Measuring soil moisture may be undertaken with nuclear probe, and installing pipes to rooting depth (aluminum pipe I think). However, many try to start and end a measurement period or year with a similar soil moisture, such as a relatively low stremflow period with no recent rain.  Water wells may also be used, and you want the water table level to be stable and the same for the balance period, with no recent rain.  Another approach might use the water pressure in representative deep rooted plants using pressure bomb, no recent rain and using the same plant water pressure to start and end study period.  Pieziometers can be used for water pressure measurement, and various other tools may be available to measure this on a specific location.  If there are uncertainties with ET, you might try comparing dormant periods with low ET, and growing periods where ET is a major component.  If you have enough soil moisture data on a specific area, you may find indicators of likely equal soil moisture from well levels, stream levels, plant water stress, or pieziometer tensions.  The highest deep seepage rates are probably during dormancy periods, when plant water stress is low and water is not captured to replenish the soil moisture used by plants.  

Dear Vahid,

In my opinion the easiest prediction can be made on the basis of soil porosity. Soil can retain water in its adsorption and capillary pores. Under field circumstances the former pores are supposed to be filled with water, while the biggest, so called gravitational pores are just drains the water downwards. Therefore, theoretically the practical water retention capacity of a soil layer equals the volume of capillary pores (0.2-10 μ equivalent diameter). You can measure/estimate it using the pF or the simplest wetting method on undisturbed soil samples. Once you get the capillary porosity data you can calculate the volume of this pore for the whole layer as water retention capacity.

Cheers

Dear Gergely Jakab

Thanks for your comment. What is your idea about using field capacity and supposing that root zone can hold water equal to field capacity. It means that if precipitation minus evapotranspiration minus runoff is greater than field capacity, the remaining amount can be considered as potential drainage.

Dear Vahid,

If you do not have the possibility to measure porosity the use of field capacity would be the optimal solution. However, as I mentioned above pores smaller than 0.2µ took part of field capacity but there were no practical water movement.

Anyway, in my opinion for modeling purposes in the field scale your theory is OK.

Hi Vahid 

Calculating the root zone water storage capacity is not as simple as it sounds, so it always helps to go back and think about the basics in terms of water flow and water storage in soils.

I take it you are interested in a simple 'bucket’ model, so one way to approach this is to think about the water balance and the various water components in the active root zone.

This includes Evapotranspiration (ET), Precipitation (P), Irrigation (I*), Runoff (R), Infiltration (I), Deep Drainage (D), Saturated Water Content (SAT), Drained Upper Limit (DUL), Wilting Point (WP), Lower Limit (LL), Plant Available Water Content (PAWC) and Readily Available Water Content (RAWC).

I have attached copies of few old reports that contain more detail about this that might be of use to you. See page 25 in the .docx file and page 25 in the .pdf file.

I am sure there are other similar reports with better explanations so worth searching the web a bit more as well. 

Hope these help

Regards

KLB

Hello Vahid,

I think the best way to calculate root zone water storage is root zone water balance equation ( see M. K. Shukla, 2014, pp 96- 97. soil physics: an introduction ) as follows:

Infiltration ( into root zone) + Capillary rise into the root zone = evaporation + transpiration + deep percolation + change of water storage of root zone .

Infiltration includes irrigation and/ or rainfall 

Capillary rise into the root zone can be from watertable 

Finally , you can measure change of water storage by gravimetric methos then multiplied by bulk density to get volumetric water content

or you can use any sensor for measuring change of water storage daily 

all the best

Dear friends

Thank you for your valuable comments. In my work, I want to estimate potential recharge i.e. the amount of water which releases from the root zoon to the deeper soil zone (vados zone). Therefore in the water balance equation the unknown variable is recharge:

Richarge=P-R-ET-S

In which p is precipitation, R is surface runoff, ET is actual evapotranspiration and S is the amount of water which remains in the soil (root zone) or the root zone water storage. The actual evapotranspiration from the whole root zone is calculated from SEBAL method using satellite imagery.  Runoff is calculated using SCS method. Finally there is some water which is retained in the root zone and cannot be drained to the deeper soil layers. I want to know how I can calculate that amount?

Yours sincerely

Vahid

Dear Vahid

If you calculate the run off with SCS method, the concept of initial abstraction (Ia=0.2S) is related to soil water content.

It could be considered as soil moisture in time(t) -SM(t)- , then calculate the SM(t+1) again. in time (t+1) the CN changes because of Antecedent Moisture Conditions.

Root zone water content (t+1)= P-R-ET-SM(t+1)+SM(t)

But if the ground water level fluctuation data is available, it is better to calculate recharge by Water Level Fluctuation (WTF) method in specific time period and calculate the S  in that time period, by data driven methods (like regression) find the factor for S. for example 5 or 10% of P, ET, or (P-ET).

the other solution is using remote sensing methods, you could calculate the soil moisture with NDVI and LST parameters and/or calculatinf the Crop Water Stress Index (CWSI).

all the best

Mohamad

Dear Mohammad Zare

I calculated the soil moisture using triangle method on remotely sensed imagery, however, the moisture is only for surface layer (for example 10 cm). how can it be generalized to the root zone?

please check the following links:

http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-06832013000300005

https://www.researchgate.net/post/Is_there_any_satellite_product_for_the_root_zone_soil_moisture_before_2010