Mark scheme for Test 2 Hydrology (6/4/09)

1. (a) (i) Describe how precipitation intensity can affect discharge in a river channel. [4]

Precipitation intensity, such as high intensity rainfall occur during convectional thunderstorms, where heavy rain lead to high infiltration capacity of the soil causing large quantities of surface runoff and a rapid rise in river discharge.

Low intensity such as light or drizzle rainfall less infiltration occurs thus less runoff occur lead to decrease in river discharge.

(ii) Briefly describe the process of interception and give reasons why it is of importance in the hydrological system. [3]

By Muhammad Izzat Zainal (block 3)

Interception storage is a process where the precipitation water will fall onto leaves before it falls directly into the ground via stemflow or throughfall. It is important in the hydrological system because it controls the amount of water that reaches the ground and control flooding to occurs. if too much water flows on the surface without

being intercepted by leaves, surface runoff occurs and may lead to flooding. Another reason is without vegetation covered, rainwater washed soil into stream (soil erosion) and will increase river bed thus increase river discharge cause flooding to occur.

(b) Using simple sketch hydrographs, explain how a change in land use in a drainage basin from woodland to urbanisation may affect river discharge.

[8]

Draw storm hydrograph(i) Urban land use-flashy storm

hydrograph (steep rising and falling limbs, shorter lag time and high peak discharge

(ii) Woodland-gentle slope graph with gentle rising and falling limbs, longer lagtime and low peak discharge.

Then you describe how it has steep rising and falling limbs-run off and infiltration rate & interception rate, permeable and impermeable ground, time to reach the stream (lagtime) then produce peak/low discharge.

(c) How can the abstraction (removal) and the storage of water by humans affect flows and stores within a drainage basin.

[10]

Abstraction occurs not only directly from channel flow (irrigation, water supplies etc.) but also from groundwater sources through wells, boreholes etc. The former will affect discharge levels whilst the latter will affect water tables, baseflow and ultimately channel flow. Storage will be seen largely in terms of reservoirs. These increase surface storage, regulate channel flow, ironing out flood peaks etc. It also deprives water from the lower parts of the catchment (e.g. Nile, Colorado etc.). Evapotranspiration rates may also increase as may groundwater supplies in part of the catchment and decrease elsewhere. Good answers will consider these elements in terms of changes to stores and flows.

2. (a) (i) Briefly define the term water balance.

[4]

Water balance is balance between inputs and outputs in hydrological cycle. The major input

is precipitation such as rain or snow. Output is where water move out of the system by process of river run-off or loss or return back to atmosphere by process of evapotranspiration from both plants and animals. Water balance can be expressed using an equation:

P (Precipitation) = Q (run-off) + E (Evapotranspitation) + /- Stores.

(ii) Describe one reason why the water balance may vary over time. [3]

Seasonal nature of precipitation, major input in hydrological cycle such as rain or snow and output which is the change of stores. For example the seasonal requirement of plants and animals through uptake and abstract water from not only surface storage such as ponds, lake or river channel or ground water storage for own use, such as

for water supplies, domestics and irrigation.

(b) Using a labelled diagram (or diagrams) describe how water moves through a catchment system. [8]

Habibah (Block 3)

The major input of the hydrological cycle is precipitation. When precipitation occur, some of the water intercepted onto leaves of vegetation (plants & trees) which shelters underlying ground to form interception storage. Some may directly fall

onto the earth’s surface and store in surface depression such as lake, puddles as surface storage. After sometimes, the plants can no more hold water, thus drip from the plants

as throughfall or run down the stem/thrunk to reach the ground by stemflow. Some of the water will infiltrate into the ground until maximum rate of infiltration is reached and held in soil as soil moisture storage. The water that cannot infiltrate will flow as surface run-off. After sometime the water that is being infiltrated will move laterally by throughflow to reach stream or downward due to gravity deep into the soil and in bedrocks via cracks and joints to form saturated rock known as groundwater storage. Groundwater will be discharge into stream by baseflow. Water will return back to atmosphere by process of evapotranspiration.

(c) Explain the ways in which human activities can affect the flows of water in a catchmentsystem. [10]

Human activities such as land use changes from woodland to urbanisation and abstraction of water can affect the flows of water in a catchment system.

In the first example of land use changes when people cut down trees in woodland areas for urbanisation the water falls directly onto the ground without being intercepted, much water overflow on land surface as run-

off thus increase river discharge. The ground which is covered with tarmac and concrete for road building and buildings, and building gutter or drains further increase surface run-off and water is quickly diverted into river increasing discharge further since no water infiltrate into impermeable ground.

In the second example human abstract water from ground water storage for water supplies by building wells and boreholes in aquifer can affect lead to fall in water table and decrease amount of groundwater. It also decrease amount of river discharge when people use up water in surface storage such as stream, pond and lake for water supplies and irrigation.

Another example is when people build dam to prevent flooding where water is store behind dam thus increases surface storage. Unfortunately it decreases the amount of discharge downstream.