Please check the following link and PDF attachments.

https://www.researchgate.net/post/CanI_calculate_soil_moisture_Index_SMI_for_my_study_area_using_Landsat_for_four_different_periods_with_out_ground_measurement

Please go through this article 

https://googleweblight.com/?lite_url=https://www.drought.gov/drought/data-maps-tools/soil-moisture&ei=rTagg_xU&lc=en-IN&geid=28&s=1&m=689&host=www.google.co.in&ts=1487393497&sig=AJsQQ1CA1Bf-j9bIRuxYlcUceIHCzX5eMA

If you do not have field data about the soil moisture in your study area, you will be able to have a approach towards this value using Topographic Wetness Index from one Digital Elevation Model. That also makes me think of the the Normalized Difference Water Index (NDWI) by using near-infrared (NIR) and short-wave infrared (SWIR) bands from Landsat 8 or Sentinel 2A. SAGA GIS (free) has a module to calculate the Soil Moisture Content.

thank u. will go with the TWI

The wetness map was generated based on coefficient derived by Baig et al 2014.

http://www.tandfonline.com/doi/abs/10.1080/2150704X.2014.915434

Dear Adeola m a

The TCW and NDMI are very useful and very accurate.

Good luck.