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__________.
Calculating Soil Sample Volume
First, determine the volume of the soil sample collected using the core sampler dimensions.
- Inside diameter ($d$) = 7.5 cm
- Radius ($r$) = $d$ / 2 = 7.5 cm / 2 = 3.75 cm
- Height ($h$) = 15 cm
- The volume ($V$) of a cylinder is calculated using the formula: $V = \pi r^2 h$
- Using the given $\pi = 3.14$:
$V = 3.14 \times (3.75 \text{ cm})^2 \times 15 \text{ cm}$
$V = 3.14 \times 14.0625 \text{ cm}^2 \times 15 \text{ cm}$
$V = 3.14 \times 210.9375 \text{ cm}^3$
$V = 662.45625 \text{ cm}^3$
Determining Water Weight and Volume
Next, calculate the weight of water in the sample and its corresponding volume.
- Weight of core sampling cylinder with moist soil = 2.81 kg
- Weight of core sampling cylinder with oven dry soil = 2.61 kg
- Weight of the core sampling cylinder = 1.56 kg
- Weight of water = (Weight of moist soil) - (Weight of oven dry soil)
- Weight of water = 2.81 kg - 2.61 kg = 0.20 kg
- Convert weight of water to grams: $0.20 \text{ kg} \times 1000 \text{ g/kg} = 200 \text{ g}$
- Assuming the density of water is 1 g/cm³, the volume of water is equal to its weight in grams.
- Volume of water ($V_{water}$) = 200 cm³
Finding Volumetric Water Content
Calculate the volumetric water content ($\theta_v$), which represents the fraction of the soil volume occupied by water.
- Volumetric water content ($\theta_v$) = (Volume of Water) / (Total Volume of Soil Sample)
- $\theta_v = V_{water} / V$
- $\theta_v = 200 \text{ cm}^3 / 662.45625 \text{ cm}^3$
- $\theta_v \approx 0.301887$
Expressing Water Depth per Meter Depth
Convert the volumetric water content into the water depth in centimeters per meter depth of soil. This value indicates how many centimeters of water would be present in one meter of soil depth.
- Water depth (cm/m) = $\theta_v \times 100 \text{ cm/m}$
- Water depth $\approx 0.301887 \times 100 \text{ cm/m}$
- Water depth $\approx 30.1887 \text{ cm/m}$
- Rounding to two decimal places, the water depth is 30.19 cm/m.
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) - In-situ volumetric soil moisture content measuring method(s) is/are:
- 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
- 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
