Hydrological cycle

31th, March, 2009

Hi, class!
Today we learned a lot!
Basically, we can divide the content we learned today into five part

1) the teacher recap on what we learned during the last lesson -infiltration
Let's see the diagram above.

In this diagram, infiltration, overland flow and water table are labeled.

When the soil is saturated, there is overland flow instead of infiltration.

2) Input & output storages & Flows

what is input in hydrological cycle? It is Precipitation! (note: not rain, must be precise!)

Basically, Output inclueds evapotranspiration and Riverflow.

For Storages, there are 2 types of storages. Below water table, there is ground water storage, above water table, there is soil moisture storage.

For Flows, there are channel flow (river), Percolation, Throughflow and Baseflow.

Ms Tang told us, Channel flow is usually river, Percolation occurs after infiltration, when water still flow downwards in soil. Percolation can result in Groundwater storage. As for Throughflow, it is the movement of Soil moisture storage. And baseflow is the movement of the Groundwater storage.

Accoding to the order (from the surface of the ground to the deep underground), there are
-Channel flow

-Percolation

-Throughflow

-Baseflow

For Baseflow, as we can see from the diagram above, it occurs below the water table. Furthermore, it can recharge the river as where water table emerges at ground surface, it forms river/lake.

3) Roles of vegetation in the "interception" component of the hydrological cycle.

Firstly, the roots plays an important role to encourage water to pass into the soil and rock.

Secondly, vegetation slows down the water flow by reducing rainsplash erosion. So what is rainsplash erosion? In my opinion, it is like that each drop of water drips on the ground and it "hit" the ground, gradually, the soil there will be more compacted. In this way, it will decrease the rate of infiltration as it becomes harder for water to go through the soil. However, when there is vegetation, each drop of water splashes, thus, there will be a less great impact on compaction of soil. Hence, the rainsplash erosion is reduced.

Thirdly, the leaves collect water and the water is then evaporated or there will be throughfall (water drips from leaf) and stemflow ( water flows along the stem).

Also, vegetation itself needs water to sustain its life, some amount of water will be stored inside the plant organs to supply water when the plant is "thirsty" and there is no water it can absorb from the outside world.

In conclusion, vegetation really plays a very important role in the hydrological cycle. For the 2 countries which locate next to each other and share very similar geological features may still have big differences in their rainfall and climate. Presence of vegetation is also a factor which affects the local weather and climate. Or we can see it in another way. Presence of vegetation is a good indicator for the local general climate. In a dessert, it is very dry, there is little vegetation. However, in a tropical place, where there is always humid, we can see much more vegetation.

4) water balance
we need to know that there is usually a water balance in nature:

(I think this is due to the regulatory force in system, it is a negative feedback)
input=output
there is one equatin
Precipitation=Evaportranspiration+Qrunoff+change in soil moisture Storage

P=E+Q+change in S (this equation is a little bit different fromt the equation in notes, but i think this one is easier to understand and remmember!:)
there is one more thing, if we want to keep the soil moisture storage constant, what can we do? -To make the soil saturated. Thus, no more water can be stored in soil. In this way, it will result in overland flow.

5) Storm Hydrographs
this is the last part which Ms Tang went through roughly. For more details, please refer to Page 22 in your notes.
The hydrograph is a graph that shows change in discharge of a river. Discharge refers to how much water is flowing through a particular point per second (unit: m3/s)
It is a good way to record precipitation and it is useful when there is flood.

Guys, I know there is a lot which we need remmember, just try to understand it first, then I think it will be much easier to remember the tedious"scientific terminologies". Actually, I realised each terminology has its own reasonable meaning behind the word/phrase, just understand it frist. (For e.g. stemflow, water flows along the stem.) I think we can try some research on some certain case studies to know the process of hydrological cycle and the importance of water balance better.

Water is a precious resource. As we are learning geography hydrology now, let's treat water as a friend, let's have a try to know her better!

-Chengxi:D