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scale approach, the natural disposition to soil erosion in the Backwater area is estimated to be in average 9.6 t ha-1 a-1. With an average of 10.7 t ha-1a-1, the potentialsoil loss in the Quyuan sub- catchment in the eastern Backwater area is even higher. Here, average annual soil losses are highest under natural conditions on the catchment scale which is generally considered to result from the higher terrain energy of the Triassic formation.

As PIMENTEL (2006) reports of a natural average annual soil loss ranging from 1 to 5 t ha-1 to be typical for mountainous regions completely covered with vegetation, the natural disposition to soil erosion in the Xiangxi catchment is concluded to be already very high. The human influence, and in particular, the interferences into the environment of the Xiangxi catchment associated to the Three Gorges Dam result in a complex system of natural and anthropogenic processes. Under the massive human influences the effects of the controlling factors are concluded to subsume and to strongly foster the natural disposition to soil erosion to considerable higher levels resulting in a distinct spatial distribution of the soil loss.

THE THREE GORGES DAM HAS AN AMBIGIOUS EFFECT ON THE SOIL EROSION RISK POTENTIAL

Compared to the natural soil erosion risk potential, the soil erosion risk potential under human impact in 2007 was estimated to be higher by a factor of about 18 accounting for an annual average soil loss of approximately 188 t ha-1 on the catchment scale. In contrast to the generally very few areas exhibiting moderate to extreme (> 25 to > 89 t ha-1 a-1) soil erosion risk potential under natural

conditions, accounting for only approximately 6% of the Backwater area, the seriously affected area under human impact is distinctly higher. According to the Chinese Soil Erosion Rate Standard, almost 41% of the total Backwater area exhibited moderate to extreme high soil erosion risk potential in 2007. This is mainly due to the crucial effect of the vegetation cover. Whereas, under natural conditions the high protection effectiveness of the closed forest vegetation cover alleviates the effect of high rainfall erosivity, high soil erodibility, and the mountainous terrain, the vegetation cover is strongly altered under land use conditions.

Besides the distinct differences between potential soil erosion under natural conditions and under land use, a shift between potential soil erosion risk under former conditions in pre-construction times of the dam (1987) and under current land use conditions in post-construction times of the dam (2007) are detectable. Before dam construction and resettlements started, the annual average soil loss in the Xiangxi catchment was estimated to be approximately 264 t ha-1 and to vary by a factor of 1.4 between annually 344.9 t ha-1 and 245.5 t ha-1 in 1987. For post-construction times of the dam, the empirical soil erosion modeling with the RUSLE revealed a distinct decrease of about 29% on the catchment scale. For the sub-units considered the soil losses vary by a factor of 1.8 between average 229.3 t ha-1 a-1 and 128.2 t ha-1 a-1 in 2007. Whereas in pre-construction times of the dam almost 45% of the Backwater area exhibited moderate to extreme soil erosion risk potential, this area percentage slightly decreased by 10% in 2007 accounting for an area of 227 km². Though, the Three Gorges Dam and associated land use changes were expected to dramatically increase the soil erosion risk potential, this impact cannot be generally concluded for the Xiangxi catchment. Here, the 'Grain for Green Program', launched in 2002 (LIU and WU, 2010), is regarded to have had a mitigating effect on the soil erosion risk potential. At least this is true for the area exhibiting extreme soil erosion that could be reduced by 36% from approximately 100 km² in 1987 to almost 64 km² in 2007. At the same time, the area under moderate to high soil erosion risk potential (> 25 to 80 t ha-1 a-1) increased by 7% from 152 to 163 km².

Against the background of the mitigation of soil erosion by the 'Grain for Green Program' that ended in 2008 (LIU and WU, 2010), the observed time period from 1987 to 2007 is considered to not entirely account for the rapid land use changes of the Three Gorges Dam. Looking at the temporal variability of the predicted potential soil losses from 2005 to 2007 based on the analysis of the fractional vegetation cover and linked crop and vegetation cover during the months of September, a distinct increase of about 24% from average 9.3 to 11.5 t ha-1 a-1 on the catchment scale can be observed. In the Backwater area the increases in average annual soil losses of about 29% during the same period are even higher. Both are assumed to indicate an accelerating effect of the Three Gorges Dam on the soil erosion risk potential. As expected, the increases are higher in the immediate reservoir area of the Xiangxi Bay where river impoundment, and thus, loss of valuable land and land use changes on the steep sloping uphill-sites are directly linked to each other. A definite proof for the

effect of the Three Gorges Dam on the soil erosion risk potential and its spatial and temporal variability, however, would require continuous analyses and monitoring with a much more 'narrow time frame' of the observation period.

Although, a minimum of at least 1,090 ha of slopes directly adjacent to the Xiangxi Bay are terraced and a distinct amount of arable land has been converted to orange orchards considered as suitable land use on steep slopes, soil erosion is a serious threat in the immediate reservoir area of the Xiangxi catchment. With approximately 128 t ha-1 a-1 in 2007, the average annual soil loss in the predominantly agriculturally used Xiangjiaba sub-catchment in the western highly dynamic Backwater area is distinctly above the average annual soil loss of 30 to 40 t ha-1 a-1 on cropland in Asian countries (PIMENTEL, 2006).

Particularly, farmland sloping greater than 5° is considered to contribute to the high soil erosion risk potential as it accounts for approximately 94% of the total cultivated area in the Xiangxi catchment and for about 98% in the Backwater area in 2007. Approximately 35 to 36% of the farmland in the Xiangxi catchment and Backwater area even occurs on slopes above the critical threshold of 25° over which cultivation and land reclamation is actually prohibited according to the Chinese Water Soil and Conservation Law (LIU, 2012). Thus, those slopes and their appropriate management are of pivotal relevance for the agricultural productivity and the sustainability in the Xiangxi catchment. Insofar, bench terraces as major recommended type of soil conservation in mountainous regions are of outstanding importance for the alleviation of the high physical vulnerability, and the cultivation of row crops, such as maize.