2. Why is water available on Earth?

Will our cup always be refilled?

Water is a renewable resource because it can be replenished through the hydrological cycle (water cycle).

The photograph shows a storm canal along Hougang Avenue 8 with weep holes at the side. 

Where do you think the water from the weep holes are from?

Water budget within a catchment

o Water surplus

o Water deficit

Rain adds water to a catchment, so precipitation, e.g. rainfall, is an input. 

Surface runoff, evaporation and transpiration decrease the amount of water in a catchment, so these processes are outputs. 

There is a net change in which the amounts of inputs and outputs change throughout the year.

When input is higher than output, there is water surplus.

When output is higher than input, there is water deficit.

The amount of water stored within a catchment changes as well.  

Singapore draws its water supply from the Linggiu Reservoir in Johor, Malaysia.

There was low rainfall (input) in the reservoir in 2015 leading to water deficit.

Source: http://www.straitstimes.com/uncategorized/singapores-water-supply-from-malaysia-reaches-critical-levels

You can download the Google Earth File below to see countries experiencing water stress.