Determine how many days until your next irrigation and how much water to apply, based on soil water-holding capacity, root depth, crop daily water use, and your management allowed depletion (MAD).
Select your soil type (sand, loam, clay), enter root zone depth, daily crop water use, allowed depletion percentage, and current soil moisture status. Get the irrigation interval in days, the net depth per irrigation, and total available water in the root zone.
A farmer watered his tomato crop every Saturday regardless of weather. In cool, cloudy weeks, he overwatered — roots rotted and yield dropped 15%. In hot weeks, the 7-day interval was too long — plants wilted mid-week. With proper scheduling: loam soil at 60 cm root depth, 5 mm/day use, 50% MAD gives 48 mm readily available water. At 5 mm/day, that's a 9.6-day interval — but adjust weekly based on weather and actual soil moisture.
✓ Expert Reviewed: This calculator and its content have been reviewed by agricultural experts. Formulas are based on standard extension service recommendations. For site-specific advice, consult your local agronomist.
Select your soil type (sand holds 80 mm/m, loam 160 mm/m, clay 180 mm/m of available water). Enter root zone depth in metres, daily crop water use (ETc, typically 3-8 mm/day in growing season), allowed depletion percentage (50% for most crops, 30-40% for sensitive ones like vegetables), and current soil moisture as % of field capacity.
The Irrigation Schedule Calculator takes the guesswork out of irrigation timing. It uses soil water balance principles: total available water × root depth × allowed depletion = readily available water. Dividing by daily crop use gives the irrigation interval. This prevents both under- and over-irrigation.
Total Available Water (TAW) = AWC (mm/m) × Root Depth (m). Readily Available Water (RAW) = TAW × MAD%. Current depletion = (1 − current moisture/FC) × TAW. Remaining usable water = RAW − current depletion. Irrigation interval (days) = remaining ÷ daily ETc. Net irrigation depth = RAW (refill to field capacity).
Loam soil (160 mm/m), 0.6 m root depth, 5 mm/day use, 50% MAD, current moisture at 60% of FC: TAW = 96 mm, RAW = 48 mm, current depletion = 40% × 96 = 38.4 mm, remaining = 48 − 38.4 = 9.6 mm, interval = 9.6 ÷ 5 = 1.9 days. Time to irrigate!
Common mistakes: using a fixed calendar schedule instead of soil-water balance, overestimating root depth (many crops only use 60-70% of potential root depth), ignoring rainfall that replenishes soil moisture, and irrigating when crops don't need it. Always verify with a soil probe or moisture sensor.
MAD is the fraction of total available water that a crop can extract from the root zone before suffering water stress. Most field crops tolerate 40-60% depletion; sensitive crops like vegetables and fruits need 30-40%. Lower MAD means more frequent but smaller irrigations. Exceeding MAD causes yield loss even if you irrigate later.
Methods include: (1) Feel and appearance — take a soil sample at root depth, squeeze it. Loamy soil forms a ball at ~50% FC; (2) Tensiometers — measure soil water tension; (3) Sensors — capacitance or gypsum blocks give % readings; (4) Neutron probe (research-grade). For most farmers, the feel method or a $50 moisture sensor is sufficient for scheduling.
Sandy soils hold only 80 mm/m of water, so they need frequent light irrigations (every 2-4 days). Loam holds 160 mm/m, allowing 5-10 day intervals. Clay holds 180 mm/m but water is tightly held, so effective available water is similar to loam. Sandy soils also have high infiltration rates but low water storage — match irrigation frequency to your soil's water-holding capacity.
Subtract effective rainfall from your soil water balance. Not all rain reaches the root zone: light showers (<5 mm) evaporate immediately; 10-20 mm of rain refills about half the root zone; >30 mm may refill it fully. Use a rain gauge and reduce or skip irrigation after effective rainfall. The USDA NRCS provides effective rainfall estimates by region.
Root depth grows with the crop: at planting, roots may be 10-20 cm; at mid-season, 60-120 cm depending on crop; late season may reach 150 cm. Use the current effective root depth (not maximum potential) when scheduling. Shallow root zones mean less stored water and shorter intervals. Adjust root depth monthly as the crop develops.