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Index
Evaporation
ET Substitution
Water Movement
in Irrigated Soils
Root Zone
Water Uptake
Pattern
Soil Moisture
Profiles
Oxygen
Total plant water use is ET crop. We talked about the "T" (transpiration) when we discussed the process of photosynthesis and how leaf stomata needed to open to let CO2 into the leaf, which allowed water vapor to leave - transpiration. Now let's look at the E part of ET crop, evaporation.
Water will evaporate from a wet soil surface at some rate which depends on the current climatic conditions, and the extent of canopy shading. Evaporation changes over time in a two-stage process.
This can be shown graphically in the illustration below.
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Total evaporation depends on both the area wetted and the frequency of wetting.
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ET crop is not largely affected by irrigation system type once full canopy cover is reached. The canopy shades the soil, minimizing evaporation.
There is some substitution between E and T. As water evaporates from the soil, it cools humidifies the plant micro-climate and thereby reduces transpiration. If evaporation is eliminated (perhaps with a sub-surface drip irrigation system), then the plant micro-climate isn't cooled or humidified, so transpiration increases.
Downward Movement
Water moves down from upper soil layers to lower layers only when the upper layers are filled to FC. Water above FC is pulled down to lower layers by gravity drainage. If an amount of water enters a particular layer and is not enough to fill that layer to field capacity, none of the water entering that layer will move below that layer.
For most soils, a day or more after an irrigation a root zone which has received exactly the amount needed to rise to Field Capacity (FC) will be at the same soil moisture content (i.e., FC) as another root zone which has received twice as much as needed. The excess above field capacity drains out of the rootzone and becomes deep percolation.
Horizontal Movement
Horizontal movement in the soil is driven by differences in moisture tension. Water will move from areas of low tension (thick films of water around the soil particles) to areas of high tension (thin films of water around the soil particles). Water does not always move from wet to dry, since soil particle size differences will influence the tension as much as the amount of water does. The SMC in a clay soil may be higher than an adjacent sandy soil, but water still move from the sandy soil towards the clay soil if the tension in the sandy soil is lower than in the clay soil.
Water also moves from the soil into plant roots in response to differences in tension. If the moisture tension inside the roots is higher than the tension of the surrounding soil, water will move from the soil into the root. That's why we schedule irrigations before the soil gets too dry. We want the soil to have a lower tension that the roots.
The plant root zone is dynamic. At germination, the actual root depth is essentially zero - quite shallow at best. However, the roots grow very quickly. If water is stored in the soil profile, above the ultimate rooting depth, it is generally assumed that the plant roots can "grow into" the water as fast as they need to. Thus simplified irrigation calculations usually assume that the root zone depth is at its ultimate value even before this is literally true.
Plants do not absorb water from the root zone in equal portions at all depths. The actual pattern varies between crop and soil layering conditions, but a useful approximation is this:
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This is known as the 40-30-20-10 rule for plant water uptake.
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Prior to an irrigation, soil at the surface may be as dry as AD (even drier than PWP). Below the RZ, the soil will probably be near FC, since the crop won't take water from below the RZ.
During an irrigation, the very top layer of soil may be saturated, but usually, some air remains in the soil even during irrigation. So soil just below this top layer will be above field capacity, but below saturation. Water moving down through the soil forms a distinct wetting front. Below the position of the wetting front, the soil remains at its pre-irrigation moisture content.
A graph of SMC vs depth in the soil might look like this.
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Roots need oxygen to function properly. High water tables can damage plant root systems and decrease yields because virtually no oxygen is available below the water table.
Maximum evaporation rate depends on climate and canopy cover
