Water distribution efficiency ($\eta_d$) measures how uniformly water infiltrates and distributes throughout the crop root zone during an irrigation application, computed as $\eta_d = 100 \times (1 - \bar{y}/\bar{d})$, where $\bar{d}$ is the average depth of water stored and $\bar{y}$ is the average numerical deviation from the mean depth. This definition and mathematical formulation are identical to Christiansen's Uniformity Coefficient ($CU$), commonly used to evaluate sprinkler and trickle irrigation uniformity. Option analysis: (A) Friction coefficient relates to pipe head loss. (B) Runoff coefficient is the ratio of runoff to rainfall. (C) Uniformity coefficient is identical in definition and formulation. (D) Drainage coefficient is the depth of water drained from an area in $24\text{ hours}$.