Crop Evapotranspiration (ETc) & Weekly Watering Schedule for 40+ Vegetables
An empirical agricultural hydrology guide translating atmospheric evaporative demand (ETo) and phenological crop coefficients (Kc) into exact weekly irrigation depths, flow rates, and timer intervals across vegetable families.
The Crop Evapotranspiration Formula: True crop water consumption (ETc) equals reference climate evapotranspiration multiplied by the stage crop coefficient: ETc = ETo × Kc. While seedlings require only 0.4 to 0.6 inches/week10 to 15 mm/week (Kc = 0.40), mature fruiting crops require 1.2 to 1.8 inches/week30 to 45 mm/week (Kc = 1.05 to 1.20), translating to exactly 0.75 to 1.12 gallons per square foot30 to 45 Litres per square metre of bed area per week.
1. The Science of Evapotranspiration (ET_o) & Crop Coefficients (K_c)
Water loss from a cropped agricultural field occurs simultaneously through two pathways: evaporation directly from the wet soil surface, and transpiration through the vascular stomata of plant foliage. Together, these processes are termed evapotranspiration (ET).
The universally accepted global standard for modeling crop water requirements is the FAO-56 Single Crop Coefficient Method:
Where ET_o is reference evapotranspiration (the water loss rate from an extensive surface of standardized cool-season grass under local solar radiation, air temperature, relative humidity, and wind speed), and K_c is the dynamic crop coefficient.
A crop's K_c value is not constant—it follows a characteristic bell-shaped phenological curve through four distinct developmental stages:
- Initial Stage (K_c,ini): Germination to 10% ground cover. Soil evaporation dominates; K_c ranges from 0.35 to 0.50.
- Crop Development Stage: Rapid vegetative expansion from 10% to 70%–80% ground cover. Stomatal transpiration accelerates rapidly.
- Mid-Season Stage (K_c,mid): Full canopy cover, flowering, and fruit development. Water consumption peaks; K_c ranges from 0.95 to 1.20.
- Late Season / Harvest Stage (K_c,end): Fruit ripening and senescence. Transpiration slows; K_c drops to 0.60 to 0.85.
2. Master Crop Water Demand & K_c Matrix (40+ Vegetables)
The table below provides calibrated Kc values, effective root zone depth in both inchescm, and peak mid-season weekly water requirements in inches per weekmm per week and gallons per plant per weekLitres per plant per week under warm summer conditions (ETo ≈ 1.5 inches/week / 38 mm/week).
| Crop Family & Species | Initial $K_c$ | Peak Mid $K_c$ | Effective Root Depth | Peak Weekly Water Depth | Critical Moisture Stage |
|---|---|---|---|---|---|
| Solanaceous Family (Nightshades) | |||||
| Indeterminate Tomato | 0.45 | 1.15 | 24 to 36 in60 to 90 cm | 1.75 in/wk (~1.09 gal/sq ft)44.5 mm/wk (44.5 L/m²) | Flowering & Fruit Expansion |
| Determinate Bush Tomato | 0.45 | 1.05 | 18 to 24 in45 to 60 cm | 1.58 in/wk (~0.98 gal/sq ft)40.1 mm/wk (40.1 L/m²) | Simultaneous Fruit Load |
| Bell & Hot Pepper | 0.40 | 1.02 | 16 to 22 in40 to 55 cm | 1.52 in/wk (~0.95 gal/sq ft)38.6 mm/wk (38.6 L/m²) | Budding & Fruit Set |
| Eggplant (Aubergine) | 0.45 | 1.10 | 20 to 26 in50 to 65 cm | 1.65 in/wk (~1.03 gal/sq ft)41.9 mm/wk (41.9 L/m²) | Continuous Flowering |
| Maincrop Potato | 0.50 | 1.12 | 14 to 20 in35 to 50 cm | 1.68 in/wk (~1.05 gal/sq ft)42.7 mm/wk (42.7 L/m²) | Stolon Initiation & Tuber Bulk |
| Cucurbit Family (Vining Crops) | |||||
| Summer Squash & Zucchini | 0.45 | 0.98 | 20 to 30 in50 to 75 cm | 1.47 in/wk (~0.92 gal/sq ft)37.3 mm/wk (37.3 L/m²) | Continuous Rapid Harvest |
| Slicing & Pickling Cucumber | 0.45 | 1.04 | 18 to 26 in45 to 65 cm | 1.56 in/wk (~0.97 gal/sq ft)39.6 mm/wk (39.6 L/m²) | Fruit Enlargement (Prevent bitter) |
| Winter Squash & Pumpkin | 0.40 | 0.96 | 28 to 38 in70 to 95 cm | 1.44 in/wk (~0.90 gal/sq ft)36.6 mm/wk (36.6 L/m²) | Vigorous Vine Run & Set |
| Cantaloupe & Honeydew Melon | 0.40 | 1.06 | 22 to 34 in55 to 85 cm | 1.59 in/wk (~0.99 gal/sq ft)40.4 mm/wk (40.4 L/m²) | Netting (Cut 2 wks prior to pick) |
| Watermelon | 0.40 | 1.08 | 24 to 42 in60 to 105 cm | 1.62 in/wk (~1.01 gal/sq ft)41.1 mm/wk (41.1 L/m²) | Rapid Fruit Swelling |
| Brassicaceae Family (Cruciferous Greens) | |||||
| Heading Cabbage | 0.45 | 1.02 | 16 to 22 in40 to 55 cm | 1.53 in/wk (~0.95 gal/sq ft)38.9 mm/wk (38.9 L/m²) | Head Firming & Expansion |
| Broccoli & Cauliflower | 0.45 | 1.03 | 16 to 22 in40 to 55 cm | 1.54 in/wk (~0.96 gal/sq ft)39.1 mm/wk (39.1 L/m²) | Curd & Florets Formation |
| Curly & Lacinato Kale | 0.45 | 0.97 | 14 to 20 in35 to 50 cm | 1.46 in/wk (~0.91 gal/sq ft)37.1 mm/wk (37.1 L/m²) | Vegetative Leaf Flushes |
| Radish (Spring European) | 0.45 | 0.82 | 8 to 12 in20 to 30 cm | 1.23 in/wk (~0.77 gal/sq ft)31.2 mm/wk (31.2 L/m²) | Hypocotyl Swelling (Fast) |
| Alliums & Umbellifers (Bulbs & Taproots) | |||||
| Bulb Onion (Dry storage) | 0.45 | 1.01 | 10 to 14 in25 to 35 cm | 1.51 in/wk (~0.94 gal/sq ft)38.4 mm/wk (38.4 L/m²) | Bulb Swelling (Dry at maturity) |
| Garlic (Hardneck / Softneck) | 0.45 | 0.95 | 10 to 14 in25 to 35 cm | 1.42 in/wk (~0.88 gal/sq ft)36.1 mm/wk (36.1 L/m²) | Scape Emergence to Clove Swell |
| Carrot (Nantes / Danvers) | 0.45 | 1.04 | 14 to 20 in35 to 50 cm | 1.56 in/wk (~0.97 gal/sq ft)39.6 mm/wk (39.6 L/m²) | Root Lengthening & Bulking |
| Beetroot (Table beet) | 0.45 | 0.96 | 14 to 18 in35 to 45 cm | 1.44 in/wk (~0.90 gal/sq ft)36.6 mm/wk (36.6 L/m²) | Root Swelling & Sugar Accumulation |
| Legumes & Leafy Greens | |||||
| Snap Bush / Pole Beans | 0.40 | 1.07 | 16 to 24 in40 to 60 cm | 1.61 in/wk (~1.00 gal/sq ft)40.9 mm/wk (40.9 L/m²) | Blossom Anthesis & Pod Fill |
| English & Sugar Snap Peas | 0.45 | 1.08 | 16 to 24 in40 to 60 cm | 1.62 in/wk (~1.01 gal/sq ft)41.1 mm/wk (41.1 L/m²) | Flowering & Pod Swelling |
| Head & Leaf Lettuce | 0.45 | 0.99 | 8 to 12 in20 to 30 cm | 1.48 in/wk (~0.92 gal/sq ft)37.6 mm/wk (37.6 L/m²) | Heading (Avoid drought stress) |
| Spinach (Savoy / Flat leaf) | 0.45 | 0.94 | 8 to 12 in20 to 30 cm | 1.41 in/wk (~0.88 gal/sq ft)35.8 mm/wk (35.8 L/m²) | Continuous Rapid Growth |
| Sweet Corn (Maize) | 0.40 | 1.16 | 24 to 36 in60 to 90 cm | 1.74 in/wk (~1.08 gal/sq ft)44.2 mm/wk (44.2 L/m²) | Tasseling, Silking & Kernel Fill |
3. Hydraulic Conversion: From Inches of Depth to Timer Minutes
The primary obstacle for home growers and agricultural operators is translating theoretical depths (e.g., "1.25 inches per week") into an exact digital irrigation controller run time (e.g., "25 minutes on Monday, Wednesday, and Friday").
The Irrigation Run Time Derivation
To determine total controller run time per watering session (T_session, in minutes):
T_session = [ V_gal / (N_emitters × Q_emitter × E_eff) ] × (60 / F_weekly)
Where A_bed is bed area in sq ft, D_water_in is target depth in inches, N_emitters is total emitter count, Q_emitter is discharge rate in GPH, E_eff is drip distribution uniformity (0.90 for precision pressure-compensating emitters), and F_weekly is irrigation events per week.
Field Example: Standard 4×8 Raised Bed
Consider a 4×8 ft raised bed (32 sq ft)1.2×2.4 m bed (2.88 m²) planted with mid-season heirloom tomatoes requiring 1.5 inches of water per week38 mm per week:
- Total Weekly Volume: 32 sq ft × 1.5 inches × 0.6233 = 29.9 gallons per week (29.9 gallons113 Litres).
- Emitter Layout: 2 lateral lines of 12-inch (30 cm)30 cm emitter tubing, totaling 16 emitters rated at 0.5 GPH (1.9 LPH)1.9 LPH each.
- Total Zone Flow: 16 × 0.5 GPH = 8.0 gallons per hour.
- Total Weekly Run Time: (29.9 gal / 8.0 GPH) × 60 = 224 minutes per week.
- Programmed Schedule: 3 watering sessions per week (Mon / Wed / Fri) of 75 minutes per session (or 2 cyclic pulses of 38 minutes each to match soil infiltration).
Frequently Asked Evapotranspiration Questions
How is vegetable crop water demand (ETc) calculated?▾
Crop water demand (ETc) is calculated using the FAO Penman-Monteith methodology: ETc = ETo × Kc. ETo represents reference evapotranspiration—the atmospheric evaporative demand of the climate based on temperature, solar radiation, humidity, and wind speed. Kc is the crop coefficient, a dimensionless factor reflecting the crop species, canopy ground cover, and developmental phenological stage.
How much water do most garden vegetables need per week?▾
During peak summer mid-season growth, most fruiting vegetables (tomatoes, peppers, squashes) require 1.0 to 1.5 inches of water per week (25 to 38 mm per week), which equals approximately 0.62 to 0.93 gallons per square foot (25 to 38 Litres per square metre). In hot, arid climates with high wind, demand can surge to 2.0 inches per week (50 mm per week).
What are the most critical moisture stress periods for vegetables?▾
Moisture stress during vegetative growth slows leaf production, but stress during reproductive stages causes irreversible yield loss. Critical stages include: flowering and fruit set in solanaceous crops (causing blossom-end rot and flower abortion), silking and ear fill in sweet corn, pod swelling in legumes, head enlargement in brassicas, and root/bulb expansion in alliums and carrots (causing splitting and woody core formation).
How do you convert weekly inches of water into drip emitter timer minutes?▾
First convert desired depth (inches) to total gallons based on bed area (1 inch of water = 0.623 gallons per sq ft). Divide total bed gallons by your drip zone total flow capacity (number of emitters multiplied by emitter GPH rating). Multiply by 60 to obtain total weekly run minutes, then divide by the desired weekly irrigation frequency (e.g., 3 runs per week).
Size Your Drip Manifold & Program Timers
Transfer your vegetable crop water volumes directly into our drip irrigation hydraulic lab to calculate friction loss, flow rates, and timer programs.
Irrigation Timer & Weekly Flow Lab
Compute total system flow load, friction head loss, zone splitting, and exact timer run minutes based on crop demand.
Soil Wetting Bulb Hydraulics Guide
Model capillary matric suction vs. gravity percolation to space emitters and tubing runs across sand, loam, and clay.
Mulch & Ground Cover Calculator
Calculate woodchip and straw mulch depth to reduce soil surface evaporation by 30% to 50% under drip lines.