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qif the ponded surface water depth is greater than potential evaporation,
the ponded surface water depth is reduced by an amount equal to the
potential evaporation and the potential evaporation is set to zero.
The salt transport processes adopted in the model are a derivation of
those reported by Nathan (1993). Salt transport is represented by simple
mass balance accounting based on complete mixing. Rainfall and
irrigation inputs are assigned fixed salinities, and complete mixing within
the soil and groundwater stores are assumed to occur.
The salinity of all internal accessions - including the groundwater
movement to and from drainage channels - is determined by the salinity
of their source (e.g. the salinity of water moving from the groundwater
store to the soil store by capillary rise is at the current salinity of the
groundwater store).
The salinity of partial area runoff and groundwater exfiltration is
evaluated to be a fixed leaching fraction of the current groundwater
salinity, in which the value of the leaching fraction is determined by
calibration. This approach reflects the fact that the salinity of
groundwater has been observed to increase with depth such that salt
concentrations are not the product of complete mixing.
The appropriateness of the conceptual numerical framework was
assessed by application of the model to both an irrigated and dryland
catchment. A description of each of the selected study areas and
simulation results are described below.
The chosen irrigated catchment was that used to calibrate the
SWAGSIM model, namely the Drain 14 catchment located in the Kerrang
Irrigation District of northern Victoria. The Drain 14 catchment has an
area of 2728 ha and is characterised by the existence of a shallow
watertable over a large proportion of the catchment that generally lies
within one to two metres of the soil surface. Refer to the SWAGSIM
model application study for a full description of the study area and input
data (Prathapar et al . 1995).
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