106
7 Effects of Conservation Practices …
Table 7.1 also shows that, TREs for soil and water were relatively lower in the
1980s or 1990s. This may be because of the changes in the conservation practices and
precipitation, especially the former, since then no other obvious nonstationarity, e.g.,
urbanization, existed. It has been mentioned above that most of the check dams, just
as other conservation practices, were built in the 1960s–1970s. They were gradually
filled up in the following 10 years, and consequently retained less soil and water
from then on. However, in recent years, some new reservoirs and check dams have
been built, and the TREs grow up therefrom.
On the other hand, enormous amounts of soil retained from small watersheds
by the conservation practices do not only form large-scale fertile farmland but also
safeguard the Yellow River against overflow. The Yellow River (as shown in Fig. 7.1),
with a total length of 5464 km, is the second longest river in China. Due to serious
silting, levees along the lower reaches have to be heightened every year, thus an
Above Ground River is formed with a riverbed 10 m higher than the surrounding
ground. In fact, most sediment in the lower Yellow River comes from the Loess
Plateau. Hence, reducing sediment pouring into the Yellow River from the Loess
Plateau is the most fundamental solution to safeguard the river (Xu et al. 2004). It
has been revealed in the above calculation that, in the Nanxiaohegou Catchment,
about 254.4 × 10
3 kg of soil per year, and a total of 10 176.8 × 10
3 kg of soil during
the period 1954–2004, had been retained by the conservation practices, which would
be a huge threat if all of them were joined into the river. Thus, we may reasonably
come to the conclusion that a big contribution has been made in relieving sediment
accumulation in the Yellow River by the conservation practices on the Loess Plateau.
7.4.4 Negative Effects of the Conservation Practices
Soil and water conservation practices might negatively influence the water cycle
process in the catchment through the following ways: (1) to bring impact on the
ecological environment and probably aggravate the problem of local water resources
in the catchment, and (2) to decrease runoff and even cut-off the Yellow River.
Some scientists, e.g., Chen et al. (2007), insisted that the long-term ecological
effects of soil and water conservation on the Loess Plateau were still not clear. In
light of the statistics, the survival rate of trees was only about 25% on the Loess
Plateau (Li 1997), and the growth quality of survival trees was also not so good due
to the poor survival conditions (Xu et al. 2004). The evapotranspiration of perennial
forest and grass vegetation was greater than or equal to the annual precipitation on
the Loess Plateau. Thus water deficit had to be compensated by absorbing water in
deep soil, which might lead to the formation of the dried soil layer, and further the
low survival rates of forest and grass (Li 1997). Nevertheless, soil loss hazards might
be alleviated if combined conservation measures, including terraces and contour
tillage, were applied in the watershed (Shi et al. 2004). Anyhow, as a widespread
structure practice, the check-dam system in gullies is one of the most effective ways
to conserve soil and water on the Loess Plateau.
7 Effects of Conservation Practices …
Table 7.1 also shows that, TREs for soil and water were relatively lower in the
1980s or 1990s. This may be because of the changes in the conservation practices and
precipitation, especially the former, since then no other obvious nonstationarity, e.g.,
urbanization, existed. It has been mentioned above that most of the check dams, just
as other conservation practices, were built in the 1960s–1970s. They were gradually
filled up in the following 10 years, and consequently retained less soil and water
from then on. However, in recent years, some new reservoirs and check dams have
been built, and the TREs grow up therefrom.
On the other hand, enormous amounts of soil retained from small watersheds
by the conservation practices do not only form large-scale fertile farmland but also
safeguard the Yellow River against overflow. The Yellow River (as shown in Fig. 7.1),
with a total length of 5464 km, is the second longest river in China. Due to serious
silting, levees along the lower reaches have to be heightened every year, thus an
Above Ground River is formed with a riverbed 10 m higher than the surrounding
ground. In fact, most sediment in the lower Yellow River comes from the Loess
Plateau. Hence, reducing sediment pouring into the Yellow River from the Loess
Plateau is the most fundamental solution to safeguard the river (Xu et al. 2004). It
has been revealed in the above calculation that, in the Nanxiaohegou Catchment,
about 254.4 × 10
3 kg of soil per year, and a total of 10 176.8 × 10
3 kg of soil during
the period 1954–2004, had been retained by the conservation practices, which would
be a huge threat if all of them were joined into the river. Thus, we may reasonably
come to the conclusion that a big contribution has been made in relieving sediment
accumulation in the Yellow River by the conservation practices on the Loess Plateau.
7.4.4 Negative Effects of the Conservation Practices
Soil and water conservation practices might negatively influence the water cycle
process in the catchment through the following ways: (1) to bring impact on the
ecological environment and probably aggravate the problem of local water resources
in the catchment, and (2) to decrease runoff and even cut-off the Yellow River.
Some scientists, e.g., Chen et al. (2007), insisted that the long-term ecological
effects of soil and water conservation on the Loess Plateau were still not clear. In
light of the statistics, the survival rate of trees was only about 25% on the Loess
Plateau (Li 1997), and the growth quality of survival trees was also not so good due
to the poor survival conditions (Xu et al. 2004). The evapotranspiration of perennial
forest and grass vegetation was greater than or equal to the annual precipitation on
the Loess Plateau. Thus water deficit had to be compensated by absorbing water in
deep soil, which might lead to the formation of the dried soil layer, and further the
low survival rates of forest and grass (Li 1997). Nevertheless, soil loss hazards might
be alleviated if combined conservation measures, including terraces and contour
tillage, were applied in the watershed (Shi et al. 2004). Anyhow, as a widespread
structure practice, the check-dam system in gullies is one of the most effective ways
to conserve soil and water on the Loess Plateau.