Assertion (A) is correct: The seasonal water requirement (evapotranspiration + application losses) for semi-dwarf wheat across India typically ranges from 300 to 600 mm (commonly 400-450 mm requiring 4 to 6 irrigations at critical stages).
Reason (R) is correct: Water requirement is not a single rigid number; it varies dynamically depending on soil texture and water-holding capacity (light sandy soils require more frequent water vs clay soils), effective seasonal winter rainfall, prevailing vapor pressure deficit, and varietal crop duration.
Because Reason (R) directly accounts for why the water requirement spans a range from 300 to 600 mm, Reason (R) is the correct explanation of Assertion (A).