W2, Water use & WUE Flashcards
Define:
Water use efficiency
WUE = yield / water use
= yield / (Δ soil moisture - growing season rainfall)
units should be t/ha/mm
Define and explain:
Vapour pressure deficit (VPD)
The drying power (evaporative demand) of the atmosphere, which affects the efficiency with which a crop or pasture can utilise water.
- Saturated vapour pressure = 100% relative humidity = total amount of water that can be held
- VPD = actual VP of air vs. saturated VP
- ↑ temp (T) = ↑ ability of air to hold water
- ↑ VPD = ↑ aridity = ↑ evaporation
- VPD drives evaporation and crop water demand
- its almost as important as rainfall (in terms of correlation with yield)
- Transpiration efficiency proportional to 1/VPD
- In other words, ↑ VPD = ↓ WUE
In winter, the VPD is low because the temp is low and humidity is high, whereas in summer the VPD is high because the temp is high and humidity is low.
The cooler and wetter an environment, the lower the VPD, and therefore the lower the water demand by crops.
Define and explain:
Plant available water (PAW)
PAW = drained upper limit (DUL) - lower limit of extraction (LLOE) / rooting depth
- DUL = field capacity
- > DUL = waterlogged
- LLOE = the driest that the crop can make the soil
- reflects the ability of the crop to extract water
- PAW = aka bucket size
Unlike the permenant wilting point (PWP = 1.5 MPa), the LLOE is affected by soil properties
- e.g. If there’s B toxicity or salinity, the plant’s ability to extract water is going to be different than the PWP.
- different crops have different LLOE (different root and soil properties = different LLOEs)
What does having the same drained upper limit (DUL) and lower limit of extraction (LLOE) suggest to you?
That the roots aren’t growing down to (or extracting water from) that depth.
Could be due to not rooting that deep or a soil constraint (e.g. B toxicity, salinity)
If you were asked to predict the yield of an area based on the French & Schultz model, how would you do it?
Yield = 20 kg grain / ha / mm rainfall
x intercept (loss from evaporation) = 110 mm (more accurate is 30-170 mm)
True or false?
Subsoil moisture is used more efficiently than shallower moisture.
True, because a higher proportion of surface/shallow moisture is evaporated, meaning the crop’s water use efficiency at that depth is reduced.
When is the crop’s WUE the lowest?
During the earliest stages of development when the canopy cover is the lowest and evaporation is highest.
True or false?
Early sowing promotes higher WUE.
True.
Early sowing into warm soil promotes root growth, meaning the plant is more likely to develop a large root system capable of accessing subsoil moisture.
Sowing early also means that when the crop is in its earliest stages of development and its water use efficiency is lowest, the environmental conditions are more conducive to conserving moisture (low vapour pressure deficit), acting as a buffer for WUE.
What management options are available to you for increasing WUE?
- Early sowing
- promotes root growth
- low VPD during early development when WUE is lowest
- Choose varieties with large/deep root systems and tolerance to subsoil constraints
- Reduce evaporation by:
- Increasing sowing rate/plant density
- Good crop establishment
- Promoting early vigour
- Adequate nutrition
- Canopy management
What are the main drivers for increasing water use efficiency?
Decreasing soil evaporation (early sowing and early vigour) and increasing transpiration efficiency by having more of the early growth happen in the cooler months when the VPD is low.
What is the effect of increasing N concentration (fertilisation) on water use efficiency (WUE)?
- ↑ [N] =
- ↓ evaporation
- ↑ crop water use
- ↑ transpiration
- ↑ transpiration efficiency (TE)
- ↑ water use efficiency (WUE)
- no change in harvest index (HI)
- ↑↑ yield
What is the effect of increasing row spacing from 20 cm to 30 cm (whilst maintaining the same plant density) on water use efficiency (WUE)?
The WUE decreases due to greater soil evaporation which, along with negatively affecting the WUE by itself, also reduces the amount of plant available water, thus negatively affecting yield.
