Campbell Scientific Sentek TriSCAN® Manual v 1.2a User Manual

Page 27

TriSCAN Manual Version 1.2a

Page 20

Climate

The most commonly recognized factor in 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 vapor 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 complex plant food compounds, through
combining 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

Rainfall is generally associated with higher humidity levels and lower solar radiation and temperatures. It
follows logically, than days on which rainfall occurs, are associated with lower water demand and use than
dry, sunnier days.

Notwithstanding the care taken to delineate macro zones, some variability in soil moisture levels is
inevitable. For example: on large properties rain events may cover only a portion of the agricultural area,
replenishing some soil reservoirs and leaving others dry.

The aspect or orientation of sloping fields can subject the crop to more or less solar radiation, wind exposure
or water run-off – all affecting crop water use.

Soils

An understanding of how soil type influences plant-soil-water dynamics, and hence irrigation scheduling is
important. Intrinsic soil properties are texture, structure, depth, soil chemistry, organic matter content, rocks
and stones and clay type. Influencing factors include compaction, salinity, water-table development,
drainage rate dynamics and topography.

Soil texture

Water storage in the soil profile depends on the soil texture. At the one end of the spectrum, sandier soils
with a low porosity (total volume of pore space) and weak adhesion forces (strength with which water and
nutrients are held), fill up and drain quickly. The nutrient holding capacity is also low and hence these soils,
in general, require smaller and more frequent irrigations and fertilizer/fertigation applications. In contrast, the
heavier clay soils have a high porosity and strong adhesion properties. Consequently, they replenish and
drain slowly and to a higher total water content than lighter (sandier) soils. Nutrient holding capacity is also
high. An infinite range of textures exist between the two extremes. Textures often change within a profile,
with the layering of different textural bands playing a large part in determining the water holding capacity of a
soil.

Soil structure

Water infiltration rates, and air and water permeability within the soil profile are closely related to the size
and distribution of soil pores (porosity). Porosity, in turn, is dependent upon the arrangement and
aggregation (binding) of sand, silt and clay particles (soil structure). Soil structure is as important as soil