Home' Grower : July 2011 Contents The South Australian Grower -- July 2011
Crop root zone
THE leaching requirement is an additional
volume to that required after refilling the soil
profile. Soils do not normally drain until close
to the full point. Thus the draining water must
compete with the other routes of water loss
from the crop root zone (shown in Figure 2).
Crop water use is the biggest user of the
Crops take up water to replace the amount
lost through the leaves and stems when they
manufacture their energy compounds by a
process called photosynthesis.
The process requires light from the sun
hence occurs during the daytime.
Consequently leaching will be more effective
at night as more of the applied water will go
Soil stored water is represented by the soil
water deficit, DR
Inputs: precipitation, P; irrigation, I;
capillary rise, CR
Outputs: evaporation, E; transpiration, T
Losses: drainage, DP; runoff, RO;
Interception losses: IL associated with
irrigation and precipitation
The amount of soil stored water increases
with root zone depth. More water is required
to leach salt beyond the root zone of deeper rooted crops than shallow rooted crops. The
depth of water in excess of the amount required to refill the soil profile to manage soil
salinity is calculated by:
The above calculation does not incorporate extra irrigation water to account for
irrigation system distribution uniformity, etc.
Support theoretical estimation of leaching requirements
with monitored data. Many irrigators already have soil
moisture monitoring systems in place.
DAILY soil water balancing tools, moisture monitoring equipment and general field
obser vations can be used to associate changes in root zone salinity with irrigation
A tool which enables calculation of the daily water balance and leaching fraction
following rainfall or irrigation is available on the PIRSA website.
Crop coefficients used to calculate water usage assume that the crop is growing under
non-limiting conditions, which is not the case for crops affected by salinity.
Thus, field obser vation is essential to refine this tool.
A case study outlining the use of monitoring tools is outlined in the next article.
• Anthony Fox is senior land management adviser, Adelaide and Mount Lofty Ranges, NRMB, and Jeanette
Chapman is compliance officer for the Environment Proection Agency.
• Details: 08 8523 7718 or firstname.lastname@example.org
Next month: Targeting salinity management within crop root zones
Figure 2: Parameters of the soil
DAW = Depth of applied water (mm)
Dr = Soil Water Deficit or depth to refill the profile, (mm)
LR = Leaching Requirement (no units)
Figure 3: Location of
sensors to assist with
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