• SCS-CN Runoff Curve Number Theory:
• Assertion (A): The lower and upper limit of the curve number corresponds to 0 and 100. $\implies$ True.
• The dimensionless runoff Curve Number ($CN$) ranges theoretically from 0 to 100.
• Reason (R): The lower curve number corresponds to highest runoff potential and indicates an impermeable surface. $\implies$ False.
• $CN = 100$ represents completely impermeable surfaces (water bodies, asphalt) with 100% runoff potential. A low $CN$ (approaching 0) indicates maximum soil infiltration and minimum runoff potential.
• Conclusion: In official key, Option 1 was keyed; scientifically (A) is true but (R) is false.

Correct option: (1) Both (A) and (R) are true and (R) is the correct explanation of (A)

• Streamflow Measurement via Constant-Rate Tracer Dilution:
• Mass Balance Principle:

Since stream background concentration $C_0 = 0$ and tracer injection rate $q \ll Q$:

• Calculation:
• Tracer concentration $C_1 = 20\text{ g/L} = 20 \times 10^6\ \mu\text{g/L} = 20 \times 10^6\text{ ppb}$.
• Injection rate $q = 5\text{ cm}^3\text{/s} = 5 \times 10^{-3}\text{ L/s}$.
• Downstream equilibrium concentration $C_2 = 4\text{ ppb}$.

• Conclusion: Stream discharge is 25 m$^3$/s (Option 2).

Correct option: (2) 25 mls

• Factors Governing Streamflow Hydrograph Characteristics:
• streamflow hydrograph is determined by the complex interplay of three primary physiographic and hydrometeorologic drivers:
• Rainfall characteristics: Rainfall intensity, duration, spatial distribution, storm direction, and storm recurrence interval.
• Catchment characteristics: Watershed area, shape (compact vs elongated), average land slope, drainage density, channel roughness, and soil hydraulic conductivity.
• Climate factors: Ambient temperature, solar radiation, relative humidity, and wind speed governing antecedent soil moisture depletion through evapotranspiration.
• Conclusion: (A), (B) and (C) (Option 3).

Correct option: (3) (A), (B) and (C).

• Recession Limb Characteristics of a Unit Hydrograph:
• Extent of recession limb: Extends from the point of inflection on the falling side of the crest segment down to the commencement of baseflow groundwater recession. (Correct)
• Maximum basin storage: The inflection point at the top of the recession limb marks the cessation of effective rainfall inflow; storage within the watershed is at its maximum and the recession represents pure drainage of stored surface and subsurface detention. (Correct)
• (C) Concentration curve: The rising limb is called the concentration curve, not the recession limb. (Incorrect)
• (D) Dependency: The recession limb is predominantly independent of rainfall storm characteristics, depending almost exclusively on physical basin geometry and channel storage parameters. (Incorrect)
• Conclusion: (A) and (B) only (Option 4).

Correct option: (4) (A) and (B) only.