Water Cycle
Water from the oceans evaporates and forms vapor in the air; through the movement of air it is brought to the land. The vapor cools down and forms droplets, which are suspended and condensed into cloud or fog in the air; the droplets grow in size and precipitates back to earth as rain. About two thirds of the rainfall evaporates into the atmosphere; the rest, besides a small portion absorbed by plants or buildings, returns to the ocean via runoff into streams and rivers to begin the cycle again.
Rainfall Unit
Rainfall unit is indicated by millimeter, representing rainfall depth; the bigger the value, the larger the rainfall volume.
Rainfall unit multiplied by the rainfall area (usually indicated by square kilometer) is the overall rainfall volume.
There are several ways to show the rainfall amount, including:
1. accumulated rainfall
using millimeter as unit, indicating the total rainfall depth during the rainfall time;
2. rainfall intensity
using mm/hr as unit, indicating rainfall depth within one hour, and the bigger the value, the more concentrated the rainfall
3. average rainfall
using mm as unit, usually using the weighted averages derived from the data of certain rainfall stations.
Types and Characteristics of Rainfall
Types and Characteristics of Rainfall in Taiwan are described as the following:
1.Convectional Rainfall:
The sun heats the ground and warm air rises with water vapor; in hot summers air circulation intensifies, and when water vapor meets cold air, it condenses into raindrops and fall to the ground, called convectional rainfall, with thefollowing characteristics:
1. Small rainfall range.
2. Rainfall in a given time(or rainfall intensity)is large; rainfall time is short.
3. Usually accompanied by thunders?
2.Orographic Rainfall:
When warm wet air is forced to rise over a mountain range, its water vapor meets cold air, condenses and precipitation occurs, called ”orographic rainfall. Its characteristics are:
1. Rainfall time is long, with small intensity.
2. Large rainfall range.
3. The rainfall is bigger on the hillside facing the wind , smaller on the lee side.
3.Frontal Rainfall:
Frontal rainfall occurs at the boundary between masses of warm and cold air; the water vapor is chilled, condenses to form rain. When the cold and warm air masses stuck, it becomes “stationary front?and produces continuously small rain,commonly called “plum rains.?Its characteristics are:
1. Large rainfall range, mostly occurs north of central Taiwan.
2. Rain often lasts several weeks, with small rainfall intensity.
3. Often occurs in April and May each year.
tropical depression; if the cyclone range is large with intense winds, it’s called typhoon. A satellite image of it is shown in Picture 2. When such depression approaches Taiwan, precipitation occurs due to its circumfluence; the rain lastsuntil it leaves Taiwan. Typhoon often occurs between April and November, and is the main cause of floods in Taiwan.Its characteristics are:
1. Big rainfall intensity, lasting several days.
2. Large rainfall range.
3. Rain accompanied by strong winds, which may cause hazards besides floods; earth-flow or stratum-slide may occur on hillside.
◆ Flood Mitigation
Floods can be divided into “water flood?and “earth flood;?the constructions for resisting or reducing floods are called “flood mitigation constructions.?Using the term “Flood Mitigation, the spirit is to emphasize human measures of flood reduction and mitigation, hoping to minimize the loss due to floods, but not to ”control?floods. The commonly-used term “flood control?should have the same meaning as “flood mitigation.?
◆ Common Measures for Flood Mitigation
1.Structural Measures
‧Flood Storage:
Build reservoirs or adjustment ponds to temporarily store the floods beyond the draining capacity of the downstream rivers; for instance, Shih-Men Reservoir and Tseng-Wen Reservoir.
‧Flood Diversion:
When the cross-section of a river channel is not big enough to drain flood, a floodway is built to divert the flood; for instance, the Erchong Floodway.
‧River Channel Dredging:
Dredge the river channel to increase the draining capacity, or reduce the roughness of river channel to facilitate water flow. It helps to drain additional flood volume, but the effect is limited.
‧Flood Embankment:
Built dikes or dams to embank floods within the designated river channels. This is the most common measure.
2.Non-structural Measures
‧Water and Soil Conservancy:
Increase greenbelts and increase the infiltration rate of rainfall into the ground. This takes time to manage and work, and the effect is limited.
‧Flood Forecast System:
Examine instant data of rainfalls and river stages, timely issue flood forecast and warning, and evacuate residents of possible flood areas to reduce loss due to floods.
‧Flood Plain Management:
Use the plain around the downstream river as the flood zone to lower the river’s flood stage and effect flood detention.