Modelling future land-use change and assessing resultant streamflow responses: a case study of two diverse Southern African catchments.
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Date
2022
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Abstract
Land-use and land cover (LULC) is a crucial constitute of the terrestrial ecosystem, impacting
on numerous fundamental processes and characteristics such as land productivity,
geomorphological process and the hydrological cycle. Assessing the hydrological impacts of
land-use and land cover changes (LULCCs) has become one of many challenges in
hydrological research. LULCCs modify hydrological processes such as evapotranspiration,
infiltration and interception, consequently impacting on the hydrological regimes of a
catchment. Understanding the implications of LULCCs on catchment hydrology is therefore
fundamental for effective water resource planning and management, and land-use planning.
Globally, numerous studies have documented the impacts of LULCCs on catchment
hydrology, however in Southern Africa there exists a knowledge gap on the impacts of
LULCCs on catchment hydrology, specifically future land-use and land cover change
(LULCC). Therefore, the aim of this study was to simulate potential future land-use within two
diverse South African catchments using an appropriate land-use change model and thereafter
to assess streamflow responses to these future land-use scenarios using the ACRU hydrological
model.
Future land-use was simulated utilizing the Cellular Automata Markov (CA-Markov) model.
The CA-Markov model is a hybrid land-use change model that integrates Markov chain, CA,
Multi-Objective Land Allocation (MOLA) and Multi-Criteria Evaluation (MCE) concepts.
CA-Markov simulated future land-use through the creation of conditional probability and
transition probability matrices, suitability images and the utilization of a CA contiguity filter
and socio-economic and biophysical drivers of LULCC. The results illustrated that within both
catchments, increasing growth of anthropogenically driven LULC classes such as urban,
agroforestry and agrarian areas inevitability contribute to the fragmentation, modification and
deterioration of natural land-cover types. The model’s reliability and capability was assessed
by running a validation, which was conducted by simulating changes between t1 (1990) and t2
(2013/14) to predict for t3 (2018). The predicted map produced for 2018 was then compared
against the actual 2018 reclassified map, which served as a reference map. The obtained kappa
values (Kstandard, Klocation and Kno) achieved during the validation were all above 80%,
thus indicating the model’s reliability and capability in successfully predicting future LULC in
the study sites. The assessment of future LULCC impacts on streamflow responses was achieved by utilizing
the ACRU model. Historical and future scenarios of land-use were utilized as inputs into a preexisting
ACRU model where all input parameters (e.g. climate, soils) remained constant with
only changes made to the land cover parameters and area occupied by each land cover. The
results illustrated that due to anthropogenic induced LULCC, the hydrological regime within
the uMngeni catchment has been altered when compared to the baseline hydrological regime.
Patterns of low (1:10 driest year) and high (1:10 wettest year) flows have changed significantly
between the baseline and 1990. However, between 1990 and the future hydrological regime
(2030 LU scenario) only a slight amplification of these impacts was evident. Mean annual
streamflow increases and decreases were present in majority of Water Management Units
(WMU’s), however, the Table Mountain, Pietermaritzburg, and Henley WMU’s illustrated
greater increases in mean annual accumulated streamflows compared to other WMU’s while
the New Hanover New Hanover and Karkloof WMU’s illustrated the greatest decreases in
mean annual accumulated streamflows.
Furthermore, results indicated that streamflow responses significantly increase in the presence
of urban land-use. The impacts become evident as streamflows cascade through the catchment.
The results also illustrated that streamflow responses were due to the nature of LULCC, viz
urban land-use, commercial forestry, and agriculture combined with the location and extent of
LULCCs.
These results are beneficial for the implementation of proactive and sustainable water resource
planning and land-use planning. Moreover, considering the simulated streamflow responses in
relation to varying land-use scenarios, it is essential that water resource planning incorporate
land-use location, nature and scale from not only the perspective of land-use effects, but also
on hydrological responses in a catchment. Given the interdependence between streamflow
responses and changes in land-use, water resource and land-use planning should not occur in
silos. Overall, this study illustrated the importance of understanding and assessing land-use and
water interactions in a water stressed region such as South Africa.
Keywords: land-use and land cover changes, hybrid land-use change model, streamflow
responses, land-use and water interactions, sustainable water resource planning.
Description
Masters Degree. University of KwaZulu-Natal, Pietermaritzburg.