Soil Water Plant Relationship Questions and Answers

Answer: B

The correct matching is 1 - c, 2 - d, 3 - a, 4 - e, 5 - b.

1. A tensiometer measures soil water tension, so it is used to monitor soil moisture.
2. A piezometer is an open tube showing the hydrostatic pressure head at a point.
3. A lysimeter is a soil-filled tank whose water balance gives evapotranspiration, the consumptive use of a crop.
4. An elbow meter measures the pressure difference across a pipe bend, from which the volumetric flow rate is found.
5. A Pitot tube converts velocity head to pressure, and is analysed with Bernoulli's equation.

Option A swaps the piezometer and Pitot tube, option C mismatches the first three instruments, and option D swaps the elbow meter and Pitot tube.

Answer: A

The depth of moisture is about 459 mm/m.

Taking 15 as the dry unit weight of the soil in kN/m³ (the stem prints kN/m²), the depth of water held per metre depth of root zone at field capacity is:

\[ \begin{aligned} d &= \frac{\gamma_d}{\gamma_w} \times F_c \times D \\ &= \frac{15}{9.81} \times 0.30 \times 1\ \text{m} \\ &= 0.459\ \text{m} = 459\ \text{mm/m} \end{aligned} \]

Forgetting the ratio \(\gamma_d/\gamma_w\) gives 300 mm/m, which is not offered. 600, 779 and 800 mm/m do not follow from these values.

Answer: A

The equation is \(AW = [(FC_{v} - PWP_{v})\,D_{r}]/100\).

Available water is the water held between field capacity, the upper limit, and the permanent wilting point, the lower limit. With both as volumetric percentages, their difference times the root zone depth gives the depth of water available:

\[ \begin{aligned} AW &= \frac{(FC_{v} - PWP_{v})\,D_{r}}{100} \\ &= \frac{(30 - 15) \times 1000}{100} \\ &= 150\ \text{mm} \end{aligned} \]

The example uses \(FC_{v} = 30\%\), \(PWP_{v} = 15\%\) and a root zone of 1000 mm. The other options subtract the wrong way round, which gives a negative value, or divide by the depth instead of multiplying.

Answer: D

The correct pair is External vapour movement- Surface evaporation of soil water.

External vapour movement is the loss of water vapour from the soil surface to the atmosphere, which is surface evaporation.

The other pairs swap causes. Saturated flow is driven mainly by gravity and hydraulic pressure gradients, while unsaturated flow is driven by the matric potential gradient, from moist to drier soil; options A and B reverse these. Internal vapour movement within the soil is driven by differences in vapour pressure, caused mainly by temperature and moisture differences, not by liquid water pressure.

Answer: B

The one that is not a category of soil water is Hydrosphere water.

Soil water is classified physically as hygroscopic water (held as a thin film on soil particles at very high tension and unavailable to plants), capillary water (held in the small pores against gravity and largely available to plants) and gravitational water (free water in the large pores that drains away under gravity).

The hydrosphere is the whole body of water on Earth, oceans, rivers, lakes, ice and groundwater, not a form of water held in the soil. ('Hydroscopic' in option A is the paper's spelling of hygroscopic.)

Answer: B

The most accurate is the Gravimetric method.

In the gravimetric method a soil sample is weighed, oven-dried at 105 degrees C to constant weight and weighed again; the loss in weight is the water. It measures moisture directly and is the standard against which all other methods and instruments are calibrated.

Tensiometers measure soil water tension, not content, and work only in moist soil. Electrical resistance devices and neutron moisture meters measure moisture indirectly and must be calibrated against gravimetric samples, although they are faster and non-destructive.

Answer: C

The correct match is a-IV, b-I, c-III, d-II.

• Movement of irrigation water from the surface into and through the soil is water intake, the term used for irrigated soils (IV).
• Downward entry of water into the soil surface is infiltration (I).
• Movement of water through a column of soil is percolation (III).
• Downward and lateral movement of water into the soil, as from canals and ponds, is seepage (II).

The common slip is to swap infiltration and percolation: infiltration is entry at the surface, percolation is movement down through the soil below it.