CHAPTER 2 LITERATURE REVIEW
2.1 Dynamics of Global Changes and Hydrology
2.1.1 Land use, Climate and Hydrology
Land use changes affect the vegetation structure, soil surface properties and alter the energy balance of a basin, thereby changing the balance between rainfall and the components of the hydrological cycle (Costa et al., 2003; D'Orgeval and Polcher, 2008; Warburton et al., 2010), such as infiltration, runoff, actual evapotranspiration and groundwater recharge. Vegetation changes, for example, alter water-use in the ecosystem, as well as infiltration rates; which results from changes in albedo, interception, rooting characteristics, stomatal response, leaf area index (Gerten et al., 2004) and changes in soil surface properties, such as leaf litter (Butterworth et al., 1999) and permeability (Giertz et al., 2005). Reduced infiltration can lead to increased velocity and volumes of runoff (Randolph, 2012), which can cause flooding, soil erosion, sedimentation and pollution of water bodies. With reduced infiltration, groundwater recharge and baseflows will be reduced, which can also lead to drying of streams, during low flows and scarcity of water for communities. Land use changes that also increase evapotranspiration, such as afforestation, reforestation and certain agricultural practices, can lead to reduced runoff (Gerten et al., 2004; Farley et al., 2005; Bren and Hopmans, 2007) and drying of water bodies, as less water is made available for groundwater recharge.
Hydrological responses to land use changes, depend on the scale of the changes, the land use type and the location of the changes within a basin (Mahe et al., 2005; Boulain et al., 2009; Warburton et al., 2012). Warburton et al. (2012) noted that land use changes have a stronger influence on streamflows at the subcatchment (10 -100 km2) than at the catchment (greater 1000 km2) scale, while different land use types influence partitioning of rainfall into baseflows and stormflows in different ways in three distinct basins in South Africa. Boulain et al. (2009) also concluded that variability of infiltration was not only caused by changes in land use, but also by its location in a Sahelian catchment. The impacts of land use changes on hydrology also depend on the climate and size of a basin (Warburton et al., 2012). In view of these geologic, scale and climatic differences, Klocking and Haberlandt (2002) stated that it is unrealistic to make generalised conclusions about impacts of land use changes on hydrology.
Evidence from previous studies, however, suggest that impacts of land use changes on hydrology are easily distinguishable at the local scale (Scott and Lesch, 1997; Seguis et al., 2004; Bren and Hopmans, 2007; Van de Giesen et al., 2011), but at large scales, the impacts
14 are not consistent (Cheng, 1999; Costa et al., 2003; Jewitt et al., 2004; Mahe et al., 2005; Li et al., 2007; D'Orgeval and Polcher, 2008). Hence, it has been suggested that to detect impacts of land use changes on hydrology accurately, spatially distributed hydrological modelling, in addition to field-based studies, using paired or single catchments, should be conducted (Klocking and Haberlandt, 2002; Olsson and Pilesjo, 2002; Zhang and Savenije, 2005; Breuer et al., 2009). For such studies, the impacts are ascertained by comparing the hydrological responses of different land use scenarios under the same climatic conditions (Hu et al., 2005). The land use scenarios depend on objectives of the study and have been developed, using techniques ranging from making assumptions (Li et al., 2007; Legesse et al., 2010) on rate of deforestation or urbanization to the use of spatially distributed land use models (Veldkamp and Verburg, 2004; Verburg and Overmars, 2009; Park et al., 2011). It is argued that distributed hydrological modelling can provide spatially distributed information on water balance components for the past, present and future scenarios, needed for effective water management. Although land use changes and their impacts occur locally, when they are ubiquitous, can aggregate, to affect the functioning of the Earth system at the regional or global scale (Steffen et al., 2004).
Climate change has also become one of the most important subjects in science due to its variable impacts on water resources (Vörösmarty et al., 2000) and the fact that it can amplify effects of land use changes on hydrology (Tong et al., 2012). Climatic changes, such as changes in the means and variability of rainfall and temperature, have the potential to alter seasonal, annual and inter-annual flow regimes of rivers, which can subsequently impact on the ecology and the environment. As rainfall patterns and temperatures change, availability of water is affected, which can change the biodiversity of ecosystems. Therefore, the challenges of climate change in water resources and environmental management cannot be overemphasized, as the ideals of sustainable development cannot be achieved without accounting for it in development planning.
Hydrological impacts of climate changes depend mainly on whether a basin is dominated by rainfall or snow, as well as its latitude and geology (Praskievicz and Chang, 2009). Hence in regions dominated by snow, changes in temperature control the hydrology (Buytaert et al., 2010), while in regions dominated by rainfall, changes in rainfall control the hydrology (Praskievicz and Chang, 2009). Also, basins with deep aquifers are more susceptible to
15 increases in temperatures, compared with those with shallow aquifers (Tague et al., 2008), as the ground water recharge rate is slower. It has also been shown that basins located in the mid- to high latitude regions and Southeast Asia (Steffen et al., 2004), will have increases in runoff as a result of climate change, while those in arid, semi-arid and some tropical regions will experience decreases in runoff (Steffen et al., 2004; Elshamy et al., 2009; Kingston and Taylor, 2010; Tshimanga and Hughes, 2012; Sood et al., 2013). However, there are some uncertainties which affect the magnitudes of hydrological impacts of climate changes. These include differences in impacts within regions (Praskievicz and Chang, 2009), as well as the choice of the climate scenarios (Praskievicz and Chang, 2009; Kingston and Taylor, 2010) and whether downscaled or raw GCM scenarios are applied. The choice of a hydrological model also account for some of the differences in impacts. For example, Cornelissen et al.
(2013) obtained different magnitudes of hydrological impacts of climate change for each hydrological model applied in a West African basin. Therefore, since impacts of climate change are location specific, to be able to develop effective adaptation measures, there is the need for locally relevant studies, especially in regions with little to no information on how climate changes will affect the hydrology and the environment.
Impacts of climate change on hydrology have been assessed by using scenario modelling, sensitivity analysis or spatial gradient analysis (Praskievicz and Chang, 2009; Peel and Bloeschl, 2011). The scenario approach uses bias corrected and downscaled GCM climate scenarios. It has been suggested that because of the scarcity of data to conduct effective GCM downscaling, the change factor method of downscaling is the best option for data scarce regions (Ruelland et al., 2012). For rainfall the ratio of the mean monthly GCM values for the future and those of the control period are multiplied by daily observed records to derive future climate scenarios, while the differences between mean monthly GCM values for the future and those of the control period are added to daily observed records to obtain future climate scenarios for temperature. The sensitivity analysis relates percentage changes in climate to percentage changes in runoff. The advantage of this approach is that it can be model-based or data driven, unlike the scenario approach, which is entirely model-based. The spatial gradient approach is based on the assumption that under the current climate the hydrological response of a basin may be similar to the responses in another basin under a different climate. Although this approach is data driven, its application is problematic since many other characteristics of a basin may not be similar (Peel and Bloeschl, 2011). Due to the lack of long-term observed
16 data to drive the sensitivity and the spatial gradient approaches, climate change impact studies on hydrology are dominated by the application of the scenario approach (Peel and Bloeschl, 2011). Several studies (Dibike and Coulibaly, 2005; Paturel et al., 2007; Ardoin-Bardin et al., 2009; Elshamy et al., 2009; Tu, 2009; Buytaert et al., 2010; Chang and Jung, 2010;
Graham et al., 2011; Mango et al., 2011; Taye et al., 2011; Kienzle et al., 2012; Ruelland et al., 2012; Teng et al., 2012; Tshimanga and Hughes, 2012; Faramarzi et al., 2013) have used the scenario approach to assess climate change impacts on water resources around the world.
Furthermore, climate and land use affect each other (Dale, 1997; D'Orgeval and Polcher, 2008), as climatic changes and human activities can change land uses (Warburton et al., 2012) and land use changes, such as vegetation dynamics, can also affect the regional climate (Xue, 1997; Wang and Eltahir, 2000) and their combined effects on hydrology is nonlinear(Li et al., 2009). Simultaneous climate change and land use changes (Figure 2.2) therefore pose a significant challenge to water resources and environmental management across scales.
However, the interactive effects of climate change and land use changes on hydrology are still not understood well in many tropical regions such as West Africa, although severe global changes have already occurred in the region and future changes are predicted to occur at a faster rate (Hulme et al., 2001; L'Hote et al., 2002; Conway et al., 2009; FAO, 2010; Jalloh et al., 2013).