This leaves an excess of > 1000 mm/year lost as surface runoff and/or deep drainage
, and with it the potential for N loss.
Assuming transient state conditions, deep drainage
was calculated as:
Robinson B, Freebaim D, Tolmie P, McClymont D (2004) How much deep drainage
occurs in the western cropping belt of southern Queensland?
To minimise deep drainage
, irrigation must be timed so that the amount of water applied does not exceed the field capacity of the soil, while maintaining sufficient soil moisture so that the plants are not water-stressed (Smith et al.
sat] of nearly 90 mm/h at high soil water contents would be very conducive to deep drainage
under high rainfall.
Summer-dominant rainfall areas such as the QMDB have been considered to have a lower risk of deep drainage
than the winter-dominant rainfall zones of southern Australia, because a greater proportion of annual rainfall coincides with high potential evaporation (SalCon 1997; Walker et al.
Conversely, deep drainage
in low, flat, coastal floodplains allows the ingress of saline water to freshwater environments.
can be determined by direct measurement (lysimetry), soil physics methods, measured (drainage by difference) or modelled soil water balance, groundwater level analysis, or using tracers such as chloride (Cl) in the soil or groundwater (Walker et al.
Use of water balance models is expanding rapidly, in estimation of biomass and crop yields where runoff estimation maybe less important, and in runoff and deep drainage
estimation where runoff estimation is important.
occurs when water passes beyond the rootzone, in one way a positive phenomenon by recharging groundwater and removing excess soluble salts from the rootzone, but also potentially negative when it leads to rising groundwater tables, particularly if these are saline.
The expectation of both increased deep drainage
and evapotranspiration has been simulated, while the important role of the lateral fluxes (sub-surface flow and runoff) has been maintained.
Zhang and Dawes 1998) was used to model deep drainage
(by difference) below each B/C horizon leachate sampler on the upper (KH8), mid (KH5), waning mid (KH3), and lower (KHI) slopes of the catchment, in 1996 and 1997 (details of the parameters measured for the modelling, including hydraulic conductivity, are in Pitman et al.