. 3. Indeed, with
reference to FAO (2013), the same pattern emerges for Njewa catchment of Lilongwe.
Discharge continued to decrease with stage in July, following the end of the
2016/2017 rainy season. Between August and toward the end of September (Julian
dates 32–89), the baseflow reached its lowest (zero). This occurred in the middle of
the dry season when streamflow is at its lowest and evaporation highest (Figs. 4 and 5).
A stage-streamflow discharge graph shows a direct linear relationship. This
means that the depth of water in the wetland determines the rate at which water is
discharged. These results correspond with findings from similar studies conducted
elsewhere such as Kambalame (2017) in Likuni and Lingadzi catchments of
Lilongwe District, Malawi, and Hadjimitsis (2010) in Cyprus. This finding implies
that impacts of the land use changes are likely to cause shifts in hydrological regimes
during both rainy season (high surface runoff leading to flooding) and dry season
(low baseflows leading to drying up of rivers).
The surface runoff coefficient was 0.65 (Table 4). This was a significant rise
compared to the ones recorded from neighboring streams in the same district such
as Lingadzi (0.2) and Likuni (0.24) (Kambalame 2017). The rise in the surface runoff coefficient can be attributed to the unprecedented conversion of land cover from
wetlands to warehouses and other buildings in the Njewa catchment. The research
demonstrates that land cover changes in Njewa catchment, Lilongwe, are leading to
reduced infiltration rates and high surface runoff coefficient, and this poses an
increased risk of flooding in the catchment (Hadjimitsis 2010; Wiyo et al. 2015).
Further, land cover changes reduce infiltration due to pavements and roofs (Wiyo
et al. 2015). The low baseflow can be related to the low infiltration rate due to high
speed of runoff and no vegetation (SADC 2016).
Therefore, Njewa catchment requires proper watershed management to handle
increased surface runoff volumes such as storm drains, large culverts, and wetland
reserve area as required.
4 In FAO documents on land use change, the term dambo is used instead of wetland particularly in
Southern Africa.
A Hydrological Assessment of Wetlands in Lilongwe Peri-urban Areas: A Case…
3 Results and Discussion
The land cover assessment showed insignificant change between 1990 and 2010;
this means that a fraction of area used for rainfed herbaceous crops in 1990 was also
used for rainfed herbaceous crops in 2000 and in 2010 (Table 3). The same was true
for built-up urban areas and cultivated wetland (dambo
4
), as indicated in the output
of Table 3. This somewhat contradicts the expected high land use change, resulting
in high discharge. This would seem to suggest that the high discharge rates (Fig. 4)
were caused by the increased urbanization that took place between 2011 and 2018.
These findings are consistent with other findings (FAO 2013; Kambalame 2017)
that show increased urbanization results in increased surface runoff (Fig. 2).
Even though the scope of the land cover analysis was limited to the year 2010,
satellite images taken between 2011 and 2018 indicate a significant shift in land use
in the wetland catchment. Land that was covered by rainfed herbaceous crops and the
wetland is significantly replaced with built-up areas as shown in Fig. 3. Indeed, with
reference to FAO (2013), the same pattern emerges for Njewa catchment of Lilongwe.
Discharge continued to decrease with stage in July, following the end of the
2016/2017 rainy season. Between August and toward the end of September (Julian
dates 32–89), the baseflow reached its lowest (zero). This occurred in the middle of
the dry season when streamflow is at its lowest and evaporation highest (Figs. 4 and 5).
A stage-streamflow discharge graph shows a direct linear relationship. This
means that the depth of water in the wetland determines the rate at which water is
discharged. These results correspond with findings from similar studies conducted
elsewhere such as Kambalame (2017) in Likuni and Lingadzi catchments of
Lilongwe District, Malawi, and Hadjimitsis (2010) in Cyprus. This finding implies
that impacts of the land use changes are likely to cause shifts in hydrological regimes
during both rainy season (high surface runoff leading to flooding) and dry season
(low baseflows leading to drying up of rivers).
The surface runoff coefficient was 0.65 (Table 4). This was a significant rise
compared to the ones recorded from neighboring streams in the same district such
as Lingadzi (0.2) and Likuni (0.24) (Kambalame 2017). The rise in the surface runoff coefficient can be attributed to the unprecedented conversion of land cover from
wetlands to warehouses and other buildings in the Njewa catchment. The research
demonstrates that land cover changes in Njewa catchment, Lilongwe, are leading to
reduced infiltration rates and high surface runoff coefficient, and this poses an
increased risk of flooding in the catchment (Hadjimitsis 2010; Wiyo et al. 2015).
Further, land cover changes reduce infiltration due to pavements and roofs (Wiyo
et al. 2015). The low baseflow can be related to the low infiltration rate due to high
speed of runoff and no vegetation (SADC 2016).
Therefore, Njewa catchment requires proper watershed management to handle
increased surface runoff volumes such as storm drains, large culverts, and wetland
reserve area as required.
4 In FAO documents on land use change, the term dambo is used instead of wetland particularly in
Southern Africa.
A Hydrological Assessment of Wetlands in Lilongwe Peri-urban Areas: A Case…