Question

i. Find out from the graph the months in which water evaporate the most from plants. Why?
ii. Are these the same months in monsoon season when the rainfall is heavy?
iii. how does the availability of more water affect the plant?
iv. In which month the rate of water evaporation is negligible? why ?​

Answer 1

Imagine an open square container, 1 m wide, 1 m long and 0.5 m high (see Fig. 59a).

Fig. 59a. An open container to collect rainwater



This container is placed horizontally on an open area in a field (see Fig. 59b).

Fig. 59b. Container placed in the field



During a rain shower, the container collects the water.

Suppose that when the rain stops, the depth of water contained in the pan is 10 mm (see Fig. 59c).

Fig. 59c. 10 mm rainwater collected in the container



The volume of water collected in the pan is:

V (m3) = l (m) x w (m) x d (m) = 1 m x 1 m x 0.010 m = 0.01 m3 or 10 litres
It can be assumed that the surrounding field has also received an uniform water depth of 10 mm (see Fig. 59d).

Fig. 59d. 10 mm rainfall on the field



In terms of volume, with a rainfall of 10 mm, every square metre of the field receives 0.01 m, or 10 litres, of rain water. With a rainfall of 1 mm, every square metre receives 1 litre of rain water.

A rainfall of 1 mm supplies 0.001 m3, or 1 litre of water to each square metre of the field. Thus 1 ha receives 10 000 litres.

QUESTION

What is the total amount of water received by a field of 5 ha under a rainfall of 15 mm?

ANSWER

Each hectare (10 000 m2) receives 10 000 m2 x 0.015 m = 150 m3 of water. Thus the total amount of water received by the 5 hectares is: 5 x 150 m = 750 m

Rainfall is often expressed in millimetres per day (mm/day) which represents the total depth of rainwater (mm), during 24 hours. It is the sum of all the rain showers which occurred during these 24 hours.