Soil

Soil storage divides into a root zone layer \(rz\) (i.e. upper soil) and a subsoil layer \(ss\) (i.e. lower soil). The two soil layers share the same soil hydraulic parameters. However, absolute storage values are different due to different thickness of the layers. Root depth (\(z_{root}\); mm) depends on land-use:

Soil hydraulic properties

Soil hydraulic parameters are calculated with the Brooks-Corey scheme (Brooks and Corey, 1966): Pore size distribution parameter \(\lambda\):

\[\lambda=\frac{1}{\frac{\log(\frac{h_{fc}}{h_{pwp}})}{\log(\frac{\omega_{fc}}{\omega_{pwp}})}}\]

where \(h_{fc}\) is soil water potential at field capacity (hPa; \(h_{fc}=63\)) and \(h_{pwp}\) is the soil water potential at permanent wilting point (hPa; \(h_{pwp}=15850\))

Pore size disconnectedness index \(m\):

\[m=b+\frac{a}{\lambda}\]

where \(a\) and \(b\) are parameters with a fixed value of 2.

Salvucci exponent \(n\) (Salvucci, 1993):

\[n=\lambda \cdot a+b\]

Effective soil water content at field capacity \(\omega_{fc}\) (-):

\[\omega_{fc}=\frac{\theta_{fc}}{\theta_{sat}}\]

where \(\theta_{fc}\) is soil water content at field capacity (-) and \(\theta_{sat}\) is soil water content at saturation (-).

Effective soil water content at permanent wilting point \(\omega_{pwp}\) (-):

\[\omega_{pwp}=\frac{\theta_{pwp}}{\theta_{sat}}\]

where \(\theta_{pwp}\) is soil water content at permanent wilting point (-).

Effective soil water content \(\omega\) (-):

\[\omega=\frac{\theta}{\theta_{sat}}\]

Air entry value \(h_a\) (i.e. bubbling pressure, hPa):

\[h_a=\omega_{pwp}^{\frac{1}{\lambda}} \cdot(-1) \cdot h_{pwp}\]

Soil water potential \(h\) (hPa):

\[h=\frac{h_a}{\omega^{\frac{1}{\lambda}}}\]

Wetting front suction \(\psi_f\) (mm):

\[\psi_f=\frac{2+3 \lambda}{(1+3 \lambda) \cdot \frac{h_a}{2} \cdot(-10)}\]

Hydraulic conductivity \(k\) (mm h-1):

\[k=\frac{k_s}{1+\omega^m}\]

with \(k_s\) saturated hydraulic conductivity (\(m h^{-1}\)).

Soil water content at \(10^{2.7}\) hPa \(\theta_{27}\) (-):

\[theta_{27}=\frac{h_a}{-10^{2.7}}^{\lambda_{bc} \cdot \theta_{sat}}\]

Soil water content at \(10^4\) hPa \(\theta_6\) (-):

\[theta_4=\frac{h_a}{-10^4}^{\lambda_{bc} \cdot \theta_{sat}}\]

Soil water content at \(10^6\) hPa \(\theta_6\) (-):

\[theta_6=\frac{h_a}{-10^6}^{\lambda_{bc} \cdot \theta_{sat}}\]

Soil moisture deficit \(\Delta \theta\) (-):

\[\Delta \theta=\theta_{sat} - \theta_{rz}\]