The observation site of precipitation should be selected in an empty and flat place around to avoid the influence of local topography and features. At present, the instruments for observing precipitation generally adopt 2cra caliber rain gauge and self-recording rain gauge.
(1) observation of artificial rainfall tube
The artificial rainfall tube is a cylindrical metal tube, as shown in Figure 3-2. When it rains, the rainwater enters the water storage bottle from the funnel. After it rains, the rainfall depth can be directly read and recorded by pouring the rainfall in the water storage bottle into a special measuring cup.
Generally, rainfall is measured by artificial rainfall tube at regular intervals, with two-stage observation system (8: and 2: daily) and four-stage observation system (8: and 14: daily) in rainy season. , 2: and 2: the next day), eight-stage system (8: , 11: , 14: , 17: , 2: , 23: and 2: , 5: the next day), and more observation times are needed when it rains heavily. If the artificial rainfall tube is used to observe the snowfall, the funnel and water storage bottle can be taken out, leaving only the outer tube as the snow bearing device.
(2) observation of self-recording rain gauge
Self-recording rain gauge mostly adopts siphon self-recording rain gauge or tipping bucket self-recording rain gauge. The working principle of the commonly used siphon self-recording rain gauge is: after the rain enters the float chamber from the rain receiver, the float is lifted and the self-recording pen is driven to record on the recording paper on the periphery of the self-recording clock. When the rainwater in the float chamber is full, the rainwater is discharged to the water storage bottle through the siphon, and at the same time, the self-recording pen drops to the starting point and continues to rise with the increase of rainfall. In this way, the rainfall process is drawn on the self-recording paper.
From the recording paper of the self-recording rain gauge, the start and end time of rainfall, the cumulative change of rainfall with time can be determined, and the rainfall intensity in different periods can also be extracted from the recording paper. However, the self-recording rain gauge cannot be directly used to measure the snowfall process.
(3) collation of precipitation data
After obtaining precipitation data, the data should be collated. The main contents include: compiling an abstract table of precipitation in flood season; Statistics of maximum precipitation in different periods; Calculate the daily, monthly, annual precipitation, etc. The daily precipitation takes 8 o'clock as the boundary, that is, the precipitation from 8 o'clock yesterday to 8 o'clock today is the daily precipitation yesterday.
According to the research, the precipitation measured by the current rain gauge or rain gauge is small due to the influence of wind, evaporation, wall adhesion and other factors, and relevant comparative observation work is being carried out. There are three types of evaporation: water evaporation, soil evaporation and plant emission.
(1) Observation of water surface evaporation
Water surface evaporation can be divided into land water surface evaporation field and floating water surface evaporation field according to the setting mode of evaporation field. The water surface evaporation observation instruments include E-61 evaporator, 8cm diameter evaporator with basin and 2cm diameter evaporator. Others include FFH-3 evaporator, floating evaporator on water, 2m and 1m large evaporation tanks, etc.
the structure of the evaporator with a sleeve basin with a diameter of 8cm is divided into two layers: an inner basin and an outer basin. The inner basin is an evaporation barrel for directly observing evaporation, which is placed in an outer basin with a larger diameter, and water is injected between the two basins to reduce the influence of ambient air temperature on the water temperature in the evaporation barrel.
E-61 evaporator is an evaporator with a set of pots buried under the ground. Its inner pot (with an area of 3cm) contains water. When it is installed, it is flat on the ground, and a water ring should be set outside the inner pot.
the evaporation observation is at 8 o'clock every day, and the evaporation of the previous day is measured. The observed value of evaporation is calculated according to the change of water surface height indicated by the measuring needle in the evaporator and the daily rainfall.
The above evaporator or evaporation pool is a water surface evaporation observation device set on land. Another kind of floating evaporator is to place the evaporator on a raft floating on the water surface, so that the environmental conditions of the evaporator are closer to the natural water body, and the observed evaporation can better represent the evaporation of the natural water body. However, some studies show that the observation accuracy of floating evaporator is not high because of the limitation of conditions and the influence of wind and waves, and the observation data of floating evaporator can only be quoted when there is no measured data of evaporation pool.
(2) Observation of soil evaporation
The observation of soil evaporation is more complicated than that of water surface evaporation, and at present, weighing soil evaporators are commonly used. It calculates the soil evaporation by measuring the weight change of clods in the evaporator within a certain period of time (usually one day), taking into account the precipitation during the observation period and the amount of water leaked from the soil, and based on the principle of water balance. At present, the commonly used instruments in China are γ γ γ и-5 soil evaporator and large-scale lysimeter. Among them, the γ γ γ и-5 soil evaporator has simple structure, convenient installation and observation, and is widely used.
γ γ и-5 soil evaporator includes two iron drums inside and outside. The inner cylinder is used for cutting and filling soil samples, with an inner diameter of 25.2cm, a depth of 5cm and a caliber area of 5cm. The outer cylinder has an inner diameter of 26.7cm and a depth of 6cm, which is used as the holder of the inner cylinder for placing the inner cylinder. In addition, there is a water collector under the inner cylinder to bear the water leakage from the soil sample in the evaporator, and a drain pipe is connected to the inner cylinder to collect the runoff generated by the soil surface of the evaporator.
(3) observation of plant emission
Plant emission is that water in soil is absorbed by plant roots and transported to the leaves, and then escapes into the atmosphere through the stomata in the intercellular space of the leaves, and the stomata have the ability to scale with the change of external conditions, which can adjust the intensity of water emission. Plant emission is not only a physical process of water, but also a physiological process of plants, so it is difficult to observe it directly. Even under the condition of experimental station, it is only of theoretical value and difficult to directly quote.
(5) collation of evaporation data
collation of evaporation data refers to calculation of daily, monthly and annual evaporation of observed values and statistics of relevant characteristic values. For the water surface evaporation measured by evaporator, because the hydrothermal conditions and wind influence of evaporator are significantly different from those of natural water, the measured evaporation is too large, so the observation results of evaporator can not be directly regarded as the evaporation value of natural water. The research of relevant units shows that the relationship between evaporation intensity and evaporation pool area becomes smaller after the diameter of evaporation pool is greater than 3.5m, so it is considered that its evaporation can represent natural evaporation. Therefore, it is more realistic to multiply the observed data of a large number of small evaporators by the conversion coefficient.