The water balance~! by zhang binghui

Hi, everyone! This is Binghui. Today it's my turn to write the blog.

Basically, what we learned today is The Water Balance.

Here is a picture of hydrologic cycle for your better understanding:


The water balance is a simple equation which describes how precipitation may be accounted for within a drainage basin.

A general water balance equation is:
P = Q + E + ΔSwhere
P is total precipitation
Q is total streamflow (runoff)
E is evapotranspiration
ΔS is the change in storage (in soil or the bedrock)

Some examples of the factors:
Q can be affected by dams, but the teacher said not very sure about it.
E can be affected by vegetation
ΔS can be affected by amount of soil and also type of soil.
The water balance is used by hydrologists to plan and manage water supply within a drainage basin. It can be used to suggest possible water shortages for which special measures like hosepipe bans can be implemented to preserve stocks. It has implications too for irrigation, pollution control and flooding. The water balance can be illustrated using a water balance graph which plots levels of precipitation and evapotranspiration often on a monthly scale.
There are some additional points:

1. Precipitation in the form of rain, snow, sleet, hail, etc. makes up the primarily supply of water to the surface. In some very dry locations, water can be supplied by dew and fog.

2. Evaporation is the phase change from a liquid to a gas releasing water from a wet surface into the air above. Similarly, transpiration is represents a phase change when water is released into the air by plants. Evapotranspiration is the combined transfer of water into the air by evaporation and transpiration.

3. Soil moisture storage refers to the amount of water held in the soil at any particular time. The amount of water in the soil depend soil properties like soil texture and organic matter content. The maximum amount of water the soil can hold is called the field capacity. Fine grain soils have larger field capacities than coarse grain (sandy) soils. Thus, more water is available for actual evapotranspiration from fine soils than coarse soils. The upper limit of soil moisture storage is the field capacity; the lower limit is 0 when the soil has dried out.

4. The change in soil moisture storage is the amount of water that is being added to or removed from what is stored. The change in soil moisture storage falls between 0 and the field capacity.

Take note of this, ΔS can be zero. There are some reasons we discussed in class:
1. It can be raining all the time caused the soil saturated, thus ΔS become zero.
2. It can also be no rain at all caused ΔS to be zero.

Lastly, I am not very sure about the question the teacher asked us in class:
Can S be reduced? The answer is yes.
But what is the reason? Can someone who understood this tell me the answer?

I think the lesson is too short, only half an hour.
These are all what we learned.
Thank you!

Binghui
31, 01, 2009