Meteorological (climatic) factors
Evaporation and precipitation are important links in the natural water cycle and are closely related to the physical state of the atmosphere. Meteorological elements, including temperature, air pressure, wind direction, wind force, humidity, evaporation and precipitation, determine the physical state of the atmosphere. This physical state of the atmosphere is called weather. The multi-year average state of weather in a certain area (expressed by the multi-year average of meteorological elements) is called the climate of this area. Meteorological and climatic factors have an important influence on the formation and distribution of water resources. Although the slowly changing climatic factors play an extremely important role in the formation of groundwater, the rapidly changing meteorological factors have a significant impact on groundwater. Among them, precipitation, evaporation and temperature have the greatest influence.
1. temperature
A certain temperature in the atmosphere is called air temperature. All complex weather changes are mainly caused by different temperature conditions. The change of temperature will directly affect the change of groundwater temperature, thus changing the gas composition in groundwater. For example, due to the increase of temperature and gas activity, some gas will escape from the water, thus reducing the content of gas components in groundwater; The decrease of gas content in water will cause the change of chemical composition of groundwater. In addition, due to the increase of heat, the evaporation of groundwater is strengthened, the water quantity is reduced and the concentration of water is increased.
2. Humidity
The water vapor content in the atmosphere is called air humidity. The water vapor content in the atmosphere varies, accounting for 0.0 1% ~ 4% of the total air, of which 70% is distributed at the height of 0 ~ 3.5 km.
Water vapor has weight, so it has pressure. Therefore, the amount of water vapor in the air can be expressed by weight or pressure. Humidity is divided into absolute humidity and relative humidity.
Absolute humidity: refers to the content of water vapor in the air in a certain area for a certain time. When the weight unit is adopted, it is expressed in grams of water vapor contained in 1m3 (g/m3) air, and the symbol is m; When the pressure unit is used, the partial pressure of water vapor contained in the air is equivalent to the height of millimeters of mercury or expressed in millibars, and the symbol is e.
The absolute humidity in the air varies greatly, which is mainly affected by factors such as temperature and surface properties. Absolute humidity is higher in warm areas and on vast water surfaces or wet soil. In low temperature areas, the absolute humidity of the air is very small.
The amount of water vapor that can be contained in the air is closely related to the temperature. The higher the temperature, the more water vapor can be contained. On the contrary, the less. The maximum amount of water vapor that air can hold at a certain temperature is called the saturated water vapor content at that temperature. It can also be expressed by weight unit or pressure unit, and the two cases are represented by symbols m and e respectively. See table 1-2 for the saturated water vapor content at different temperatures.
Table 1-2 Saturated water vapor content at different temperatures
Absolute humidity can only explain the amount of water vapor in the air at a certain moment, but can't explain whether the water in the air is saturated. So there is the concept of relative humidity.
Relative humidity: the ratio of absolute humidity to saturated water vapor content is relative humidity (R). that is
General hydrogeology
Although the absolute humidity of air is constant, when the temperature drops, the relative humidity will increase. When the relative humidity reaches 100%, it means that the water vapor in the air has reached saturation. The temperature at which water vapor in the air reaches saturation is called dew point. When the temperature is lower than the dew point, excess water vapor will condense and produce precipitation.
Step 3 precipitate
When the air temperature is lower than the dew point, the excess water vapor in the air will condense and fall to the surface in the form of liquid or solid, which is called precipitation. The meteorological department uses a rain gauge to measure precipitation, which is expressed by the millimeters of the water layer height obtained by laying the total precipitation in a certain area for a certain period on the ground. If the annual precipitation in a certain area is 1 000 mm, it means that the amount of precipitation in this area is flat on the horizontal area of this area, and the height of the water layer is 1 000 mm. ..
Precipitation is one of the main links in the water cycle. The amount of precipitation in a region determines the richness of water resources in the region and has an important impact on the formation of groundwater resources. Atmospheric precipitation seeps into the ground, which is the most common recharge and the most important source of groundwater. The intensity of atmospheric precipitation recharge mainly depends on two aspects: one is the intensity and duration of atmospheric precipitation (especially rainfall and snowfall); On the other hand, it is the local infiltration conditions, such as lithology and thickness of vadose zone, topography and vegetation. If the rainfall per unit time (rainfall intensity) is large and lasts for a long time, there will be more groundwater that may be replenished; When the infiltration conditions are good, such as the surface rock and soil permeability is good, the terrain is flat and the vegetation is good, the infiltration is strong and the groundwater is replenished.
Different types of rainfall have different recharge to groundwater.
Rainstorm: short duration and high intensity. According to the practice of meteorological department, the rainfall on that day is greater than 50mm or 12h is greater than 30mm, which is called rainstorm. This kind of rain generally covers a small area and the rainfall process is short. Generally speaking, most rainfall can't penetrate into the ground and become surface runoff, which often washes the surface strongly and even changes the original structure of the surface. However, in the flat exposed gravel layer area and the area with good vegetation coverage, quite a lot of water still seeps into the ground.
Rain in Mao Mao: The duration is not long, the rainfall is small and the raindrops are small. This kind of rain often evaporates when it rains, which has little significance for groundwater recharge.
Outbreak of rain: long duration, low intensity and large coverage area. Under the condition of suitable surface conditions, this kind of rain can replenish groundwater in large quantities, which is of great significance to groundwater recharge.
Rainstorm: It lasts for a long time and has a large average intensity, which often leads to surface floods. Its impact on groundwater is also significant. It often destroys the original surface structure and poses a threat to mines and some projects.
When analyzing the recharge function of atmospheric precipitation, we should not only consider the absolute precipitation, but also consider the nature (such as duration and intensity), form (liquid and solid) and type of precipitation. During hydrogeological investigation, monthly average, annual average and multi-year average data of precipitation should be collected.
evaporate
At room temperature, the process of water entering the atmosphere from liquid to gas is called evaporation. Evaporation in nature can be carried out on water surface, rock and soil surface and plant branches and leaves. Therefore, according to the different evaporation properties, it can be divided into three types: water surface evaporation, soil surface evaporation and leaf surface evaporation. Evaporation is still expressed in millimeters of water layer thickness.
(1) water surface evaporation
Water surface evaporation refers to the evaporation of surface water in a certain area within a certain period of time. Evaporation is measured by water pan, and its value is expressed by evaporation, indicating the size of evaporation in an area.
Water surface evaporation is affected by many factors, which are related to evaporation surface temperature, air saturation deficit, wind speed and air pressure. The higher the evaporation surface temperature, the greater the saturation loss, the greater the wind speed, the lower the air pressure, the faster the evaporation speed and the greater the evaporation capacity.
(2) soil surface evaporation
Soil surface evaporation refers to the evaporation of soil surface moisture in a certain period of time. Soil surface evaporation is not only related to air temperature, saturation deficit, wind speed and air pressure, but also related to groundwater burial conditions and soil properties. Generally, when the local groundwater is shallow, due to the capillary action of soil, the groundwater is sucked to the surface and the evaporation increases; The deeper the burial, the smaller the evaporation. The finer the soil particles, the more water the soil layer often keeps, and the greater the evaporation.
(3) Leaf evaporation
Leaf evaporation refers to the evaporation of water on the leaves of a plant in a certain area within a certain period of time, and its evaporation process is called transpiration (evapotranspiration).
It must be noted that the evaporation provided by the meteorological department can only explain the relative intensity (evaporation degree) of evaporation, and does not represent the actual evaporation.
Finally, the relationship between air pressure and groundwater and the concept of humidity coefficient are introduced.
The pressure exerted by atmospheric mass on the ground is called air pressure, which is usually expressed by the height of millimeter mercury. The air pressure in the standard state is 760mmHg, which is equivalent to about 105Pa.
The difference of air pressure in different places causes airflow, cold and warm air confrontation and rainfall. Affected by monsoon, rainfall in eastern China is mostly concentrated in summer, and it is cold and dry in winter. The change of air pressure can affect the fluctuation of groundwater level, thus causing the change of spring flow. If the air pressure drops, the spring flow will increase.
Humidity coefficient (KB) refers to the ratio of annual precipitation (X) to annual evaporation (Z) (water surface evaporation) in an area.
General hydrogeology
Humidity coefficient reflects the abundance and shortage of water and the dry and wet characteristics of climate in a region. The greater the KB, the richer the water in this area; On the contrary, the stronger the evaporation, the less water. The former is beneficial to the formation of groundwater, while the latter is not. The relationship between humidity and humidity coefficient in this area is as follows:
General hydrogeology
General hydrogeology