Step 1: Understand the operation of a cylinder infiltrometer.
A cylindrical ring is inserted into the soil, water is applied within it, and the rate of water level decrease is recorded. This provides a one-dimensional (vertical) infiltration rate, but it is subject to errors.
Step 2: Assess the impact of each variable on the measurement.
- A. Cylinder diameter: Water can spread laterally underground from the cylinder. This lateral spread is proportionally less significant for larger diameter cylinders. Consequently, a smaller diameter ring will result in a greater overestimation of the true vertical infiltration rate compared to a larger one. Diameter influences the outcome.
- B. Thickness of cylinder: A thick cylinder wall causes increased soil disturbance during insertion, potentially altering the soil structure surrounding the ring and affecting the measured infiltration rate. Thickness is a relevant factor.
- C. Metal composition of cylinder: Provided the metal is inert and does not interact with water or soil (e.g., steel, aluminum), its composition will not significantly affect the physical process of infiltration. This variable is irrelevant.
- D. Cylinder installation depth: The cylinder must be inserted sufficiently deep to prevent subsurface leakage and to contain vertical flow. Inadequate depth can introduce errors. Installation depth is crucial.
Step 3: Determine the appropriate combination.
Factors A, B, and D can all impact the measured infiltration rate. Factor C cannot.