You can follow the reference as given below:

R. G. Allen, L. S. Pereira, D. Raes, and M. Smith, Crop evapotranspiration - Guidelines for computing crop water requirements, FAO Irrigation and drainage paper no. 56, 1998.

You can follow the reference as given below:

R. G. Allen, L. S. Pereira, D. Raes, and M. Smith, Crop evapotranspiration - Guidelines for computing crop water requirements, FAO Irrigation and drainage paper no. 56, 1998.

Dear Colleague,

in soil water balance you need available water content (AWC), or readily available water (RAW), which is somehow what you asked: soil moisture storage. But it doesn't meter is it raining or not, this is a permanent state of the certain soil. 

AWC = RAW, which is close to 50% of totally available water (TAW) in the soil.

TAW = FC - PWP  (Field capacity - Permanent wilting point)

So, to calculate AWC you need to know FC and PWP. These water capacity points are determined in the laboratory. If you don’t have lab. equipment FC and PWP can be estimated based on soil texture, structure and organic matter content.

For this, I would recommend frequently used United States Department of Agriculture software SPAW:

https://www.nrcs.usda.gov/wps/portal/nrcs/detailfull/national/?&cid=stelprdb1045331

https://www.apsim.info/portals/0/apsoil/soilmatters/pdf/Mod4.pdf

https://www.apsim.info/Portals/0/APSoil/Estimating%20PAWC-a%20methodology_Ver1.1.pdf

https://www.researchgate.net/post/What_is_the_best_way_to_determine_soil_moisture_content_using_soil_samples_I_do_not_have_the_budget_for_a_probe_from_approximately_10cm_soil_depth

Water balances are sometimes done on a water year basis, selecting the dry time each year where the soil moisture changes are less, negating this issue.  It falls apart if you happen to have some big storms near the end of the water year to rewet soils.  It is best to measure the soil moisture, and to some extent, information from wells and streamflow may be helpful indicators if you are not measuring soil moisture.  Soil type differences exhibit variance in how much water they store and make available to plants or evaporate from surface organics that can hold up to 4 times their weight in water.  Studies of how the soils hold, make water available, transmit water from your area are helpful, but I have seen some conditions such as following a drought or wildfire when the surface can be saturated, and soils below are power dry.  As suggested already, there are ways to measure or estimate soil moisture in study areas or catchments.  Sharing ideas with your soils, hydrology and geology professors may help you refine your research if you are unable to measure or estimate soil moisture changes for short term water balance calculations.

thanks for all :)