Meteorological (climate) factors

Evaporation and precipitation, important links in the water cycle in nature, are closely related to the physical state of the atmosphere. Meteorological elements include temperature, air pressure, wind direction, wind force, humidity, evaporation and precipitation, which determine the physical state of the atmosphere. The physical state of the atmosphere represented by meteorological elements is called weather. The multi-year average state of weather in a certain area (represented by the multi-year average of meteorological elements) is called the climate of the area. Meteorological and climatic factors have an important impact on the formation and distribution of water resources. Although slowly changing climatic factors play an extremely important role in the formation of groundwater, rapidly changing meteorological factors have a significant impact on groundwater. Among them, precipitation, evaporation and temperature have the greatest impact.

1. Air temperature

The atmosphere has a certain temperature called air temperature. All complex weather changes are mainly caused by different temperature conditions. Changes in air temperature will directly affect changes in groundwater temperature, and changes in water temperature will change the gas composition in groundwater. For example, as the temperature increases, gas activity increases, and part of the gas will escape from the water, thereby reducing the content of gas components in the groundwater; the reduction in the content of gas components in the water will also cause changes in the chemical composition of the groundwater. In addition, due to the increase in heat, the evaporation of groundwater is strengthened, the amount of water is reduced, and the concentration of water is increased.

2. Humidity

The content of water vapor in the atmosphere is called air humidity. The water vapor content in the atmosphere varies from 0.01% to 4% of the total air volume, 70% of which is distributed at an altitude of 0 to 3.5km.

Water vapor has mass, so it has pressure. Therefore, the amount of water vapor in the air can be expressed by mass or pressure. Humidity is divided into two types: absolute humidity and relative humidity.

Absolute humidity: It is the water vapor content in the air at a certain time in a certain area. When the mass unit is used, it is expressed in grams of water vapor contained in 1m3 of air (g/m3), and the symbol is m; when the pressure unit is used, it is the partial pressure of water vapor contained in the air equivalent to the number of millimeters of the height of the mercury column or in millimeters. Ba means, the symbol is e.

The absolute humidity in the air varies greatly, mainly affected by factors such as temperature and ground surface properties. In warm zones and over vast water bodies or moist soil, the absolute humidity is greater. In areas with low temperatures, the absolute air humidity is very small.

The amount of water vapor that can be accommodated in the air is closely related to the temperature. The higher the temperature, the greater the amount of water vapor that can be accommodated, and vice versa. At a certain temperature, the maximum amount of water vapor that can be accommodated in the air is called the saturated water vapor content at that temperature. It can also be expressed in mass units (M) or pressure units (E). The saturated water vapor content at different temperatures is shown in Table 1-2.

Table 1-2 Saturated water vapor content at different temperatures

Absolute humidity can only indicate the amount of water vapor in the air at a certain moment, but cannot indicate whether the moisture in the air has reached saturation. , therefore, there is the concept of relative humidity.

Relative humidity (r): the ratio of absolute humidity to saturated water vapor content. That is,

Introduction to Hydrogeology

Although the absolute humidity of the air remains unchanged, when the temperature drops, the relative humidity increases. 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 the dew point.

3. Precipitation

When the temperature of the air is lower than the dew point, excess water vapor in the air will condense and fall to the surface in liquid or solid form, which is called precipitation. Precipitation is expressed in millimeters of thickness of the water layer. If the annual precipitation in a certain area is 1000mm, it means that the amount of water falling in the area is spread over the horizontal area of ??the area, and the thickness of the water layer is 1000mm.

Precipitation is one of the main links in the water cycle. The amount of precipitation in a region determines the abundance of water resources in the region and has an important impact on the formation of groundwater resources. Atmospheric precipitation penetrates into the ground and recharges groundwater most commonly. It is the most important source of groundwater. The strength of the recharge effect of atmospheric precipitation mainly depends on two aspects: on the one hand, the intensity and duration of atmospheric precipitation (especially rainfall and snowfall); on the other hand, the local infiltration conditions, such as the lithology and lithology of the vadose zone. Thickness, topography, vegetation, etc. If the rainfall (rainfall intensity) per unit time is large and lasts for a long time, the amount of groundwater that may be recharged will be large; when the infiltration conditions are good, such as surface rock and soil with good water permeability, flat terrain, and good vegetation, the infiltration effect will be The stronger it is, the more groundwater will be recharged.

Different types of rainfall recharge groundwater differently.

1) Heavy rain: short in duration but high in intensity. According to the practice of the meteorological department, rainfall greater than 50mm on the same day or greater than 30mm in 12 hours is called a heavy rain. This kind of rain generally covers a small area and the rainfall process is short. Generally speaking, most of the rainfall does not have time to penetrate into the ground and becomes surface runoff. It often washes away the surface strongly and even changes the original structure of the surface. However, in areas with flat, exposed gravel layers and areas with good vegetation coverage, considerable water can still seep into the ground.

2) Drizzle: short duration, small rainfall, and small raindrops. This kind of rain often evaporates easily while falling, and it has very little recharge to groundwater and is of little significance.

3) Heavy rain: it lasts for a long time, has low intensity, and covers a large area. When the surface conditions are appropriate, this kind of rain can recharge a large amount of groundwater, which is of great significance to the recharge of groundwater.

4) Heavy rain: It lasts for a long time and has high average intensity. It often causes ground floods and has a significant impact on groundwater. It also often destroys the original surface structure and poses a threat to mines and certain projects.

Our country has a vast territory and complex terrain, and the distribution of precipitation in various regions is extremely uneven. In general, precipitation gradually decreases from coastal to inland areas; precipitation in the south is greater than in the north; and precipitation in mountainous areas is often greater than in nearby plain areas. In the central mountain range of Taiwan, the average annual precipitation is more than 3000mm; the annual precipitation in the Yangtze River Basin is more than 1000mm; the precipitation in the Yellow River Basin is mostly 500mm; the precipitation in the northwest region is less than 250mm; the Tarim Basin has less than 50mm; The rainfall in Qiang year is less than 5mm, making it the driest region in my country.

Precipitation in my country is mainly concentrated in summer, with the highest amount in July and August. This situation is most obvious in Northeast and North China.

When analyzing the recharge effect of atmospheric precipitation, not only the absolute amount of precipitation should be considered, but also the nature of precipitation (such as duration, intensity), precipitation form (liquid, solid) and type of precipitation, etc. . During hydrogeological surveys, monthly average, annual average and multi-year average data of precipitation should be collected.

4. Evaporation

The process of water changing from liquid to gaseous state and entering the atmosphere at normal temperature is called evaporation. Evaporation in nature can occur on water surfaces, rocky soil surfaces, and plant branches and leaves. Therefore, according to the different properties of evaporation, it can be divided into three types: water surface evaporation, soil surface evaporation and leaf surface evaporation. Evaporation is still expressed in millimeters of thickness of the water layer.

(1) Water surface evaporation

refers to the evaporation of water on the surface of surface water bodies in an area within a certain period of time. The amount of evaporation is measured using an evaporating dish on the water surface, and its value is expressed in terms of evaporation, which represents the evaporation capacity of an area.

The amount of evaporation from the water surface is affected by many factors. It is related to the temperature of the evaporation surface, air saturation difference, wind speed, air pressure, etc. The higher the temperature of the evaporation surface, the greater the saturation difference, the greater the wind speed, and the lower the air pressure, the faster the evaporation rate and the greater the evaporation volume.

(2)Soil surface evaporation

refers to the evaporation of water on the soil surface within a certain period of time in an area. In addition to air temperature, saturation difference, wind speed, and air pressure, soil surface evaporation is also related to the burial conditions of groundwater and soil properties. Generally, when the groundwater is buried shallowly, the capillary action of the soil will draw the groundwater to the surface and increase the evaporation; when the groundwater is buried deeper, the evaporation will be smaller. The finer the soil particles and the more moisture the soil layer regularly holds, the greater the amount of evaporation.

(3) Leaf evaporation

Refers to the evaporation of water from the leaves of certain plants in an area within a certain period of time. The evaporation process is called transpiration (evapotranspiration).

It must be noted that the evaporation amount provided by the meteorological department can only indicate the relative intensity of evaporation (evaporation degree), and it does not represent the actual amount of evaporated water.

The pressure exerted by the mass of the atmosphere on the ground is called air pressure, often expressed in millimeters of mercury height. Under standard conditions (at sea level with an air temperature of 0°C and a latitude of 45°), the air pressure is 760mm of mercury column height, which is approximately equivalent to 105Pa.

Differences in air pressure in various places cause air flow, and cold and warm air collide, forming rainfall. Eastern my country is under the influence of monsoon climate, so rainfall is mostly concentrated in summer, while winter is cold and dry. Changes in air pressure can affect the rise and fall of groundwater levels, causing changes in spring flow. If the air pressure drops, the flow of spring water will increase.

The humidity coefficient (KB) refers to the ratio of annual precipitation (X) to annual evaporation (Z) (water surface evaporation value) in an area.

Introduction to Hydrogeology

The size of the moisture coefficient reflects the abundance and shortage of moisture in a region and the wet and dry characteristics of the climate. The larger the KB, the richer the water in the area; on the contrary, the evaporation is strong and the water is lacking. The former is conducive to the formation of groundwater, while the latter is not conducive to the formation of groundwater. The relationship between the humidity level and the humidity coefficient in an area is as follows:

Introduction to Hydrogeology

The above various meteorological data can be collected from weather stations in various places. These data are necessary when conducting hydrogeological surveys. It can help analyze the formation of groundwater and predict changes in groundwater.

After meteorological data are collected, they must be sorted. There are two types of maps. One is a contour map, which is a plan map used for a large area (this kind of map is very useful for hydrogeologists). less tidying up); the other is the change process curve chart (Figure 1-2).

Figure 1-2 The relationship between changes in diving water level and meteorology in Beijing