It is calculated at weekly interval. But dry spell between first moisture to current determine the drought.

Root length, biomass, Sterility%, leaf rolling etc. are the Drought related indices.

Drought indices assimilate data on rainfall, snowpack, streamflow, and other water supply indicators into a compre- hensible big picture. A drought index value is typically a single number, far more useful for decision-making than raw data. The Standardized Precipitation Evapotranspiration Index (SPEI) is an extension of the widely used Standardized Precipitation Index (SPI). The SPEI is designed to take into account both precipitation and potential evapotranspiration (PET) in determining drought.

The indicators and indices listed colored are based on the IDMP Handbook on Drought Indicators and Indices and have been drawn from IDMP and partner literature and online searches. The SPEI is a multiscalar drought index based on climatic data. It can be used for determining the onset, duration and magnitude of drought conditions with respect to normal conditions in a variety of natural and managed systems such as crops, ecosystems, rivers, water resources, etc

Kindly sir have a look at this paper.

https://journals.ametsoc.org/view/journals/apme/53/10/jamc-d-14-0032.1.xml

We have worked with drought indices:

Article Systematization of the calculation of the Standardized Preci...

Dear Dr Miyuru Bandara Gunathilake . See the following useful link: https://agrimetsoft.com/faq/Which%20meteorological%20drought%20indices%20just%20use%20daily%20data%20for%20calculating%20that%20index

The rainfall data should be input in monthly . Kindly see also the following useful RG link: Article Computation of the Standardized Precipitation Index (SPI) an...

Also check please the following very good link: https://spei.csic.es/home.html

Please, have a look at the following link:

http://www.dmcsee.org/uploads/file/70_drought_indices_a_ceglar.pdf

The Standard Precipitation Index (SPI) is a relatively new drought index based only on precipitation. It's an index based on the probability of precipitation for any time scale. Some processes are rapidly affected by atmospheric behavior, such as dry land agriculture, and the relevant time scale is a month or two. Drought indicators describe drought conditions and include precipitation, temperature, stream-flow, ground and reservoir water levels, soil moisture, and snowpack. The prediction of precipitation plays an important role in the prediction of other types of drought, since the water deficit may propagate to other components of the hydrological cycle.