What are the characteristics of the environmental conditions of the small arch shed?

(1) The thermal energy of arch shed with low temperature depends entirely on the radiant energy of the sun, just like the solar ridge. When the outside light is strong, the greenhouse temperature rises rapidly; On cloudy days, strong winds, low temperatures or at night, the greenhouse temperature drops rapidly or very low. The temperature change is obviously influenced by the external environmental conditions and seasons.

Generally, the heating capacity of small arch shed is only 3 ~ 6℃, and the maximum heating capacity can reach 15 ~ 20℃, so the high temperature hazard in the shed is easy to occur in sunny days. Under cold and night conditions, the greenhouse temperature is only 1 ~ 3℃ higher than that outside, so it is easy to frost when it meets cold current. After the small arch shed is covered with grass grating, the temperature can be increased by more than 2℃, which is 4 ~ 8℃ higher than that in the open field. When the outdoor temperature is-15℃, the lowest temperature in the shed is -3℃, which is about 12℃ higher than that in the open field. When the outside temperature is lower than-10℃, the small arch shed is not suitable for production.

Seasonal variation of temperature of small arch shed: Because the temperature of small arch shed is obviously affected by seasons, the lowest temperature season beyond its service life is also the lowest temperature season of small arch shed. The temperature of the small arch shed used in early spring is getting higher and higher with the development of the season; The temperature of the small arch shed for autumn extension cultivation is getting lower and lower with the development of the season. At night, when the small arch shed is covered with heat preservation and cold protection, the frequency of minimum temperature and extreme minimum temperature is less and higher than that of outdoor.

When the small arch shed is used for early spring cultivation, the seedlings and production of cold-resistant and semi-cold-resistant vegetables can be carried out when the outdoor temperature is above-10℃. When the outside temperature is above -5℃, the seedlings and production of warm-loving vegetables can be carried out. If the night covering material of small arch shed has good thermal insulation, the service life of small arch shed will be advanced in early spring and delayed in autumn. It is the lowest temperature in the shed and the lowest temperature that crops can tolerate that ultimately determines the service life of the small arch shed.

When the small arch shed is used to delay the cultivation of warm vegetables in autumn, when the temperature in the shed drops below 15℃, it should be covered with grass fences to keep warm and cold. When the temperature drops to 2 ~ 5℃, prepare to pull out seedlings; When cultivating cold-resistant vegetables, when the greenhouse temperature drops to 8 ~ 10℃, cover with straw mats to prevent cold; When the greenhouse temperature drops to about 0℃, harvest or overwinter.

Daily variation of temperature in small arch shed: small arch shed has small space, weak buffering capacity and large daily variation of shed temperature. When there is no wind on sunny days, the warming effect is obvious, and the greenhouse temperature can reach 40℃; The temperature is very low on cloudy days, cold days, rainy days, snowy days and windy days, which can be below 0℃.

On sunny days, the temperature difference of small arch shed is large, which can reach 20 ~ 30℃; The temperature difference is small on cloudy days. The small arch shed began to heat up after seeing the light in the morning, and the highest temperature appeared around noon 13. In the afternoon, the solar radiation gradually weakened and the temperature began to drop. After covering with grass, the cooling rate decreases, which is slower than the outside temperature. At night, the temperature drops to the lowest before sunrise, which is the lowest temperature of the day. After uncovering, the temperature rises again, forming a new cycle. The above is the general law of daily variation of temperature in small arch shed.

Difference of temperature distribution in small arch shed: Due to different intensities of illumination, radiation, conduction and convection in small arch shed, local temperature difference is formed in small arch shed, resulting in uneven temperature distribution.

In the morning, the temperature in the shed was relatively uniform before the cover was uncovered. Because the soil radiates heat into space, the temperature at different levels is different. Due to convection, the roof temperature is higher and the ground temperature is lower before uncovering in the morning, with a difference of 3 ~ 4℃ (Figure 1-6).

Figure 1-6 shed isotherm (1sunny and cloudy on April 8, 964)

(Vegetable cultivation editor of Beijing Agricultural University, 1980)

After opening, the temperature in the shed rose rapidly due to the warming effect of sunlight, and the temperature distribution in the shed changed. The part with the strongest light forms a high temperature area. Around noon, the high temperature area moved to the ground, and the soil began to store heat, and the temperature in the lower layer was higher than that in the upper layer. The temperature difference in the horizontal direction is more obvious than that in the vertical direction, and the area with the largest temperature difference is 30 ~ 40 cm from the ground. After covering grass at night, due to the adjustment of convection phenomenon, the temperature in greenhouse tends to be stable and the temperature distribution becomes uniform. At 2 1, the temperature of the lower layer is still higher than that of the upper layer. From 2 1 to 7: 00 the next day, the temperature of the upper layer is higher than that of the lower layer before the cover is uncovered.

When crops grow tall in the small arch shed, the airflow in the shed is relatively stable, and the temperature distribution in the shed becomes uniform when there are no crops. However, due to the small space of the small arch shed and the large local temperature difference, attention should be paid to ventilation and cooling in cultivation management, and coverage should be strengthened at night to prevent excessive heat loss and aggravate the uneven temperature distribution in the small arch shed.

Soil temperature of small arch shed: The soil temperature in the shed changes with the change of shed temperature, and it is also influenced by seasonal, weather and daily changes, resulting in uneven temperature distribution. The change of soil temperature is far less prominent and relatively stable.

In early spring, the ground temperature in the shed is generally lower than that in the shed. The recovery of soil temperature is slower than that of air temperature, mainly due to the large specific heat of soil and slow temperature rise. Therefore, the limiting factor of planting crops in small arch shed in early spring is soil temperature, not air temperature. Only when the soil temperature in the small arch shed is stable above the minimum temperature required for the growth of the planted crops can the crops be planted safely. For example, the safe planting period of cucumber in a small arch shed in early spring is when the soil temperature in the shed is stable at 12℃, while the safe planting period of tomato is when the soil temperature is stable at 10℃. Generally, the soil temperature is based on the surface soil temperature of 10 cm before uncovering.

The temperature limiting factor of the small arch shed in autumn is air temperature. A lot of solar energy is stored in the soil in autumn, which makes the soil temperature obviously higher than the air temperature in autumn and winter. Because the specific heat of air is obviously less than that of soil, when the cold current comes in autumn and winter, the most obvious cooling is air, not soil, so the freezing injury of autumn crops is mainly caused by the sudden drop of temperature. When the temperature drops to the lowest temperature of crop growth limit, let the crops spread from the air on the shelf to the ground. The ground temperature is higher, and the daytime temperature is higher, so crops can avoid the cold current and continue to grow for a period of time, so that fruits can be matured and marketed one after another.

Generally, the daily variation law of soil temperature in small arch shed is that the minimum temperature rises gradually in the morning under the heating of sunlight, and reaches the highest point around 14 in the afternoon, that is, the highest temperature in a day. Generally speaking, the highest soil temperature appears about 1 hour later than the air temperature. In the afternoon, it gradually decreases with the decrease of light and temperature. Because of the large specific heat of soil, the decline rate is slower than the temperature. The speed after covering is slightly slower than that without covering. The next morning, the soil was as low as the temperature.

The diurnal variation of soil temperature in small arch shed is similar to that in air temperature, which changes greatly on sunny days and relatively little on cloudy days. The change range of grass cover is obviously smaller than that of shed without grass cover. The diurnal variation of surface soil temperature in small arch shed is greater than that in deep soil. For example, when the diurnal range of soil temperature is 6℃ for 5 cm in sunny days, the diurnal range of soil temperature at 20℃ is only 3℃. Cloudy days become smaller, which are 3.6℃ and 65438 0.5℃ respectively.

The average ground temperature of semi-arch circular arch greenhouse 10 cm in Beijing area is 4 ~ 5℃. Temperature in the first half of March10 ~11℃; In late March, it was 14 ~ 18℃. It can be seen that cold-resistant vegetables such as cabbage can be planted in the small arch shed in early March; Cucumber and other warm vegetables can be planted in late March.

(2) The arch shed with light can increase the temperature of the border by solar radiation. The light quantity and light distribution of small arch shed directly affect the temperature and temperature distribution in the shed. The new film has good light transmittance, generally above 90%; When water droplets adhere, the light transmittance drops to 50% ~ 60%; After 15 days of use, only about 60% of the polluted membrane was found. The light transmittance of heavily polluted films and some old films is less than 30% of that of new films, even lower than 10%.

The light in the small arch shed is not only the dependent factor of the greenhouse temperature rise, but also the leading factor of crop photosynthesis in the shed. Insufficient light, low temperature in greenhouse and weak photosynthesis of crops are characterized by slow growth and even stiff seedlings. Therefore, the new anti-pollution, anti-aging and anti-static membrane should be selected for the small arch shed, and the old membrane is strictly prohibited. If possible, it is best to choose the drip-free dustproof film used in energy-saving solar greenhouse.

The lighting quantity of small arch shed is mainly affected by the size of lighting surface, which has little to do with the shed type, and mainly affects the heat dissipation at night. The larger the shed surface, the more heat dissipation at night, and the faster the temperature drops. When the space of small arch shed is not the limiting factor, the shed surface area should be reduced as much as possible to avoid excessive heat dissipation and rapid cooling at night. A small arch shed with a large shed surface does not mean that it has a large lighting surface, and the lighting amount of small arch sheds with the same frame width and different heights is basically the same. Therefore, the height of the small arch shed can meet the growth of crops, and it is not appropriate to increase it blindly.

During the day, the temperature of the light-seeing part and the top of the small arch shed is higher; The temperature is higher than the ground temperature; The temperature rises quickly and the ground temperature rises slowly; The horizontal illumination gradually decreases from the south to the north; The illuminance in the vertical direction gradually decreases from top to bottom; When there are crops, the uniformity of light distribution becomes worse; The light distribution in the north-south direction of the small arch shed is more uniform than that in the east-west direction. Therefore, in the growth period of crops in small arch shed, the light should be increased to the maximum extent to avoid shading between crops. Under the condition of not affecting thermal insulation, try to choose the north-south shed direction.

(3) The moisture-proof plastic film has strong air tightness. When the greenhouse is sealed, the air in the greenhouse is isolated from the outside world, and the water evaporated from the ground and plants are all enclosed in the arched shed, which is due to the high relative humidity of the air in the arched shed and all the film-covered protection facilities. When the outside air temperature is lower than that in the shed, the plastic film is easy to condense and water drops appear.

Generally, the air relative humidity of small arch shed can reach 70% ~100%; It can keep 40% ~ 60% during daytime ventilation; When the shed is closed for a long time, it can be more than 100% and coagulated. The relative humidity of the air in the small arch shed decreases with the increase of temperature and increases with the decrease of temperature. The relative humidity decreases on sunny days and increases on cloudy days. The change of absolute humidity (absolute water content) of air in small arch shed is just the opposite of relative humidity, and it increases or decreases with the increase or decrease of temperature. When the temperature decreases, the reduction of absolute water content makes the excess water in the air condense on the cold interface, such as the film, and the relative humidity of the air reaches more than 100%, resulting in supersaturation. It leads to the decrease of light transmittance and the aggravation of crop leaf diseases. Ventilation should be done in time on sunny days to eliminate excess water and avoid frequent condensation.

Keeping humidity relatively stable and avoiding too low or too high is an important condition for disease prevention and treatment. High temperature, high humidity, low temperature and high humidity in the small arch shed are all favorable conditions for crop diseases. Strengthen ventilation, dehumidification and cooling to avoid high temperature and high humidity environment and ensure the healthy growth of crops.