How to convert the deposition of radionuclides on surface from the atmosphere ($Bq/m^2 d$) to precipitation ($Bq/L$) on surface? These are results from CROM tool and you can check the SRS-19 book.
or the concentration of these radionuclides in the atmosphere ($Bq/m^3$) to precipitation on water ($Bq/L$)?
If I have a value of concentration activity from the atmosphere in Bq/m^3 and I want it in Bq/L as precipitation of this radionuclide on the surface of the water.
What parameters do I need to convert it or what equations may help?
with knowing that I have a value of deposition of this radionuclide in Bq/m^2 day
 A: Usually, you would use a (for example straight-line Gaussian) model to estimate the atmospheric dispersion first. Then wet deposition can be easily included in the model.
If you already know the activity concentration in the air, we can skip the dispersion model.
Nevertheless, you still need the wind speed $u$ and the washout coefficient $\Lambda$. The washout coefficient can be estimated as follows.
$$\Lambda=\Lambda_0\cdot\left(\frac I{I_0}\right)^\kappa$$
where
$\Lambda$ is the washout coefficient in $\mathrm{s^{-1}}$,
$I$ is the precipitation rate in $\mathrm{mm\ h^{-1}}$, e.g. $I=5\ \mathrm{mm\ h^{-1}}$,
$\Lambda_0$ is the reference washout coefficient for the reference precipitation rate $I_0$, e.g. $\Lambda_0=7\times10^{-5}\ \mathrm{s^{-1}}$ for particulate aerosols,
$I_0$ is the reference precipitation rate $I_0=1\ \mathrm{mm\ h^{-1}}$, and
$\kappa$ is the exponent $\kappa=0.8$ for particulate aerosols.
Since we are skipping the atmospheric dispersion model, we may directly calculate
$$D=\frac{c\cdot \Lambda}{u}$$
where
$D$ is the deposited surface activity in $\mathrm{Bq\ m^{-2}}$,
$c$ is the airborne activity concentration in $\mathrm{Bq\ m^{-3}}$,
$\Lambda$ is the above-mentioned washout coefficient in $\mathrm{s^{-1}}$, and
$u$ is the wind speed in $\mathrm{m\ s^{-1}}$.
