Hydrology instrument — what does a lysimeter measure? A lysimeter is primarily used to measure which hydrologic quantity for a vegetated plot under controlled conditions?

Introduction / Context:
Estimating water loss to the atmosphere is crucial for irrigation design, water resources planning, and drought assessment. A lysimeter isolates a block of soil and vegetation to directly measure the mass change due to water use, providing high-quality evapotranspiration (ET) data for calibration of crop coefficients and hydrologic models.

Given Data / Assumptions:

• Soil monolith and vegetation are enclosed within a container resting on a weighing system or drainage measurement setup.
• Inputs (rainfall/irrigation) and outputs (drainage/overflow) are recorded.
• Mass change over time corresponds to ET under controlled boundary conditions.

Concept / Approach:
For weighing lysimeters, ET over a time interval is computed from mass balance: ET = Inputs − Outputs − ΔS where ΔS is the change in storage measured by weight change (mass/ρ). Drainage lysimeters track percolation to infer ET combined with storage changes. Unlike Class-A pans (which measure free-water evaporation), lysimeters capture plant transpiration plus soil evaporation, i.e., actual ET for the crop and soil system studied.

Step-by-Step Solution:
Record rainfall/irrigation volumes and mass changes.Measure percolation/drainage leaving the lysimeter.Compute ET by conservation of mass for the control volume.

Verification / Alternative check:
Compare lysimeter-derived ET with reference ET from Penman-Monteith using local weather data; derive crop coefficients Kc = ET_c / ET_o.

Why Other Options Are Wrong:
Infiltration can be studied but a lysimeter's primary output is ET.
Evaporation only pertains to pans or bare-soil trays; lysimeters include plant transpiration.
Radiation requires pyranometers, not lysimeters.
Groundwater recharge is inferred, not directly measured, and only in drainage-type configurations.

Common Pitfalls:

• Ignoring edge effects and micro-climate differences between lysimeter and surrounding field.
• Neglecting careful calibration of load cells in weighing lysimeters.

Final Answer:
Evapotranspiration (combined evaporation and plant transpiration)