Classification of atmospheric drought on the basis of the thermohygrometric coefficient of air dryness

According to the sources of the World Meteorological Organization, more than twenty meteorological drought indices are currently used in practice. However, these indexes have a number of disadvantages: i) they are based either on the ratio of long-term precipitation (year, season, month) to evaporation, or vice versa, ii) some of them (for example, SPI) lose their physical meaning in the absence of precipitation during certain months, seasons and the year as a whole, iii) there are large errors in the calculation of indicators that include total evaporation, iv) they do not allow to estimate drought over short time intervals (days, decades), which is important for solving many applied problems.

Given the above circumstances, in this study we propose and justify the use of a new classification of atmospheric drought (AD) based on the thermohygrometric coefficient (THC, ‰): K=(T-τ)/T, where Т – τ = Δ – dew point temperature deficit, Т – air temperature in Kelvin. THC was calculated for the daytime period, when the values of air temperature and vapor pressure (VP) reached maximum. An analysis of the relationship between air temperature, VP and THC values for various gradations of AD intensity was performed on the basis of data between 1961 and 2008 for six meteorological stations in Uzbekistan with different physical-geographical conditions. Oppressing effect of air temperature and humidity on various crops was considered, when identifying the criterion of THC for weak, moderate, strong and very strong AD.

With a weak AD, THC lies in the range of 76-90‰, with a moderate one - 91-105‰, with a strong one - 106-120‰, with a very strong one - more than 120‰. The relationship between air temperature and VP for various AD gradations, regardless of the physical-geographical region, shows that the AD intensity for all gradations lies within the temperature range of 25-47°C, while the VP varies significantly for the same gradations. Weak and moderate AD can occur at an air temperature of about 25°C with a VP in the range of 5.3-7.5 hPa and a humidity deficit of 24-26 hPa. At a temperature of 30°C and above, AD of any intensity can be observed. Very strong AD occurs at a very low air moisture content (at 30°C, the VP is below 3.8 hPa, at 35°C it is below 5.3 hPa, and at 40°C it is below 7.5 hPa). In this case, humidity deficit is above 42 hPa, 53 hPa and 67 hPa, respectively.

Based on the data obtained, a nomogram was constructed to determine various gradations of AD intensity. Its peculiarity is that the calculation of THC values is no longer required to determine the AD intensity gradation. The AD intensity is determined by simply finding the point of intersection of the air temperature value and the value of the VP corresponding to a given moment in time.

This classification of AD makes it possible to create a ground-based monitoring system for air aridity, as well as its forecasting. The usage of this method eventually is going to lead to a rational management of water resources.