Assertion (A) is correct: Draining excess water from cold, waterlogged soils accelerates soil warming in spring, raising soil temperature by $2 - 5^\circ\text{C}$ and promoting earlier seed germination and root growth.
Reason (R) is correct and provides the exact thermodynamic explanation: Liquid water has an exceptionally high volumetric heat capacity ($C_w = 4.18\times 10^6\text{ J}\cdot\text{m}^{-3}\cdot\text{K}^{-1}$ or $1.0\text{ cal/cm}^3\cdot^\circ\text{C}$) compared to mineral soil solids ($C_s \approx 2.0\times 10^6\text{ J}\cdot\text{m}^{-3}\cdot\text{K}^{-1}$) and air ($C_a \approx 1.2\times 10^3\text{ J}\cdot\text{m}^{-3}\cdot\text{K}^{-1}$). By removing water and replacing it with air, drainage drastically lowers the overall volumetric heat capacity of the soil, requiring far less solar thermal energy to achieve the same temperature rise.

Infiltration dynamics in soils exhibit two distinct phases:
Initial infiltration rate is largely soil surface-controlled and matric suction-driven (governed by surface roughness, cracks, initial moisture, and crusting).
As the wetting front advances downward and the soil profile becomes progressively saturated, suction gradients diminish to zero, and flow is driven purely by gravity.
Under prolonged ponding, the steady-state (final/basic) infiltration rate ($f_c$) equals the saturated hydraulic conductivity of the least permeable sub-surface layer within the soil profile.

In-situ field measurement of saturated hydraulic conductivity ($K_{fs}$):
The Guelph Permeameter is a constant-head well permeameter designed specifically for measuring in-situ field-saturated hydraulic conductivity ($K_{fs}$), matric flux potential, and sorptivity at any desired depth in the vadose zone above the water table.
A double-ring infiltrometer measures surface infiltration capacity ($f$), not specific localized 3D conductivity.
A neutron probe measures volumetric soil water content ($\theta$).
A piezometer measures hydraulic head / water table elevation below the saturated zone.