Crop

Irrigating carrots on sands in Western Australia

The essentials

Carrots are a major vegetable crop on the sandy soils of the Swan Coastal Plain.
These sands have very low water holding capacity. When evaporation exceeds rainfall, efficient irrigation is important for producing high yield and quality
A 16 week summer carrot crop grown on sandy soil near Perth may require 900 to 1100 mm of irrigation which is 9,000 to 11,000 kilolitres or 9 to 11 megalitres of water per hectare.

Note that: 1000 litres = 1 kilolitre, 1000 kilolitres = 1 megalitre and 1000 megalitres = 1 gigalitre

Efficient irrigation management is the key to efficient nutrient use. Hence efficient irrigation is needed to minimise environmental impact.
Carrots are considered to be relatively salt sensitive and similar in this regard to lettuce, onions and strawberries.
Two key requirements: An efficient irrigation system and a simple scheduling approach.
The amount of irrigation needed depends on the planting time, soil type, crop vigour and weather conditions
A good approach is to schedule using pan evaporation and adjust for the crop growth stage (using crop factors), then check with soil moisture measuring devices
A crop factor, which varies with crop growth stage is used to determine the proportion of evaporation the crop should receive. For example, a crop approaching maturity with a crop factor of 1.3 should receive 1.3 times pan evaporation.
Evaporation figures can be supplied from direct measurement from a pan evaporimeter or can be calculated from weather station data.
Long-term pan evaporation figures can be used to guide irrigation. However using actual daily evaporation is more accurate. Using actual daily evaporation figures and a crop factor (see Figure 1) the daily water requirement and then the system run time to apply this amount can be calculated as follows:
- Water requirement [mm] = (Crop factor) x (daily Evaporation mm)
- Run time (mins) = (Application rate [mm/hour]/60) x (Water requirement [mm])
Soil moisture probes should always be used to check irrigation adequacy

Introduction

Irrigating efficiently is important for achieving high yield and quality and for minimising effects on the environment. Under-irrigation will compromise yields while over-irrigation can affect root quality, promote excessive fertiliser use, and possibly result in pollution of groundwater and surface water.

Water quality

Carrots are considered to be relatively salt sensitive along with lettuce, onions and strawberries.

Ideally, water quality for carrots should be below 1.0 deciSiemens/m or 550 mg/L total dissolved salts (TDS). On sandy soils and with frequent irrigation, higher salt levels would be tolerated without yield penalty because the soil is not drying seriously between irrigations and it is easy to leach salts below the root zone. In WA carrots are irrigated with water generally in the range from 400 to 800 mg/L TDS. In hot weather, carrot germination has reportedly been reduced with irrigation water above 1,400 mg/L TDS.

The impact of irrigation water quality on carrot yield and quality has not been studied in Australia. It is likely that in some areas, carrot yield is reduced by salt in irrigation water however the impact will depend on climate, soil type and irrigation management. Additional water should be applied to leach salt where irrigation water quality is marginal.

Crop water requirement versus planting time

On loamy soils near-mature carrots are irrigated to apply close to evaporation replacement while on coarse sandy soils up to 1.3 to 1.4 times evaporation are required.

The following table shows the average water requirement for a Stefano crop growing in coarse sand near Perth, Western Australia.

Table 1. Estimate of average carrot (variety Stefano) crop water requirement (mm of irrigation) on coarse sandy soil near Perth, WA for 1999 –2004. (Note that 100 mm of irrigation is equivalent to 1 Megalitre of water per hectare)

Planting date	Predicted harvest date	Total irrigation (mm)	Planting date	Predicted harvest date	Total irrigation (mm)
1-Jan	26-Apr	971	1-Jul	5-Dec	846
8-Jan	5-May	928	8-Jul	9-Dec	870
15-Jan	15-May	879	15-Jul	14-Dec	911
22-Jan	25-May	823	22-Jul	18-Dec	936
29-Jan	4-Jun	773	29-Jul	21-Dec	948
5-Feb	14-Jun	727	5-Aug	25-Dec	975
12-Feb	25-Jun	671	12-Aug	28-Dec	988
19-Feb	5-Jul	635	19-Aug	1-Jan	1007
26-Feb	16-Jul	594	26-Aug	5-Jan	1027
4-Mar	26-Jul	549	2-Sep	8-Jan	1033
11-Mar	6-Aug	521	9-Sep	12-Jan	1050
18-Mar	16-Aug	503	16-Sep	16-Jan	1065
25-Mar	26-Aug	486	23-Sep	20-Jan	1071
1-Apr	4-Sep	476	30-Sep	24-Jan	1081
8-Apr	14-Sep	475	7-Oct	29-Jan	1082
15-Apr	23-Sep	487	14-Oct	3-Feb	1091
22-Apr	1-Oct	503	21-Oct	8-Feb	1099
29-Apr	10-Oct	538	28-Oct	14-Feb	1111
6-May	17-Oct	551	4-Nov	20-Feb	1104
13-May	25-Oct	582	11-Nov	26-Feb	1103
20-May	1-Nov	616	18-Nov	5-Mar	1106
27-May	8-Nov	660	25-Nov	13-Mar	1105
3-Jun	14-Nov	705	2-Dec	20-Mar	1088
10-Jun	20-Nov	748	9-Dec	28-Mar	1076
17-Jun	25-Nov	780	16-Dec	6-Apr	1054
24-Jun	30-Nov	817	23-Dec	15-Apr	1017
			30-Dec	24-Apr	975

Irrigation frequency and timing

Irrigation - Carrots In the warmer months on the sandy soils of the Swan Coastal Plain carrot crops are irrigated daily and in hotter weather are irrigated twice per day.

To maximise the use of off-peak power in summer, pre 8.00 am waterings can be applied. These waterings should apply no more than 8 mm to a crop that has been watered within the previous 24 hours and will usually be limited to less than 6 mm to avoid excessive drainage beneath the root zone. When more that 8 mm/day of irrigation is required, this should be applied in two waterings separated by more than 4 hours. Soil moisture will be monitored with soil moisture probes such as sand (low-tension) tensiometers or TDR probes.

Irrigation for crop establishment

During land preparation prior to planting, water application will be kept to a minimum to minimise leaching losses of residual or mineralised soil N.

During the early stages of crop establishment, the soil surface needs to be kept moist to ensure even germination and to prevent heat induced girdling of young carrot seedlings. This is achieved by multiple light waterings on hot days. During these early stages, total daily water application should not exceed 90 % of Epan in order to minimise nutrient leaching.

Crop factors

Plants use increasing amounts of water as they grow. To account for this when irrigating 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 (HAL/AUSVEG project VG95010). Figure 1 shows the crop factors (pan evaporation replacement rates) that produced optimum yields for carrots watered twice per day in summer. From about 35 days after sowing in late spring/summer, carrot water requirements increase from about 85% of pan evaporation to 1.30 times pan evaporation near maturity for a moderately vigorous crop and to 1.4 times for a high vigour 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.

Figure 1. Crop factor curve for irrigating carrots on sand (McKay, Calder & Deyl, 2000)

Table 2. Pan evaporation replacement rates for a moderately vigorous carrot crop growing in summer in yellow (Karrakatta) sand on the Swan Coastal Plain, Western Australia
Crop interval (days after sowing)	Root crown diameter (mm) A	Pan replacement factor
0 -24	<1	<0.8
25 to 34	<1	0.85
35 to 48	1 - 7	1.15
49 to 61	7 - 18	1.25
62 to 83	18 – 28	1.35
84 to harvest	>28	1.40

Root diameter based on a crop density of 75 plants/m2.

Calculating crop water requirement

See Calculating water requirement

Soil moisture monitoring with tensiometers

See Sand tensiometers