Two foundational papers on drought https://doi.org/10.1038/267192a0 and https://doi.org/10.1080/02508068508686328 indicating still-relevant challenges for answering your questions.

Ilan Kelman thank you for u attention!

Dear Maftuna Karimboeva,

The references provided by Ilan Kelman are very useful as background and fundamental material for the definition of droughts. In addition, you might further think about the context of drought in your study. What is the final aim of your study and what is the most appropriate definition of drought connected to that aim?

Once you have a clear drought definition, you can further think about the operationalisation of this definition, e.g. which variables need to be measured and/ or simulated at which spatial and temporal scales to assess drought and possible historic and future changes in drought.

Good luck,

Martijn

Drought is a relative term used to indicate periods of moisture deficit, either in climatic situations or in soil science referring to shortage of moisture for a given crop. Thus in climatic Science, absence of rainfall for more than two weeks is officially deemed a drought in Britain, whereas in Westland, New Zealand, no rain in 24 hours is regarded as a drought! In arid and semiarid areas, the term becomes even more subjective, e.g., at Suez in Egypt, the average annual precipitation is a mere 5 cm/a. On the other hand, 12-15 cm is normal when it does rain!

In soil science, pf values are used to indicate moisture levels and the wilting point for sunflowers forms the basis of estimates of dryness. However, if you study the actual wilting point of different plants, there is tremendous range of values starting with extremely low values for cacti and desert plants going up to virtually waterlogged conditions for mangroves trees. The latter use pneumatophores to carry air down through the waterlogged soils to their roots.

See: Journal of Ecology, 48: 97-105 and Journal of Soil Science, 25; 448-460.

Stusrt.