**6. Prospect and conclusions**

In Africa the superimposition of climatic changes and human activities is accompanied by a serious degradation of environmental conditions generally. The impact of this change involves certain thresholds which depend on the intensity and duration of meteorological events, the condition of the vegetation cover, the physical and chemical properties of the soil system and of the geomorphic settings. High rates of change occur in regions where large areas are affected by human intervention and where factors such as a high relief, steep slopes and a strong coupling between hillslopes and rivers support a rapid response. Slope failure in colluvial deposits, and erosion of hillwash and aeolian deposits indicate the important role of forms and deposits which are inherited from the past. Long-term processes, such as deep weathering, can contribute to the humanly-induced instability of hillslopes, once intrinsic thresholds are exceeded due to a continuous lowering of the shear strength or the increase in soil thickness (Shroder, 1976).

Studies on the impact of climatic changes on erosion processes in the late Pleistocene and the early Holocene indicate that a complete adjustment to the changed conditions requires a simultaneous response of all landscape components throughout a period of time that is long enough, to overcome the inertia of the geomorphic system (Thomas, 2004). With respect to the time frame of change in the vegetation-soil systems, these adjustments are considered to have been accomplished within a period of 103 to 104 years (Thomas, 2004, 2006). The expected rates of response point to the temporal and spatial differences between natural changes and humanly-induced change. Human interference is capable of changing the vegetation cover and the hydrologic regimes of extensive areas within a relatively short time. Repeated biomass burning in the savanna and rain forest zones coupled with intensified land-utilisation activity resulted in a degradation of the vegetation-soil system in several areas and often initiated an array of self-reinforcing processes.

Predictions of IPCC (2001) on the climatic development in Africa suggest that the climate is likely to get warmer, while the total amount of rainfall will not change significantly. However, a higher number of days with heavy rainfall is likely. These changes may affect the biota, the land use pattern and the hydrologic regimes. In the alpine Usambara Mountain area of East Africa (Tanzania) the lower replacement of montane forest trees seems to have been accompanied by general global warming over the last 100 years (Binggeli, 1989, Hamilton and Macfadyen, 1989). As a result of global warming, a general decline in the extent of the Afroalpine areas is likely (Taylor, 1999). The predicted increase in heavy rains may promote the increase of runoff, whilst the decrease of soil moisture is likely to bring with it edaphic aridity and an increase in erodibility (Beckedahl, 2002). This may result in a reinforcement of soil erosion. The decline in the number of rain days, on the other hand, may promote vegetation decay and leave more areas unprotected from heavy rainfalls. However, land use changes seem to have a much greater impact on susceptibility to soil erosion (Beckedahl, 2002, Valentin et al., 2005).

The increase in population in Africa is expected to result in an extension of the area cultivated land, even in steeply sloping mountainous regions. The impact of change in the climate and the intensified land use are likely to cause a reinforcement of degradation processes in the landscapes and may result in a lowering of the carrying capacity of land. However, predictions on future rates of change also depend on socioeconomic processes and political decisions. The devastating impact of desertification in the Sahel was not only a result of drought but was also associated with one of the highest population growth rates in the world.

#### **7. References**

268 International Perspectives on Global Environmental Change

However, we have a poor understanding of the turnover of carbon in quantitative terms in the savannas due to the complex interaction of weathering, soil formation, vegetation and litter production and different reaction-times. Finally, a full assessment of the climatic impact of biomass-burning depends also on the reliability of the data and on the quality of

In Africa the superimposition of climatic changes and human activities is accompanied by a serious degradation of environmental conditions generally. The impact of this change involves certain thresholds which depend on the intensity and duration of meteorological events, the condition of the vegetation cover, the physical and chemical properties of the soil system and of the geomorphic settings. High rates of change occur in regions where large areas are affected by human intervention and where factors such as a high relief, steep slopes and a strong coupling between hillslopes and rivers support a rapid response. Slope failure in colluvial deposits, and erosion of hillwash and aeolian deposits indicate the important role of forms and deposits which are inherited from the past. Long-term processes, such as deep weathering, can contribute to the humanly-induced instability of hillslopes, once intrinsic thresholds are exceeded due to a continuous lowering of the shear

Studies on the impact of climatic changes on erosion processes in the late Pleistocene and the early Holocene indicate that a complete adjustment to the changed conditions requires a simultaneous response of all landscape components throughout a period of time that is long enough, to overcome the inertia of the geomorphic system (Thomas, 2004). With respect to the time frame of change in the vegetation-soil systems, these adjustments are considered to have been accomplished within a period of 103 to 104 years (Thomas, 2004, 2006). The expected rates of response point to the temporal and spatial differences between natural changes and humanly-induced change. Human interference is capable of changing the vegetation cover and the hydrologic regimes of extensive areas within a relatively short time. Repeated biomass burning in the savanna and rain forest zones coupled with intensified land-utilisation activity resulted in a degradation of the vegetation-soil system in

Predictions of IPCC (2001) on the climatic development in Africa suggest that the climate is likely to get warmer, while the total amount of rainfall will not change significantly. However, a higher number of days with heavy rainfall is likely. These changes may affect the biota, the land use pattern and the hydrologic regimes. In the alpine Usambara Mountain area of East Africa (Tanzania) the lower replacement of montane forest trees seems to have been accompanied by general global warming over the last 100 years (Binggeli, 1989, Hamilton and Macfadyen, 1989). As a result of global warming, a general decline in the extent of the Afroalpine areas is likely (Taylor, 1999). The predicted increase in heavy rains may promote the increase of runoff, whilst the decrease of soil moisture is likely to bring with it edaphic aridity and an increase in erodibility (Beckedahl, 2002). This may result in a reinforcement of soil erosion. The decline in the number of rain days, on the other hand, may promote vegetation decay and leave more areas unprotected from heavy rainfalls. However, land use changes seem to have a much greater impact on susceptibility

case studies.

**6. Prospect and conclusions** 

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**14** 

*China* 

**Assessment of the Impact of Land-Use** 

**Types on the Change of Water Quality in** 

**Wenyu River Watershed (Beijing, China)** 

Land use-land cover (LULC) change is one of the major environmental changes occurring around the globe. Water quality is one of such factors affected by LULC change, since it is a key component of a healthy watershed where it integrates important geomorphic, hydrologic, and some of the biological processes of a watershed (Hem, 1985). Alteration of any one of these processes will affect one or more water quality parameters (Peterjohn and Correll, 1984). Hydrologists and aquatic ecologists have long known that the surface across which water travels to a stream or a lake has a major effect on water quality. Accordingly, the relative amounts of particular types of land use-land cover (LULC) in a watershed will affect water quality as well (Griffith, 2002). Therefore, the change in land-use and

The importance of the interrelationships between LULC and water quality is reflected by the increased recognition over the past two decades that non-point source (NPS) pollution has come into being the major environmental concern (Loague *et al,* 1998; Sharpley and Meyer, 1994; Griffith, 2002). Pollutants affecting water quality may come from point or nonpoint sources. Point pollution can be easily monitored by measuring discharge and chemical concentrations periodically at a single place. In the past several decades, the major efforts and funding of water pollution control programs focused on the point sources management, and the magnitude of the point source pollution problem has been reduced in many cases. However, NPS pollution presents great challenges because of their dispersed origins and the fact that they vary with the season and the weather, in addition to the fact that non-point inputs are often overlooked by human beings. Land cover influences water quality because land cover determines the type and quantity of NPS pollutants that may enter the water

There are a lot of studies examining non-point source pollution focused on the effects from runoff over the agricultural land and concluded that agricultural coverage strongly

management practices will give rise to the considerable impact on water quality.

**1. Introduction** 

body.

Yuanzhi Zhang1,2 and Yufei Wang1

*2Laboratory of Coastal Zone Studies,* 

*Institute of Space and Earth Information Science,* 

*Shenzhen Research Institute of CUHK, Shenzhen,* 

*1Yuen Yuen Research Centre for Satellite Remote Sensing,* 

*The Chinese University of Hong Kong (CUHK), Shatin, Hong Kong* 

