Eight different irrigation treatments were tested in each experimewntal year.
Climate, soil moisture content, crop development, pests, weeds and other aspects of the crop were recorded.
Eight irrigation treatments were conducted in 2010, 2011 and 2012, under a polytunnel. Those treatments were named A to H (Table 4). Irrigation events were triggered according to the available water content (AWC) within the rooting zone (Appendix C). Irrigation trials were readjusted after 2010 trials, slightly modifying the schedule through the season, and rejecting the poor performing/unsuitable schedules. Thus in 2011 and 2012, stress was induced during different stages of the crop.
Each irrigation treatment had three replicates (Appendix C.2 for plot layout).
The experimental considered onions of the Arthur cultivar, at a targeted density of 52 plants per m2. Onions were drilled and harvested on the dates shown in Table 5. The polytunnels’ shelter were installed in in April/May (Table 5). After these dates, irrigation was the only water input. More details on the experiment are provided in Lacey & Ober (2011and 2012).
63 Table 4 Irrigation regimes for the trials in Broom’s Barn in 2010, 2011 and 2012 (Source: Lacey & Ober 2011; Lacey & Ober 2012).
2010 2011 2012
A1 Typical A2 Typical, end season stress A3 Typical, end season stress B1 Typical, no extra stress B2 Typical with mid & end
season stress B3 Typical with mid & end season stress
C1 Typical, no extra stress,
extended C2 Typical with early & end
season stress C3 Typical with early & end season stress
D1 Less more often, no extra
stress D2 Less more often, no stress D3 Less more often, no stress E1 Less more often, no extra
stress, extended E2 Less more often, end
season stress E3 Less more often, end season stress F1 Excess F2 Less more often, mid &end
season stress F3 Less more often, end season stress, late irrig.
G1 Stress G2 Less more often, early &
end season stress G3 Less more often, then typical, end stress H1 No irrigation H2 Stress all season H3 Stress all season
Table 5 Drilling, harvesting and rain shelter installation dates for onions cv.
Arthur in the experimental station Broom's Barn (Source: Lacey & Ober 2011;
Lacey & Ober 2012).
Year Planting date Harvest date Rain shelters installation
2010 18th March 13th September 28th April
0.566495⁰; 70 m asl), Suffolk, UK. This experimental station is located in one of the most important areas of onion production. The experimental trials were situated under polytunnels.
64 Climate data collection
Climate conditions were recorded under the polytunnel’s shelter by an automatic weather station. This would measure daily temperature (max/min), rainfall (until polytunnel installation), relative humidity, radiation, and wind speed. Figure 9 shows the recorded mean monthly temperature, rainfall and reference evapotranspiration (ETo).
Figure 9 Mean monthly rainfall and ETo (mm) and average temperature (⁰C) recorded in Broom's Barn Research Centre in 2010, 2011 and 2012 under the
polytunnel.
65 Soil data
To simulate soil water movement and retention, information about the different soil horizons – including soil texture and thickness -, and the presence (if any) and depth of any restrictive layer (compaction) were required. Soil texture analyses established that the soils consisted of loamy sand (Lacey & Ober 2011; Lacey & Ober 2012). Hydraulic conductivity and soil water content at saturation (SAT), field capacity (FC), permanent wilting point (PWP), and total available water (TAW) depend on soil texture. FC and SAT were experimentally established in-field, following the methodology explained in section 2.6 used by Zekri and Parsons (1999). According to the soil water content at PWP and SAT the equivalent “soil texture” was determined (Allen et al. 1998) and PWP and TAW determined for that texture.
On the basis of the soil water content at saturation and field capacity, the trials’
soils were categorised as a mixture of sandy loam and loamy sand textures (Allen et al. 1998). For the modelling, three soil horizons of 0.1 m each were considered. This soil depth was chosen for the profiles, as soil water content was measured at the depths of 0.1, 0.2 and 0.3 m in each of the experimental plots. Sandy loam and loamy sand water hydrologic characteristics are shown in Table 6. FC and SAT measurements were compared to soil water content given for different soil textures and modelled soil texture decided by approximation.
Table 6 Soil moisture content at permanent wilting point (PWP), field capacity (FC), saturation (SAT) in %, and hydraulic conductivity at saturation in mm per
day, for sandy loam and loamy sand soils (Allen et al. 1998).
PWP (%) FC (%) SAT (%) KSAT (mm d-1)
Sandy loam 10.0 22.0 41.0 500
Loamy sand 8.0 16.0 38.0 800
Average soil texture for each irrigation treatment and each of their 0.1 m top layers are given in Table 7.
66 Table 7 Average soil texture for each soil layer and irrigation treatment (LS:
Loamy sand; SL: Sandy loam).
Irrigation treatment
0-0.1m 0.1-0.2m >0.2m
A LS SL SL
B LS LS SL
C LS SL SL
D LS SL LS
E LS LS SL
F SL SL LS
G LS SL SL
H LS LS SL
It has to be noted that due to the soil texture determination, uncertainty in the simulated water balance could arise.
Soil water data collection
Soil moisture content was recorded with capacitance probes (Decagon 10HS sensors) at depths of 0.1, 0.2 and 0.3 m every 15 min in each plot. The irrigation schedule was based on the calculation of water depletion based on those readings. The amount of water required in the root zone to bring the soil back to its field capacity is known as soil water depletion (Allen et al. 1998). The irrigation thresholds and application rates for each of the treatments are shown in Appendix C.
Crop data collection
Canopy cover estimations were made using light interception records, measured by a hand-held spectral radiometer (Skye Spectrosense 2) to determine the percentage of green cover at a specific point in each plot (Lacey
& Ober, 2011). Measurements were conducted weekly. In 2010 measurements were taken 31, 61, 68, 76, 85, 92, 97, 105, 112, 119, 124, 132, 140, 52, and
67 181 days after drilling, and in 2011: 73, 85, 87, 102, 113, 127 and 143 days after planting. These data were used during the AquaCrop model calibration and validation stages. Root depth was estimated from the capacitance probe data, thus acquiring values of 0.1, 0.2 or 0.3 m (same as depth of moisture probes).
Lacey and Ober (2011; 2012) measured biomass through the growing season..
Plant biomass (fresh weight per plant) was determined approximately every 4 weeks. A sub-sample of 10 randomly selected plants per plot was weighed.
Final green yield was recorded for each of the treatments, as well as, vigour, fall-over, senescence and the effect of some pests and diseases.
Experimental yield records were given as tonnes of fresh green yield per ha and transformed into tonnes of dry matter (DM) per hectare by multiplication with a dry matter conversion factor determined experimentally (11-13%). Experimental yield is higher than the yield harvested on farms, therefore a correction factor of -15% was applied to the yield data following advice from Lacey (pers comm). In experimental trials, every bulb and small plant is collected; however, on commercial farm small bulbs (less than 40 mm) are left on the field. This diameter corresponds to the minimum that harvesters can lift.