Drip (trickle) irrigation delivers measured volumes of water and dissolved soluble fertilizers directly to the root zone under low pressure through emitters.
It achieves the highest water application efficiency (85-95%) and saves 40-70% water compared to surface methods.
Because initial installation and capital investment costs are high, it is economically best justified and most profitable for water-scarce regions producing high-value cash crops such as fruit orchards, polyhouse vegetables, sugarcane, and grapes.

Bio-drainage (biological drainage) is the vertical reclamation and lowering of high water tables using fast-growing, deep-rooted, highly transpiring vegetation (especially tree species like Eucalyptus tereticornis, Casuarina, Populus, and Acacia).
Unlike conventional subsurface tile drainage which generates saline effluent requiring safe disposal, bio-drainage removes excess groundwater solely via biological evapotranspiration into the atmosphere without producing drainage wastewater.

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).

Water Use Efficiency (WUE) of irrigation methods:

• Pitcher (clay pot) irrigation achieves exceptionally high field water efficiency ($\gt 90-95\%$) through continuous ultra-slow auto-regulated sub-surface seepage directly into the root zone without any evaporative or percolation losses.
• Drip (trickle) irrigation provides $80-90\%$ application efficiency by applying water directly at the plant root zone under low pressure.
• Sprinkler irrigation typically achieves $60-70\%$ efficiency, as water is sprayed through the air where wind drift and evaporation reduce efficiency.
• Traditional surface (flood/furrow) irrigation has the lowest efficiency ($30-45\%$) due to substantial deep percolation, seepage, and surface runoff losses.