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Infiltration is a key component to hydrological process. We have introduced a simple infiltration model where infiltration rate (infil in mm) depends on the soil water content of the first layer as following:

SW₍₀₎ is the total soil water content and UL₍₀₎ is the soil water content at saturation of the first layer in mm. WPW₍₀₎ is the soil water content at wilting point in mm. P is the precipitation of the day in mm. The surplus water, which has not infiltrated is assumed as surface runoff.

In the code, Sibyll also implemented the GREEN_AMPT approach, which she considers to be better. However it is commented in the code waterbalance.c (line 121) and waterbalance.c (line 159) because it requires data on duration of rain and rain intensity, which are not available in sufficient quality at the global scale. This approach, however, can be used for regional applications, where better data may be available.

Daily evaporation from bare soil E_S) occurs at the simulated fraction of the grid cell (1-f_v) not covered by vegetation (determined annually), which may extend towards the plant-covered area according to the daily status of PFT-specific leaf phenology. Evaporation declines linearly as the soil dries, a simple approach that is suitable for large scales (Huang et al., 1996).

where wr30 represents the relative moisture in the upper 30 cm of the soil column, where soil moisture is taking into account to 2/3 and the second layer to 1/3. The evaporated water is taken to 2/3 from the first layer and to one third from the second layer, while it changes the soil moisture of the whole layer. α represents the Priestley-Taylor-coefficient of 1.32. E_S is limited by the energy which is already used by interception, sublimation and transpiration see Evapotranspiration.

We have introduced a new percolation scheme to be able to simulate the free water in the soil bucket. Percolation is calculated by the storage routine technique (Arnold et al., 1990) as used in regional hydrological models e.g. SWIM.

where FW_(t,i) and FW_(t+1,i) are the soil water content between saturation and field capacity at the beginning and the end of the day for all soil layers i. Δt is the time interval 24 h here and TT_(i) determines the travel time through the soil layer in hours:

where HC_(i) is the hydraulic conductivity of the layer in mm h^-1:

where SC_(i) is the saturated conductivity in mm h^-1 and SW_{(t, i)} is the total soil water content of the layer in mm and UL_(i) is the soil water content at saturation in mm. Thus percolation can be calculated by subtracting FW_(t,i) from FW_(t+1,i) for all soil layer:

Percolation is limited by soil moisture of the lower layer similar to the infiltration approach.

Excess water over the saturation produce lateral runoff and at the base the seepage can be assumed as groundwater runoff (output MSEEPAGE).

• MSWCi is the fractional saturation of soil water content swc in layer i per month.
  This fraction is the portion of the waterholding capacity whc (= field capacity fc - permanent wilting point pwp).
  To have the fraction of the pore space filled with water pwf, take swc · whc + pwp.
  The volumentric water content vwc can be calculated by dividing the pore water fraction by the water content at saturation wsat: vwc = pwf / wsat.
• MDISCHARGE is monthly discharge in hm3/d
• MRUNOFF is monthly runoff in mm/month.
  It is the sum of the three components:
  • MRUNOFF_SURF, the surface runoff (mm/month) that occurs when the upper soil layer is saturated and,
  • MRUNOFF_LAT, the lateral runoff from all layers (mm/month) that occur when the respective soil layer is saturated
  • MSEEPAGE, the seepage water (mm/month) that percolates through the lowest soil layer. This contributes to groundwater recharge but is not identical to that.

Soilwater describes the routine:

waterbalance.c

There is still not all published, see #Evaporation

Sibyll Schaphoff, Jens Heinke

Evapotranspiration