Challenges in modelling hydrological responses to impacts and interactions of land use and climate change.
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2012
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Abstract
To meet society’s needs for water, food, fuel and fibre the natural land cover throughout the
world has been extensively altered. These alterations have impacted on hydrological responses
and thus on available water resources, as the hydrological responses of a catchment are
dependent upon, and sensitive to, changes in the land use. Similarly, changes in the climate
through enhanced carbon dioxide (CO2) levels in the atmosphere have resulted in increased
temperature and altered precipitation patterns that alter hydrological responses. In combination,
land use change and global climate change form a complex and interactive system, whereby both
human influences and climate change manipulate land use patterns, and changes in land uses
feed back to influence the climate system, with both impacting on hydrological responses.
Relatively few studies have been undertaken examining the combined impacts of climate change
and land use change on water resources, with no consensus emerging as yet as to combined
influence of land use change and climate change on hydrological responses and the role of
geographical characteristics in determining the overriding influence. There is, however,
agreement that the effect on hydrological responses will be amplified. Given that South Africa is
currently water stressed and considered to be highly exposed to climate change impacts, an
understanding of hydrological responses to the complex interactions between land use and
climate change is crucial to allow for improved integration of land use planning in conjunction
with climate change adaptation into water resources management.
To determine the sensitivity of land use to changing climate, a sensitivity study assessing the
potential impacts of climate change on the areas climatically suitable for key plantation forestry
species was undertaken. Under sensitivity scenarios of climate change the climatically optimum
areas for specific forest species were shown to shift, with optimum areas changing in extent and
location between and within South Africa’s provinces. With potential for shifts in land use due to
climate change shown, the imperative to improve understanding of the dynamics between land
use and climate change as well as the subsequent impacts on hydrological responses was further
established.
For the assessment of climate-land use-water interactions, a process-based hydrological model,
sensitive to land use and climate, and changes thereof, viz. the daily time step ACRU model was
selected. In order to increase the confidence in results from the model in a study such as this, its
representation of reality was confirmed by comparing simulated streamflow output against
observations across a range of climatic conditions and land uses. This comparison was
undertaken in the three diverse South African catchments chosen for the study, viz. the semi-arid,
sub-tropical Luvuvhu catchment in the north of the country, which has a large proportion of
subsistence agriculture and informal residential areas, the Upper Breede catchment in the winter
rainfall regions of the south, where the primary land uses are commercial orchards and
vineyards, and the sub-humid Mgeni catchment along the eastern seaboard, where plantation
forestry is dominant in the upper reaches, commercial plantation sugarcane and urban areas in
the middle reaches, and urban areas dominate the lower reaches. Thus, in effect a space for time
study was undertaken, thereby reducing the uncertainty of the model’s ability to cope with the
projected future climate scenarios. Overall the ACRU model was able to represent the high, low
and total flows, and thus it was concluded that the model could be used with confidence to
simulate the streamflows of the three selected catchments and was able to represent the
hydrological responses from the range of climates and diversity of land uses present within the
catchments.
With the suitability of the model established for the theme of this research, the understanding of
the complex interactions between hydrological responses and land use could be improved. The
hydrological responses of the three selected catchments to land use change were varied. Results
showed that the location of specific land uses within a catchment plays an important role in the
response of the streamflow of the catchment to that land use change. Furthermore, it was shown
that the contributions of different land uses to the streamflow generated from a catchment are not
proportional to the relative area of those land uses, and the relative contribution of the land use to
the catchment streamflow varies with the annual rainfall of the catchment.
With an improved understanding of the dynamics between land uses and hydrological responses,
the impacts of climate change on hydrological responses were assessed prior to analysing the
combined impacts on land use and climate change. Five plausible climate projections from three
coupled atmosphere-ocean global climate models covering three SRES emissions scenarios
which were downscaled with the RCA3 regional climate model and adjusted using the
distribution-based scaling (DBS) approach for bias correction were used as climate input to the
ACRU model, with future projections applied to a baseline land cover scenario compared to
historical climate applied to the same baseline land cover scenario. No consistent direction of
change in the streamflow responses was evident in the Mgeni and Luvuvhu catchments.
However, decreases in streamflow responses were evident for all five scenarios for the Upper
Breede.
With an understanding of the separate impacts of land use and climate change on hydrological
responses, an analysis of the combined impacts was undertaken to determine which changes
were projected to be of greater importance in different geographical locations. Results indicated
that the drier the climate becomes, the relatively more significant the role of land use becomes,
as its impact becomes relatively greater. The impacts of combined land use and climate change
on the catchments’ streamflow responses varied across both the temporal and spatial scales, with
the nature of the land use and the magnitude of the projected climate change having significant
impacts on the streamflow responses.
From the research undertaken, the key results were
• that the climatic variable to which plantation forestry species are most sensitive is
rainfall;
• that optimum growth areas for plantation forestry are projected to shift under changing
climates, having a potentially significant impact on the landscape and thus on the
hydrological responses from the landscape;
• that the daily time-step, physical-conceptual and process-based ACRU model is
appropriate for use in land use change and climatic change impact studies as shown
through a space for time study;
• that the contributions of different land uses to the streamflow generated from a catchment
is not proportional to the relative area of that land use and that, as the mean annual
precipitation of a subcatchment decreases, so the disparities between the relative areas a
land use occupies and its contribution to catchment streamflow increases;
• that specific land use changes have a greater impact on different components of the
hydrological response of a catchment;
• that land uses which currently have significant impacts on catchment water resources will
place proportionally greater impacts on the catchment’s water resources if the climate
were to become drier; thus the drier the climate becomes, the more relatively significant
the role of land use becomes;
• that when considering any hydrological impacts of land use change, climate change or
combined land use and climate change, assessments need to consider the scale where the
localized impacts may be evident, the progression of the impacts as the streamflow
cascades through the catchment, as well as the impacts at the whole catchment scale
where the accumulation of the effects through the catchment are evident; and lastly
• that each catchment is unique with its own complexities, feed forwards and feedbacks,
thus each catchment will have a unique threshold as to where land use change or climate
change begins to have a significant influence of the hydrological response.
Given these complex interactions between land use, climate and water, there is a growing
imperative to improve the understanding of the movement of water within catchments, to be
receptive and adaptive to new concepts and information, and to developing resilient and adaptive
water management strategies for the future in a way that minimises the risks and maximises the
benefits to potential impacts of climate change.
Description
Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2012.
Keywords
Hydrologic models., Climatic changes--Environmental aspects--South Africa., Watershed management--Environmental aspects--South Africa., Forests and forestry--Environmental aspects--South Africa., Theses--Hydrology.