Irrigation Engineering & Crop Water Requirement

Supplying the right water at the right time. Learn reference evapotranspiration (FAO-56 Penman–Monteith), crop coefficients and crop ET, effective rainfall and net/gross irrigation requirement, irrigation efficiencies, surface irrigation (basin, border, furrow), sprinkler and drip system design (emitters, laterals, uniformity, hydraulics, pump sizing), fertigation, deficit irrigation and automation — with Python for ET₀, crop ET and a drip-system run time.

Irrigation uses about 70% of global freshwater withdrawals (and ~80% in India). Estimating crop water needs accurately and delivering water efficiently is the core of irrigation engineering.

Drip and sprinkler (micro-irrigation) can achieve 75–95% application efficiency versus 40–60% for flood irrigation — saving water, energy and fertiliser while raising yields.

An analogy

Crop ET is the plant's daily 'water bill', set by weather (sun, heat, wind, dryness) and crop stage. Irrigation design is like choosing the payment method — flood irrigation is paying in cash with lots of change dropped on the floor; drip is an exact bank transfer.

Crop water requirement

ET₀ is the ET of a reference grass surface — depends only on weather (radiation, temperature, humidity, wind). Crop ET = K_c × ET₀, with K_c varying through initial, development, mid-season and late stages (FAO-56 tables). Subtract effective rainfall and soil storage to get net irrigation; divide by efficiency for gross.

Irrigation methods

Choose by soil, crop, water, topography and cost:

Drip and sprinkler hydraulics

Design aims for high uniformity (EU > 90%): limit pressure variation along laterals to ±10% (flow variation) using friction calculations (Hazen–Williams with Christiansen's F factor), pressure-compensating emitters on slopes, and proper sub-main sizing. System components: pump, filters (screen, disc, sand media), fertiliser injector, valves, mainline, sub-mains, laterals, emitters, flush valves and controllers.

Scheduling and advanced strategies

Scheduling by soil moisture sensors, ET-based water balance (CROPWAT, AquaCrop), plant sensors or satellite ET. Deficit irrigation (regulated deficit, partial root-zone drying) saves water with small yield loss, guided by the FAO yield-response factor K_y: 1 − Y_a/Y_m = K_y(1 − ET_a/ET_m). Automation via IoT controllers, solar pumps and canal automation improves delivery.

Formulas

Worked example: Crop ET

Given: ET₀ 5 mm/day, K_c 1.15

Find: ET_c.

Answer: 5.75 mm/day

Worked example: Gross requirement

Given: Seasonal ET_c 450 mm, effective rain 100 mm, drip η 0.9

Find: IR_g.

Answer: 389 mm (3890 m³/ha)

Worked example: Pump power

Given: 15.4 L/s, head 35 m, η 0.65

Find: Power.

Answer: 8.1 kW (≈ 11 hp)

Penman–Monteith ET₀ and drip design

import math
# Crop water requirement with FAO-56 Penman-Monteith ET0 (daily) and drip design
T, RHmean, u2, Rn, G, z = 30.0, 55.0, 2.0, 15.0, 0.0, 200.0    # C, %, m/s, MJ/m2/day
P = 101.3 * ((293 - 0.0065 * z) / 293) ** 5.26
gamma = 0.000665 * P
es = 0.6108 * math.exp(17.27 * T / (T + 237.3))
ea = es * RHmean / 100
delta = 4098 * es / (T + 237.3) ** 2
ET0 = (0.408 * delta * (Rn - G) + gamma * 900 / (T + 273) * u2 * (es - ea)) / (delta + gamma * (1 + 0.34 * u2))
print(f"reference ET0 = {ET0:.2f} mm/day")
Kc = 1.15                                   # mid-season tomato
ETc = Kc * ET0
print(f"crop ETc = {ETc:.2f} mm/day")
# Drip system for 1 ha: 1.2 m row spacing, emitters 0.3 m apart, 2 L/h, wetted fraction 0.6
area = 10000.0
emitters = (area / 1.2) / 0.3
gross_mm = ETc * 0.6 / 0.9                  # wetted-area reduction, 90% efficiency
vol_L = gross_mm * area
hours = vol_L / (emitters * 2.0)
print(f"emitters {emitters:.0f}, daily gross depth {gross_mm:.2f} mm, run time {hours:.2f} h/day")
print(f"system flow {emitters*2/3600:.1f} L/s")
# reference ET0 = 6.32 mm/day
# crop ETc = 7.26 mm/day
# emitters 27778, daily gross depth 4.84 mm, run time 0.87 h/day
# system flow 15.4 L/s

On a hot, fairly dry day ET₀ is 6.3 mm; mid-season tomatoes use 7.3 mm/day. Drip wets only ~60% of the area and loses little, so gross application is 4.8 mm/day — under an hour of running with 27 800 emitters. The 15.4 L/s system flow sizes the pump, mainline and filters (often split into shifts to use a smaller pump).

Common mistakes

Applications

Key takeaways

Going further