Drought can be defined as the lower availability of water compared to the average water availability. Nowadays, drought is more frequent due to climate change, lack in precipitaion , heat waves etc. Drought can persist for days, weeks, months and even longer. Drought impacts the environment, economy, and society. Environmental effects include drying of wetlands, forest fires, loss of biodiversity, etc.; economic issues include loss of water supply, loss of agriculture, etc.; and social effects include the loss of food supply, direct effects on health due to heat waves, increase in food cost, water scarcity, etc.

Drought is generally categorized into three types: meteorological, hydrological, and agricultural drought. Meteorological drought can be defined as the condition when there is less precipitation than average over a long period. Hydrological drought is the condition when the availability of water reserves is below the normal threshold. Agricultural drought is the condition when there is a shortage of water for agriculture and crop yields are affected.

Agriculture drought is the most prominent sector affected by drought because of its dependence on water resources and the soil moisture reserve at various stages of plant growth. Agricultural droughts have a possible impact on food security in drought prone areas. The information about the agricultural drought can be useful for farming systems, irrigation and availability of the water resources.

The satellite based drought indices is very valuable, it can provide drought indices at a global spatial and temporal scale. The two main requirements to calculate agricultural drought indices- at a global scale- from satellite based information are: First, data about the availability of soil moisture at the root zone, and second, the presence data for a long period of time to account for the severity of the drought condition.

The main aim of this product is to make available a drought index generated from the SMOS level-4 Root Zone Soil Moisture product available on CATDS derived from the SMOS level-3 surface soil moisture. SMOS level-4 Root Zone Soil Moisture product provides root zone soil moisture data at a depth of (0-100) cm for the period of 2010-present.

SMOS Level-4 Root Zone Soil Moisture named as SMOS-RZSM is the root zone SM product provides SM information at root zone i.e. the depth of 0-100 cm with a spatial resolution of 40 km on a daily temporal scale. SMOS-RZSM product is derived from the SMOS level-3 surface SM by the diffusion process. SMOS-RZSM product is available on CATDS from 2010 to present.

The algorithm used to calculate the drought index for a given period, either weekly or monthly, from the SMOS-RZSM data. Narasimhan et al. (2005) used the simulated soil moisture data from the SWAT index to calculate the drought index based on the weekly soil moisture deficit.

Here, SMOS-RZSM data to calculate the drought index using the following steps:

1. Median, maximum and minimum of root zone soil moisture value is calculated for each week (or month) in a year using the data spanning from 2010-2023. They are represented as MSM_1,MSM_2…..MSM_52 (or MSM_1, MSM_2 ….. MSM_12).

2. Using these statistics we calculate Soil Water Deficit (SWD)

SWD removes the seasonal variability and makes the drought index spatially comparable to different climatic conditions.

3. We use SWD calculated in step 2 to derive SMDI.

To calculate the SMDI incrementally, the SMDI for 1st week (or month) is initialized and then incremented.

The range of values for SMDI lies between -4 to +4, with -4 representing extreme dry conditions and +4 representing extreme wet conditions

The output is the SMOS level-4 Drought Index (SMOS-DI) product, produced monthly for ascending and descending overpasses separately, from 2010 till the present. SMOS-DI products are available in netCDF format and projected on the EASE-Grid version 2, which is an equal area grid, characterized by various pixel size, being 25 km x 25k m at 30 deg of latitude. and is used for all SMOS products at CATDS.