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Steps of crop water requirement calculation

Short answer

Compute reference evapotranspiration (ET0), multiply by crop coefficient Kc for ETc, add percolation/leaching needs, subtract effective rainfall and soil moisture contribution to obtain net irrigation requirement. Convert to gross requirement using efficiency; the peak demand period governs canal design.

Write down the steps for crop water requirement calculation.

Crop water requirement (CWR) is the total water needed by a crop from sowing to harvest, equal to the consumptive use plus losses and special needs, supplied by rainfall, soil moisture and irrigation.

Steps of calculation

  1. Collect climatic data (temperature, humidity, sunshine, wind, rainfall) and the crop calendar (sowing, growth stages, harvest).
  2. Compute reference evapotranspiration EToET_o by Penman, Blaney-Criddle, Hargreaves, or pan evaporation method (ETo=KpEpanET_o = K_p E_{pan}).
  3. Choose the crop coefficient KcK_c for each growth stage from FAO-24/56 tables.
  4. Compute crop evapotranspiration:
ETc=Kc×EToET_c = K_c\times ET_o
  1. Add other needs: percolation and seepage (rice), land preparation, nursery, leaching requirement, and water for planting.
  2. Compute effective rainfall PeP_e (75% dependable rainfall).
  3. Compute net irrigation requirement:
NIR=ETc+other needs−Pe−soil moisture contributionNIR = ET_c + \text{other needs} - P_e - \text{soil moisture contribution}
  1. Compute gross irrigation requirement using efficiency:
GIR=NIRηa ηcGIR = \frac{NIR}{\eta_{a}\,\eta_c}
  1. Convert depth to discharge: Q=GIR×ATQ = \dfrac{GIR\times A}{T} or express as l/s/ha.
  2. Find the peak demand period which governs the design capacity of canal and structures.

✓ Answer checked by Claude Opus 5.5 (high effort)

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