Methods for In-situ Volumetric Soil Moisture Content
In-situ measurement refers to determining the soil moisture content directly within the soil profile without removing the sample. Volumetric soil moisture content represents the volume of water per unit volume of soil.
Analysis of Measurement Methods
- Neutron Probe (Option A): This method uses a radioactive source to emit fast neutrons. These neutrons collide with hydrogen nuclei (primarily in water molecules) in the soil, slowing them down. A detector counts the slowed (thermal) neutrons, and the count rate is calibrated to relate to the volumetric water content. It is an in-situ method.
- Tensiometer (Option B): A tensiometer measures soil water potential or matric potential (tension). It indicates how tightly water is held by the soil particles, not the actual volume of water present. Therefore, it does not directly measure volumetric soil moisture content.
- Time Domain Reflectometry (TDR) (Option C): TDR uses electromagnetic wave propagation along rods inserted into the soil. The speed of the wave is affected by the soil's dielectric permittivity, which is highly dependent on the water content. TDR provides a direct in-situ measurement of volumetric soil moisture content.
- Piezometer (Option D): A piezometer is used to measure groundwater levels or pore water pressure (hydraulic head) in aquifers or soil layers. It is not designed for measuring soil moisture content within the unsaturated zone.
Conclusion
Based on the analysis, the Neutron probe and Time Domain Reflectometry are established in-situ methods for measuring volumetric soil moisture content.
Correct options: A, C.
Important Questions from Soil Moisture Measurement
- An undisturbed soil sample is collected from a field with core cylinder having an internal diameter of $60\text{ mm}$ and length of $100\text{ mm}$. The weights of moist soil and oven dried soil are $0.48\text{ kg}$ and $0.44\text{ kg}$, respectively. Assuming density of water as $1000\text{ kg.m}^{-3}$, the water depth (in metres per metre depth of soil) is ________
(Rounded off to two decimal places) - The elevations of pressure gauge and porous cup of the tensiometer installed in unsaturated zone are 145.8 m and 144.2 m, respectively. Pressure measured at the gauge is $-19.62\times10^3$ $N$ $m^{-2}$. The specific weight of water is 9810 $N$ $m^{-3}$. The estimated pressure at the porous cup is ________ $N$ $m^{-2}$.
- Tensiometer installed in the soil measures
Undisturbed soil sample is collected from a field when the soil moisture is at field capacity. The inside diameter of the core sampler is 7.5 cm with a height of 15 cm. Weight of the core sampling cylinder with moist soil is 2.81 kg and that with oven dry soil is 2.61 kg. The weight of the core sampling cylinder is 1.56 kg. Assuming $\pi = 3.14$, the water depth in centimeter per meter depth of soil (rounded off to two decimal places) is__________.
- Match the following items between Column-I and Column-II with the most appropriate combinations:
Column-I Column-II 1) Neutron Probe P) Open channel flow 2) Pressure Plate Apparatus Q) Deep percolation 3) Tipping Bucket R) Aquifer parameters 4) Current Meter S) Soil moisture 5) Pumping Test T) Rainfall intensity 6) Lysimeter U) Soil-moisture characteristic curve
