Implementation in the Gravity Flow method

Adding the GA method to the Gravity Flow model is slightly more complicated because the GA method assumes a uniform, infinite soil column.

Many GF models include a fine discretization near the ground surface to cap­ture the infiltration dynamics. However, the GA method is an approximation to these infiltration dynamics and makes the refined grid unnecessary. Using multiple layers within the GA's depth of infiltration would often result in varia­tions of water content with depth, which makes the infiltration calculations dif­ficult.

Multiple UZ layers are handled by reducing the amount of infiltration depend­ing on the actual storage available in the depth of infiltration. In other words, the amount of infiltration and depth based on the average water deficit in the GF layers in the root zone is calculated and then MIKE SHE calculates if the infiltration exceeds the actual available storage in the depth of infiltration. If it does then MIKE SHE only infiltrates the available storage. This is a reasona­ble physical approximation for the total amount of infiltration, but might under­estimate the rate of infiltration if the upper layers are very dry.  

The GF module allows you to have multiple soil types in the same column. Multilayer GA models are technically possible, but this is not implemented in MIKE SHE. If there are multiple soil types within the infiltration depth, a Warn­ing is issued. The infiltration is  only calculated based on the soil type in the uppermost UZ layer.

The GA infiltration with the GF method proceeds as follows

1.       Calculate UZdeficit based on entire UZ deficit and an estimate of the cur­rent recharge to SZ

2.       If Case 1,

a) Calculate time to ponding

b) if time to ponding is greater than the time step, infiltrate all the rain­fall in the time step

c) If time to ponding is less than the time step, infiltrate only the allowed amount.

d) Recalculate the water contents in the layers, assuming, each layer is successively 100% saturated before the layer beneath receives water.

3.       If Case 2,

a) Calculate available storage in the

b) If time to ponding is greater than the time step,

i) Calculate the maximum rainfall

ii) Calculate the maximum depth, and then calculate the maximum actual storage in the layers down to the max depth

iii) Infiltrate either (i) or (ii)

iv) Update all water contents assuming that each successive layer is 100% saturated.

c) If time to ponding is less than the time step,

i) infiltrate only the allowed amount

ii) Update all water contents assuming that each successive layer is 100% saturated

iii) Then add the excess rainfall to OL

Internal infiltration rates

The rate of exchange between the layers is currently governed by Ks ( ). This is unchanged, except that during the GA infiltration, it is ignored.  

Bypass Flow

The handling of bypass flow directly to SZ is not affected by the GA imple­mentation.

Overland-groundwater leakage coefficient

If a leakage coefficient is specified that is lower than the specified Ks then the lower value is used.