ನೀರಾವರಿ ವೇಳಾಪಟ್ಟಿ ಕ್ಯಾಲ್ಕುಲೇಟರ್

Irrigation scheduling: when and how much

The two questions every irrigation planner answers — when to irrigate andhow much — are the operational heart of farm water management. Wrong answers cost yield (deficit irrigation at critical growth stages) and money (over-irrigation wastes pumping cost, leaches nutrients, encourages disease). FAO-56 Chapter 7 codifies the soil-water-balance approach used by extension services worldwide and by ICAR-IIWM Bhubaneswar for India.

Four soil-water concepts

• Field Capacity (FC): water held against gravity after 2-3 days of drainage. Sandy loam ~22%; clay loam ~30%; clay ~38% volumetric.
• Permanent Wilting Point (PWP): water held so tightly to soil particles that plants cannot extract it. Sandy loam ~10%; clay loam ~15%; clay ~22%.
• Total Available Water (TAW): TAW = (FC − PWP) × root depth. The plant's "water bank account".
• Management Allowable Depletion (MAD): the fraction of TAW you let the crop spend before refilling. Wheat 0.5, paddy 0.2 (continuously wet), cotton 0.55, mango 0.35.

The product RAW = MAD × TAW is the "Readily Available Water" — the working balance. Divide RAW by daily ETc to get irrigation interval (days). Multiply by application efficiency (drip 0.9, sprinkler 0.75, surface 0.55) to get the gross depth to apply.

Worked example: Punjab wheat alluvium

Sandy clay loam: FC 28%, PWP 14%. Wheat root depth at mid-stage 60 cm. Today's measured soil moisture 20%. ETc = 5 mm/day. MAD 0.5.

• TAW = (28 − 14)/100 × 600 mm = 84 mm
• RAW = 0.5 × 84 = 42 mm
• Interval at 5 mm/day = 42 / 5 = 8.4 days
• Net depth to refill = (28 − 20)/100 × 600 = 48 mm
• Gross depth at flood efficiency 0.6 = 48 / 0.6 = 80 mm; at drip 0.9 = 53 mm

The wheat farmer therefore irrigates every 8-9 days, with ~50 mm net (~5 lakh L/ha) per event. Over a 140-day season this is 6-7 irrigations totalling 350-380 mm — close to the CWR calculation. A drip-fertigated wheat field with daily 4-6 mm small applications uses 30-40% less water at higher yield.

How to measure soil moisture cheaply

Tensiometers (₹600-1,200 each) — gauge soil-water tension; deploy 2 per ha at 30 and 60 cm depth. Gravimetric (oven-dry) — most accurate but laborious. Dielectric sensors (Decagon, METER) — ₹8,000-25,000, very accurate. Feel-and-appearance (USDA chart) — free, learnable in one season, good ±5% accuracy. KVK extension officers train farmers in feel-and-appearance during the kharif planning cycle.

Critical growth-stage irrigations

Not all irrigations are equal. ICAR-IARI yield-deficit trials quantify the critical stages — moments when water deficit costs disproportionate yield:

• Wheat: crown root initiation (20 DAS) and booting/grain-filling (80-100 DAS)
• Paddy: tillering, panicle initiation, flowering
• Cotton: squaring, boll-development
• Maize: knee-high to tasselling and silking
• Mustard: branching and pod-filling

Schedule the calculator's interval to ensure no deficit during these windows even at the cost of a slight surplus in non-critical periods. The IW/CPE method (Irrigation Water / Cumulative Pan Evaporation ratio) is a simpler proxy: irrigate when cumulative pan-evaporation reaches a threshold (e.g., 60 mm for wheat, 90 mm for chickpea).

Tying scheduling to drip and pump design

The scheduling calculator's gross depth feeds the Drip Sizing calculator: peak ETc determines daily run-time at the system's lph. The pump HP is sized for peak demand day. If your computed interval is 8 days at 50 mm gross, you can either run the pump 8 hours every 8 days OR 1 hour every day with daily fertigation — the latter is the modern micro-irrigation preference because of more uniform soil moisture and steady nutrient delivery.

Reading the calculator output

The widget returns four numbers: TAW (mm), RAW (mm), irrigation interval (days), and net + gross application depths. Recompute weekly as ETc changes through the season and current moisture moves. Pair with the IMD AgroMet ETo forecast to plan a week ahead.

Sources

Allen R.G. et al. (1998) FAO-56 Chapter 7 — Irrigation Scheduling; ICAR-IIWM Bhubaneswar Soil-Water Characteristics for Indian Soils 2022; USDAFeel and Appearance Method NRCS 2018; ICAR-IARI critical-stages yield-deficit trials 2023.