Abstract
Vapour Pressure Deficit (VPD) is an important indicator of atmospheric water demand and plant transpiration. In greenhouse production, changes in VPD can influence the rate at which crops lose water through transpiration.
This research explores whether accumulated VPD can be used as a practical environmental indicator for estimating moisture loss from a crop production system.
The long-term objective is to investigate whether accumulated VPD can contribute to irrigation scheduling decisions by linking atmospheric demand with observed water loss.
1. Research Objective
The primary objective of this research is to investigate the relationship between accumulated VPD and moisture loss in a controlled crop production environment.
Can accumulated atmospheric water demand, represented by VPD, be related to the amount of water lost from the crop production system?
Establishing such a relationship could provide a basis for developing climate-driven irrigation strategies that respond to actual atmospheric demand rather than relying only on fixed irrigation schedules.
2. Why VPD?
Vapour Pressure Deficit represents the difference between the amount of moisture the air can hold at saturation and the actual amount of moisture present in the air.
Higher VPD generally represents greater atmospheric demand for water, while lower VPD represents lower atmospheric demand.
Where:
- VPD = Vapour Pressure Deficit
- SVP = Saturation Vapour Pressure
- RH = Relative Humidity
3. Experimental Approach
The research approach combines environmental monitoring with direct measurement of water movement through the crop production system.
4. Methodology
The experimental concept uses changes in growing-media weight to estimate moisture loss.
Step 1 — Initial Measurement
The growing container and media are weighed to establish an initial reference weight.
Step 2 — Water Application
A known quantity of water is added to the growing system.
Step 3 — Drainage Measurement
Drainage is measured to determine how much of the applied water leaves the system.
Step 4 — Final Weight
The growing system is weighed again after the observation period.
Step 5 — Climate Data
Temperature and relative humidity data are collected and used to calculate VPD.
Step 6 — Accumulated VPD
VPD values are accumulated over the observation period to represent the total atmospheric demand experienced by the crop.
5. Observed Water Loss
Preliminary observations from a greenhouse production environment demonstrated substantial variation in daily water loss.
| Date | Observed Water Loss |
|---|---|
| 8/22 | 162 L |
| 8/23 | 224 L |
| 8/24 | 217 L |
| 8/25 | 126 L |
These observations demonstrate that water loss can vary considerably between observation periods, highlighting the importance of environmental conditions when developing irrigation strategies.
6. Daytime and Nighttime VPD
Preliminary observations were also considered separately for daytime and nighttime periods.
Observed water loss: approximately 175 L
Observed water loss: approximately 135 L
These preliminary observations illustrate that the duration of an observation period alone does not fully describe crop water loss. Atmospheric demand and crop physiological activity must also be considered.
7. Research Concept
The central hypothesis is that accumulated VPD may provide a useful environmental predictor of moisture loss.
8. Expected Analysis
Experimental observations can be analysed by comparing accumulated VPD against measured moisture loss.
A regression model can then be developed to determine whether accumulated VPD explains a meaningful proportion of the observed variation in moisture loss.
Depending on the experimental dataset, linear or non-linear regression models may be evaluated.
9. Discussion
VPD provides a way of describing atmospheric demand, but crop water loss is influenced by multiple factors.
These may include radiation, temperature, relative humidity, crop development, canopy size, ventilation, irrigation status, growing-media characteristics and plant physiological responses.
Therefore, accumulated VPD should be considered as one component of an irrigation decision system rather than an isolated measurement.
The long-term objective is to combine climate data, accumulated VPD and observed crop water use to develop data-driven irrigation recommendations.
10. Conclusion
This research investigates the potential relationship between accumulated VPD and moisture loss in greenhouse crop production.
Preliminary observations demonstrate considerable variation in water loss between observation periods. Further controlled measurements are required to quantify the relationship and establish a reliable predictive model.
The research forms part of the broader ClimaSense objective of connecting environmental sensing, agricultural data analysis and practical crop-management decisions.
11. Future Research
- Collect continuous VPD and moisture-loss measurements.
- Compare accumulated VPD with actual crop water use.
- Evaluate different crop growth stages.
- Investigate different greenhouse climate conditions.
- Develop regression-based water-loss prediction models.
- Investigate VPD-based automated irrigation scheduling.