Science
Blaney-Criddle Crop Evapotranspiration (ETo, ETc) & Net Irrigation
Calculate FAO Blaney-Criddle reference evapotranspiration ETo = p * (0.46 * T_mean + 8) (mm/day), crop water demand ETc = K_c * ETo, and monthly Net Irrigation Requirement MAX(0, 30 * ETc - P_eff).
Estimate monthly crop water demand when only temperature and latitude are measured!
Full Penman-Monteith stations require solar radiation, wind speed, and vapor-pressure sensors—yet thousands of rural agricultural basins record only daily air temperature! The FAO Blaney-Criddle method estimates daily Reference Grass Evapotranspiration (ET_o, mm/day) from mean monthly temperature T_mean (C) and mean daily percentage of annual daytime hours p (e.g., p = 0.28% in summer mid-latitudes): ET_o = p * (0.46 * T_mean + 8)!
Multiplying ET_o by the growth-stage FAO Crop Coefficient K_c (e.g., 1.15 for mid-season corn/maize) gives daily Crop Evapotranspiration ET_c = K_c * ET_o (mm/day), and subtracting monthly effective rainfall P_eff (mm) from 30-day crop demand yields Monthly Net Irrigation Requirement: NIR = MAX(0, 30 * ET_c - P_eff)!
| Season & Crop Stage (Daytime Share p = 0.28, K_c = 1.15) | Mean Temp T_mean (C) | Reference ETo / Crop ETc | 30-Day Net Irrigation (P_eff = 35 mm) |
|---|---|---|---|
| Peak Summer Mid-Season Corn | 25.0 C | ETo = 5.46 | ETc = 6.28 mm/d | 188.37 - 35 = 153.4 mm |
| Mild Spring Growth | 15.0 C | ETo = 4.17 | ETc = 4.80 mm/d | 143.93 - 35 = 108.9 mm |
| Monsoon Month (P_eff = 220 mm) | 25.0 C | ETo = 5.46 | ETc = 6.28 mm/d | MAX(0, -31.6) = 0.0 mm (Rainfed!) |
FAO Blaney-Criddle Reference Evapotranspiration ETo (mm/day)
Evaluates p * (0.46 * T_mean + 8) from daytime-hour percentage B2 (0 < p < 1) and mean air temperature B3 (C).
=IF(OR(B2 <= 0, B2 >= 1, (0.46 * B3 + 8) <= 0), "Invalid Daytime Share or Subfreezing Temp", ROUND(B2 * (0.46 * B3 + 8), 2))30-Day Monthly Net Irrigation Requirement NIR (mm)
Computes MAX(0, 30 * K_c * ETo - P_eff) using crop coefficient B4 and effective monthly rainfall B5 (mm).
=ROUND(MAX(0, (30 * B4 * B2 * (0.46 * B3 + 8)) - B5), 1)Try the example
Use the original values or change them to test your understanding.