Campbell Scientific Sentek EnviroSCAN Access Tube Installation Guide - Manual (Page 7)

Sit e Selection
© Sent ek Pt y Lt d Page 4
Micro zone selection determines the position of access tubes in relation to the crop and irrigation system.
Micro zone selection considers the:
• Area of root zone and canopy spread
• Water distribution uniformity (sprinkler pattern)
• Moisture pattern of drip irrigation
• Surface, topographic and soil anomalies
The consideration of these factors will assist in finding the best representative position or site for access tube
placement within the macro and micro zones.
Macro and micro zone selection is described in greater detail in the following pages. If you require further
information, consult your Sentek reseller and/or a trained agronomist.
Important factors you should know that affect crop water use
All the factors listed below can have an impact on the way the water is stored in the soil and on the way that
plants use that water. They affect transpiration and evaporation rates and have a direct impact on irrigation
scheduling. In macro zone selection, it is important to consider the way these factors influence water use in
a particular area or zone:
• Climate
• Soils
• Crop
• Cultural
management
• Irrigation
system
Climate
The most commonly recognized factor influencing the amount of crop transpiration is the weather.
Temperature
Crops need to draw up water to compensate for water use through transpiration (water loss through the
leaves) and evaporation (water loss from the surface of soil and leaves). The demand increases with
increasing temperature up to a maximum threshold for each crop (when the stomata close and
photosynthesis stops).
Humidity
Atmospheric demand for transpiration and evaporation is relative to the humidity (amount of water vapour in
the air). The higher the humidity level, the lower the demand.
Wind speed
Crop transpiration and evaporation increase with increasing wind speed, creating an increased water
demand. At higher wind speeds, transpiration eventually decreases due to stomata closure, but evaporation
increases.
Solar radiation
On sunny days, crops can synthesize more basic sugars and more complex plant food compounds, through
the combination of atmospheric carbon dioxide and soil-derived water, than on cloudy days. Although crops
vary in their sensitivity of photosynthetic response, they all require access to greater amounts of soil water.
Rainfall
Rain is generally associated with higher humidity levels and lower solar radiation and temperatures. It
follows that days on which rainfall occurs are associated with lower water demand and use than dry sunny
days.