Hydrometeorological precipitation forecast

As far as precipitation forecasting is concerned, hydrometeorology is no different from meteorology. Hydrometeorological rainfall (or snowmelt) forecast is a professional forecast aimed at the actual needs of river flood control, reservoir flood control, profit-making dispatching and engineering construction. Rainfall (snow melting), flood and flood are both intrinsically related and fundamentally different. Rainfall (snow melting) is not equal to flood, and it will only cause flood under certain basin underlying surface and water system conditions. Flood is not equal to flood, which is caused by many reasons. Therefore, hydrometeorological forecast attempts to closely link meteorological conditions such as atmospheric circulation with hydrological characteristics, and combine rainfall weather model with flood model (snowmelt rate and its runoff should be considered in snow-covered areas). Generally, when rainfall forecast is carried out, the areas where rainstorm and flood may occur in the future are predicted according to factors such as basin landform, basin water regime, quality and standard of water conservancy projects and the relationship between rainfall and runoff. Identify and judge the possibility of catastrophic floods in the basin; After the flood, forecast the flood development trend and the incoming water in the reservoir area. In order to improve the accuracy of rainstorm area, location and time forecast, combined with meteorological satellite, meteorological radar and conventional meteorological observation data, rainstorm monitoring and short-term forecast were developed (see weather forecast). The forecast time limit is several hours to more than ten hours, and the forecast accuracy is high. It is possible to combine rainfall forecast with flood forecast completely, thus extending the time limit of flood forecast and improving the accuracy of flood forecast.

Possible maximum precipitation (PMP)

PMP refers to the theoretical maximum precipitation that may last for a certain period of time in a particular river basin. This precipitation is very important for the design and application of large-scale water conservancy projects. Generally speaking, the maximum possible flood (PMF) should be used as the dam protection standard for these projects. One of the methods to calculate the possible maximum flood is to determine the possible maximum precipitation first. There are many ways to determine the possible maximum precipitation, which can be summarized into two types. One is the frequency analysis of rainstorm (or snowmelt), that is, according to the measured and investigated rainstorm (or snowmelt) data, the precipitation with extremely rare frequency is calculated, which is generally called statistical method. The other is to draw up a reasonable model according to the basic factors of rainstorm-water vapor and dynamic conditions, so as to maximize the indexes of these influencing factors, take their acceptable physical upper limit in meteorology, and then combine these indexes to form a more serious but acceptable time series in meteorology and hydrology, which is generally called meteorological genetic method. In addition, there is the rainstorm displacement method. Since 1975, the estimation of possible maximum precipitation in China has developed rapidly. 1977 compiled the isoline map of possible maximum 24-hour rainfall in China (trial version) and the corresponding isoline map of measured and investigated maximum 24-hour rainfall in China, isoline map of annual maximum 24-hour rainfall in China and isoline map of annual maximum 24-hour variation coefficient in China.

Water surface evaporation and total evaporation in river basin

Water surface evaporation refers to the water surface evaporation of large water bodies in a certain area. Generally, evaporators are used to measure water evaporation. However, due to the different natural conditions between the evaporator and the actual water body, the evaporation measured by the instrument is generally greater than that of the natural water surface, and it varies with the form, installation method and season of the vessel. Therefore, it is necessary to calculate the conversion coefficient of evaporator through experiments to estimate the actual evaporation. In addition, the water surface evaporation can also be indirectly calculated according to the observation data of evaporation control factors, that is, through some meteorological and hydrological factors such as water and heat balance and water balance.

The total evaporation of river basin is also called land evaporation, which is generally expressed by E, and refers to the sum of water evaporation, soil evaporation, plant emission, ice and snow evaporation and phreatic evaporation in a river basin or region. Usually, it is indirectly obtained from the difference between the average annual precipitation (P) and the runoff (R) in the basin, and E = P-R. The total evaporation in the basin is restricted by the possible evaporation and water supply conditions (that is, the degree to which water can be replenished on the evaporation surface). In arid and semi-arid areas, due to the scarcity of precipitation, the evaporation rate may greatly exceed the water supply capacity, and the total annual evaporation in the basin is close to or equal to the annual precipitation. In humid areas, the total evaporation in the basin is close to or equal to the water evaporation in this area. Land evaporation in semi-humid areas is between the above two situations, that is, it is controlled by water supply conditions or possible evaporation. As far as the ocean and the mainland are concerned, the evaporation on the ocean is greater than the precipitation, and the evaporation on the mainland is less than the precipitation, so there must be a net transport of water from the ocean to the mainland.