Waterlogging and Drainage Questions and Answers

Answer: C

The incorrect statement is that waterlogging increases the capillary water available to plants. In a waterlogged soil the pores are filled with water and air is shut out, so roots cannot get oxygen, and plants do not benefit from the extra water. The other effects occur: the soil becomes colder, since the water has a high heat capacity and evaporates; salts are brought to the surface by capillary rise and deposited as water evaporates; and weeds and other wild plants grow in the wet soil, which cuts crop yield.

Answer: D

The one that is not a cause is Use of heavy machinery, resulting in no soil compaction.

Salinity and sodicity develop when salts accumulate in the root zone. Irrigation mismanagement (excess water raising the water table), poor land levelling (water ponding and evaporating in low spots) and leaching without drainage (salts moved down but not removed) all cause it.

The last option describes no compaction, which is not a mechanism of salt build-up, so it is not a cause. (Compaction from heavy machinery can worsen drainage, but the option as worded rules that out.)

Answer: C

The key gives \(C_{s} = C \cdot Q/ Q - (C_{u} - \rho_{eff})\), read as \(C_s = \frac{CQ}{Q - (C_u - \rho_{eff})}\).

The salt brought in by irrigation is \(C \times Q\). Plants and evaporation remove water but leave the salt behind; of the consumptive use \(C_u\), the part met by useful rainfall \(\rho_{eff}\) does not come from irrigation. So the water left in the soil to carry the salt is \(Q - (C_u - \rho_{eff})\), and the salt concentration of the soil solution is the salt divided by that water.

The options with \(Q^{2}\) or \(C^{2}\) have no physical basis, and the form with a factor 2 in the denominator halves the true concentration.

Answer: A

The cause that is not artificial is Deep percolation from rainfall.

Waterlogging occurs when the water table rises close to the root zone. Rain soaking down through the soil is a natural process, so it is a natural cause.

The other three are man-made. Roads, railways and canals built across natural drainage lines block them; unlined canals lose large amounts of water by seepage into the surrounding land; and over-irrigation of land at higher elevations builds up hydraulic pressure that pushes water into lower areas. Each comes from human works, which is why they are classed as artificial causes.

Answer: C

The option that is not a functional part is the Storage system.

A surface drainage system removes excess water from the land surface. Functionally it has three parts: the collection system (field drains and ditches that gather the water), the disposal system (larger channels that carry it away) and the outlet (the point where it is discharged into a stream or natural channel).

Storing water is the opposite of what a drainage system is for. Storage belongs to water harvesting structures such as farm ponds, so it is not a functional part of surface drainage.

Answer: B

The method is Bio-drainage.

Bio-drainage uses deep-rooted, fast-growing trees that transpire large amounts of water, such as Eucalyptus, Casuarina and Acacia, to pump groundwater out of waterlogged land through their roots and leaves. It lowers the water table without channels or pumps.

Mole drainage forms unlined channels in clay subsoil with a mole plough. Tube drainage uses perforated pipes, and gravity drainage relies on surface or subsurface flow by gravity. None of them uses trees.

Answer: B

What is most required is Drainage.

In humid regions heavy rainfall regularly exceeds what the soil can absorb and what crops can use. The excess water raises the water table and saturates the root zone, so removing it by surface and subsurface drainage is the main water management need.

Waterlogging is the problem drainage solves, not something required. A sluice is a gate for controlling water flow in a channel, and evaporation is a natural loss that cannot be relied on to remove large rainfall excesses.

Answer: C

The one that is not a surface drainage system is Vertical systems.

Surface drainage removes excess water from the land surface through shaped land and shallow channels. Its systems include the bedding system (land formed into raised beds with furrows between), random and parallel ditch systems, and furrow (field ditch) systems.

Vertical drainage lowers the water table by pumping from wells. It is a form of subsurface drainage, not surface drainage.