Irrigation Scheduling in a Drying Vineyard Block
A 4.2 ha Shiraz block on sandy loam is irrigated by drip at 4 L/h per emitter, 1.5 m vine spacing and 2.8 m row spacing. Mid-January, the grower notes midday leaf curling and a soil probe reading of 18% volumetric water content at 30 cm, falling to 12% at 60 cm. Reference evapotranspiration for the coming week is forecast at 7.5 mm/day, and the crop coefficient for this growth stage is 0.65. The allocation licence allows 1.8 ML for the remainder of the season, and the pump can deliver 22 kL/h. Rainfall of 6 mm is forecast in four days. The grower wants to know whether to extend run times now or wait for the rain.
1. Calculate the block's approximate daily crop water use and estimate how many hours of pumping per day would replace it. Show your reasoning.
Read the worked answer
Crop water use (ETc): ETc = ETo × Kc = 7.5 mm/day × 0.65 = 4.875 mm/day.
Volume per day: 4.875 mm over 4.2 ha. 1 mm over 1 ha = 10 kL, so 4.875 mm × 4.2 ha × 10 = ~205 kL/day.
Pumping hours: 205 kL ÷ 22 kL/h ≈ 9.3 hours/day.
Check against emitters: Emitters per ha = 10,000 ÷ (1.5 × 2.8) ≈ 2,381. At 4 L/h that is ~9,524 L/h/ha = 9.5 kL/h/ha, or ~40 kL/h for the block - well within pump capacity, confirming the pump is not the limiting factor.
Interpretation: Replacing full ETc needs ~9 hours of running daily, which is a large but feasible schedule. The deeper dry layer (12% at 60 cm) signals the vines are already drawing down stored water, so the calculation understates stress risk if only surface readings are used.
2. Given the 1.8 ML remaining allocation and the forecast rain, recommend an irrigation strategy for the next week and justify it.
Read the worked answer
Budget check: 1.8 ML = 1,800 kL. At ~205 kL/day full replacement, the allocation lasts ~8.8 days - the whole remainder of the season cannot be met at full ETc. The grower must deficit-irrigate deliberately rather than accidentally.
Rain credit: 6 mm forecast × 4.2 ha × 10 = ~252 kL, equivalent to about 1.2 days of full ETc - useful but not a substitute for irrigation, especially since 6 mm may not penetrate to 60 cm on sandy loam.
Recommended strategy:
- Do not wait idly for the rain; apply a moderate deficit now (e.g. 60–70% of ETc, ~6 hours/day) to arrest the midday curling while conserving allocation.
- After the rain event, re-probe at 30 and 60 cm; if the deep layer has not improved, prioritise a longer, slower run to push water past 30 cm rather than frequent short runs.
- Reserve a buffer of at least 400–500 kL for veraison and ripening, when stress most affects quality and when heatwaves are likely.
Justification: Matching application to measured soil deficit and forecast ETc prevents both over-irrigation (wasting a capped resource) and sustained severe stress (yield and fruit-quality loss). The plan is adaptive because it responds to post-rain probe data rather than a fixed calendar.