Field instruments: Rainfall simulators are primarily used to determine which hydrologic parameter in infiltration studies?

Introduction / Context:
Rainfall simulators reproduce controlled rainfall intensities on test plots to study soil–water interactions. They are invaluable for infiltration, erosion, and runoff initiation research under reproducible conditions.

Given Data / Assumptions:

• Adjustable rainfall intensity and drop size distribution.
• Known plot area, slope, and soil condition.
• Collection of runoff and measurement of infiltration.

Concept / Approach:
By applying known rainfall rates and measuring surface runoff onset and volume, the infiltration capacity (time-varying infiltration rate) can be derived as rainfall rate minus measured runoff rate, adjusted for storage.

Step-by-Step Solution:
Apply simulated rainfall of known intensity i(t).Measure runoff R(t) from the plot.Compute infiltration rate f(t) ≈ i(t) − R(t)/A, accounting for initial storage when necessary.In this way, infiltration capacity curves are obtained.

Verification / Alternative check:
Results often align with infiltration models (e.g., Horton-type f(t) curves) under similar soil conditions, validating simulator-based measurements.

Why Other Options Are Wrong:
Evaporation and precipitation totals are not directly measured with simulators; runoff is a by-product but the main target is to infer infiltration behavior.

Common Pitfalls:
Neglecting plot edge effects, splash losses, or nonuniformity of simulated rainfall, which can bias infiltration estimates.

Final Answer:
Infiltration capacity (infiltration rate).