Crop factors
Plants use increasing amounts of water as they grow. To account for this, a crop factor is used to reflect the amount of water the crop uses at a particular age or growth stage.
Crop factors for carrots have been derived from line source experiments on coarse yellow Karrakatta sands at Medina Research Station (HIA project VG95010). Figure 1 shows the crop factors (pan evaporation replacement rates) that produced optimum yields at two nitrogen levels for carrots watered twice per day.
In this case the 240kg N/ha treatment produced greater than 70t/ha root yields and can be considered a crop of moderate vigour. From about 35 days after sowing in late spring/summer, carrot water requirements increase from about 0.8 times pan evaporation to 1.3 times pan evaporation (130% Epan) near maturity for a moderately vigorous crop, and to 1.5 times (150% Epan) for a high vigour (320kg N/ha) crop.
Crop factors for vegetables on sandy soils are high because of the combination of high water use, high vapour pressure deficits in summer and the rapidly declining hydraulic conductivity of the coarse textured soil as moisture content declines.
Irrigation scheduling
A sound approach is to schedule irrigation using average long-term daily pan evaporation rates and crop factors. A more accurate approach is to schedule irrigation based on the previous day’s actual evaporation where these figures are available from live weather stations. Soil moisture monitoring devices should be used in conjunction with irrigation scheduling to monitor the adequacy of applied irrigation. Crop factors for carrots are shown in Figure 1 and summarised for various growth stages in Table 2.
| Crop interval (days after sowing) | Root crown diameter (mm) | Crop factor |
|---|---|---|
| 0-24 | <1 | <0.8 |
| 25–34 | <1 | 0.85 |
| 35-48 | 1-7 | 1.15 |
| 49-61 | 7-18 | 1.25 |
| 62-83 | 18-28 | 1.35 |
| 84 to harvest | >28 | 1.40 |
Calculating crop water requirement
Average daily pan evaporation for each week of the year is plotted in Figure 2 and listed in Table 3. Crop water requirement will be calculated according to the following formula:
Crop water requirement (mm/day) = (Crop factor) x (daily Epan)
Example calculation: Carrot crop has reached 10mm root diameter stage. What is the average water requirement for the second week in November?
Water requirement = 1.25 (from Table 2) x 6.6 (from Table 3)
= 8.25mm/day
These calculated figures will be used only as a guide for irrigation scheduling.
During periods of hotter or cooler than average weather, adjustments to water application will be made and should be guided by soil moisture probe readings from within the crop.
Acknowledgments
This information was originally authored by Allan McKay.