100
7 Effects of Conservation Practices …
representing the ratio of the amount retained by a certain practice to the total amount
retained by all practices in the catchment in a year.
The amount of soil or water aroused by rainfall in the catchment before the conservation practices were built, namely the transport amount of soil or water, includes
two parts: (1) the amount of soil or water lost from the catchment, which was determined according to the flow rate and water sampler of the runoff at the catchment
outlet, and (2) the amount of soil or water retained by the conservation practices:
W = R a × A
(7.1)
where W is the amount of water (m
3 /a) or soil (10
3 m
3 /a) retained by a certain practice
per year; R a is the retention ability for water (m
3 /(km
2 a)) or soil (10
3 kg/(km
2 a))
of the practice in the catchment; and A is the surface area of the practice (km
2 ). The
area of each practice is based on the statistical yearbook of the catchment, and it is
also corrected in accord with other data, e.g., the annual plan or summing-up reports,
remote sensing data, data of land use investigation, data of general investigation on
soil and water conservation measures, and planning maps.
Since the catchment area is as large as 36.3 km
2 , a regional disparity exists as
rainfall and runoff are discovered in different rainfall observation stations. Thus, the
retention ability of a type of practice in the catchment should be corrected as it is
transferred from the test data at a plot scale:
R a = R a1 · α · X
(7.2)
where R a1 is the retention ability for water (m
3 /(km
2 a)) or soil (10
3 kg/(km
2 a))
of a certain practice investigated in the plot test; X is the rainfall correction factor
for different catchments, and X = 1 since the runoff plots were locally set up in the
Nanxiaohegou Catchment; α is the rainfall correction factor for different locations
in the catchment, α = P a /P a1 , where P a is the precipitation monitored at the test
plot, and P a1 is the average precipitation in the catchment. In this study, the amount
of soil or water retained by a type of practice is quoted from the experimental results
reported in the book chapter by Tian et al. (2008), and then the retention ability for
water or soil of a certain practice is inversely computed according to Eq. (7.1) on the
basis of the amount of retained soil and the area of the practice.
The amount of transported or retained soil nutrients on the land of each conservation practice could be calculated by multiplying the amount of soil retained by the
practice with the statistic average nutrient content of the land, so that
W n = W s × C n /1000
(7.3)
where W n is the amount of transported or retained total nitrogen (TN), total phosphorus (TP) or SOM per year, 10
3 kg/a; W s is the amount of transported or retained
soil per year, 10
3 kg/a; and C n is the statistic average content of TN, TP or SOM
in the soil of the land, g/kg. Since the Loess Plateau is largely in arid or semiarid
area, the flow rate of the perennial river in the representative small watersheds, e.g.,
7 Effects of Conservation Practices …
representing the ratio of the amount retained by a certain practice to the total amount
retained by all practices in the catchment in a year.
The amount of soil or water aroused by rainfall in the catchment before the conservation practices were built, namely the transport amount of soil or water, includes
two parts: (1) the amount of soil or water lost from the catchment, which was determined according to the flow rate and water sampler of the runoff at the catchment
outlet, and (2) the amount of soil or water retained by the conservation practices:
W = R a × A
(7.1)
where W is the amount of water (m
3 /a) or soil (10
3 m
3 /a) retained by a certain practice
per year; R a is the retention ability for water (m
3 /(km
2 a)) or soil (10
3 kg/(km
2 a))
of the practice in the catchment; and A is the surface area of the practice (km
2 ). The
area of each practice is based on the statistical yearbook of the catchment, and it is
also corrected in accord with other data, e.g., the annual plan or summing-up reports,
remote sensing data, data of land use investigation, data of general investigation on
soil and water conservation measures, and planning maps.
Since the catchment area is as large as 36.3 km
2 , a regional disparity exists as
rainfall and runoff are discovered in different rainfall observation stations. Thus, the
retention ability of a type of practice in the catchment should be corrected as it is
transferred from the test data at a plot scale:
R a = R a1 · α · X
(7.2)
where R a1 is the retention ability for water (m
3 /(km
2 a)) or soil (10
3 kg/(km
2 a))
of a certain practice investigated in the plot test; X is the rainfall correction factor
for different catchments, and X = 1 since the runoff plots were locally set up in the
Nanxiaohegou Catchment; α is the rainfall correction factor for different locations
in the catchment, α = P a /P a1 , where P a is the precipitation monitored at the test
plot, and P a1 is the average precipitation in the catchment. In this study, the amount
of soil or water retained by a type of practice is quoted from the experimental results
reported in the book chapter by Tian et al. (2008), and then the retention ability for
water or soil of a certain practice is inversely computed according to Eq. (7.1) on the
basis of the amount of retained soil and the area of the practice.
The amount of transported or retained soil nutrients on the land of each conservation practice could be calculated by multiplying the amount of soil retained by the
practice with the statistic average nutrient content of the land, so that
W n = W s × C n /1000
(7.3)
where W n is the amount of transported or retained total nitrogen (TN), total phosphorus (TP) or SOM per year, 10
3 kg/a; W s is the amount of transported or retained
soil per year, 10
3 kg/a; and C n is the statistic average content of TN, TP or SOM
in the soil of the land, g/kg. Since the Loess Plateau is largely in arid or semiarid
area, the flow rate of the perennial river in the representative small watersheds, e.g.,