Water conservation by ecosystem is one of the important service functions provided by ecosystem for human beings. The functions of water conservation in ecosystem mainly include intercepting precipitation, enhancing soil infiltration, inhibiting evaporation, relieving surface runoff and increasing precipitation. These functions directly affect the change of river water level in the form of "time" and "space". From the time point of view, the runoff time can be extended, or the river water can be replenished in low water, and the flood flow can be reduced when the flood occurs, thus playing a role in regulating the river water level; In space, the ecosystem can transform the surface runoff generated by rainfall into soil experience and underground runoff, or return water to the atmosphere through evaporation and transpiration to carry out large-scale water circulation and redistribute atmospheric precipitation on land.

Beijing is one of the big cities in China that are seriously short of water. The per capita water resource is only 400 cubic meters, which is 15% of the average level in China and 4% of the average level in the world, ranking behind the hundred capitals of more than 20 countries in the world. Including overseas water, Beijing can use about 4.2 billion cubic meters of natural water resources, and the city's average annual water consumption is about 4 billion cubic meters. At present, it can be maintained, but it is very prone to water crisis in dry years. Through the efforts of governments at all levels and forestry departments in Beijing for many years, 33,200 hectares of forest land have been added in Miyun County in the upper reaches of Miyun County Reservoir in the past 30 years, and 25,900 hectares have been added in 10. As an important water source protection site of Miyun Reservoir, the value of forest water conservation in Yunfeng Mountain Nature Reserve mainly includes three parts, namely, the value of forest to retain precipitation, the value of increasing available water on the surface and the value of purifying water quality.

The field test results show that the water conservation capacity of each tree species forest land is Populus davidiana, Betula platyphylla, broad-leaved mixed forest, Quercus mongolica, Robinia pseudoacacia, Pinus tabulaeformis, Larix gmelinii and Platycladus orientalis in the upper reaches of Miyun Reservoir in Beijing.

The average soil water storage capacity of water conservation forest (non-capillary pore water storage) is 44. 56 tons /( 100 ha/year), 35,333 tons /( 100 ha/year) higher than that of 9230 tons /( 100 ha/year). Eight tree species forest lands were measured: Robinia pseudoacacia forest, Pinus tabulaeformis forest, Quercus mongolica forest, Populus davidiana forest, Betula platyphylla forest, Larix gmelinii forest and Tilia amurensis-dominated broad-leaved mixed forest. The results show that the soil water storage capacity is higher than that of barren slope. The increase of soil water storage in forest land is the result of the increase of soil porosity. The measured total porosity of forest soil is 47. 5% ~ 63.2%, or 6. 4% ~ 22. It is 65438 0% higher than that of grass slope. Non-capillary porosity of forest soil gradually decreases with the change of soil depth. For example, the non-capillary porosity in the 0-40 cm soil layer of Populus davidiana forest decreased from 25% to 6. 8%, while the slope of grassland in 0-20 cm soil layer only decreased by 65,438 0.5%.

Therefore, the calculation formula of forest soil water storage in Beijing is

Study on biodiversity and its protection in Yunfeng Mountain Nature Reserve, Beijing

Where: y is the total water storage capacity of forest soil (cubic meters or tons);

Ciai-soil water storage capacity (ton/hectare) and woodland area (hectare) of the I-th tree species respectively;

N- number of tree species.

See Table 4. 1 for the area of main tree species and soil water storage capacity in Beijing.

Table 4. 1 regional tree species and soil water storage capacity

Note: Others include uncultivated afforestation land and trees around villages and towns, but do not include economic forest land, sparse forest land and nursery land.

According to the survey results, if there are no tree species with measured data, the average value of each tree species is used for calculation.

From Table 4. 1

Study on biodiversity and its protection in Yunfeng Mountain Nature Reserve, Beijing

According to estimates, the average annual precipitation in Beijing is 65 1mm, and the precipitation from June to August accounts for about 70% of the annual precipitation. The total forest area with known water conservation capacity is 28. 82 square kilometers, including woodland, shrub land, uncultivated afforestation land and trees around villages and towns. Therefore, as long as the secondary precipitation does not exceed 0, it is possible to keep all the precipitates. 035m = 35mm。 Based on this, we assume that the forest can retain precipitation several times a year, that is, it can retain precipitation several times.

4.2. 1 Rainfall Conservation Value of Forest

According to the water balance of forest area, the total water conservation of forest is calculated, and the total water storage of forest is the difference between precipitation, evapotranspiration and other consumption in forest area. The formula is

T = A

In which: t-forest water storage capacity, cubic meters;

A—— Rainfall area of forest, cubic meters;

P- precipitation, mm;

E e———— Evapotranspiration, mm;

C-surface runoff, mm, because the surface runoff in forest area is very small and can be ignored.

The forest area of Yunfeng Mountain Nature Reserve is 26. 97 square kilometers, the average annual total precipitation is 65 1mm, and the evapotranspiration in forest areas accounts for 60% of the total annual precipitation. rule

Study on biodiversity and its protection in Yunfeng Mountain Nature Reserve, Beijing

The calculated forest water storage capacity is 7.22× 106 cubic meters. The value of forest water storage source is equivalent to the value of a reservoir with the same capacity, and the construction cost of reservoir water storage 1 cubic meter is used to calculate the price. According to the survey, the current unit storage cost is 5.7 14 yuan/m3. Therefore, the value of forest precipitation in Yunfeng Mountain Nature Reserve is 401.2000 yuan.

4.2.2 Forests increase the value of surface water.

The value of forest to increase effective surface water can be calculated by the following formula:

Study on biodiversity and its protection in Yunfeng Mountain Nature Reserve, Beijing

Where: V———— effective surface water increased by forest, cubic meters;

Si-the area of the I-th tree species, hectares; H0, Hi—— rainfall retention capacity per unit area of control plot and tree species, respectively,

Cubic meters/hectare;

n———— 1 1 .

Take the measured H0 = 92.30m3/ha (grassland), then

Study on biodiversity and its protection in Yunfeng Mountain Nature Reserve, Beijing

According to the difference of water storage capacity between woodland and grassland, it is found that the effective surface water increased by forest is 3. 00 yuan /m3, the value of surface water increased by forest is 6.5438+0.47 million yuan.

4.2.3 Water quality value of forest purification

According to the data provided by the School of Soil and Water Conservation of Beijing Forestry University, the water quality protected by water conservation forest meets the domestic water standard. Therefore, the clean water price of water conservation forest is 3. 00 yuan /m3. It can be set that 60% of the precipitation accumulated in the forest area is used for transpiration and growth of trees, and the rest is underground runoff. Therefore, the value evaluation of forest purified water quality is as follows

7.22×106× 3.00 = 216.6 million yuan.

The total value of forest water conservation is 2.5810.9 million yuan.