Chapter 1 · Unit 1-3
How do models describe water? What makes Taiwan's mountains and rivers different?
Textbook pp. 8–10
Goals
This unit uses real data
The 3D terrain of Taiwan comes from SRTM satellite elevation data, about 280 m per cell. We will recompute the textbook's numbers ourselves and compare.
1.3 Hydrologic modeling
Another split: space
Lumped model: the whole watershed is one uniform unit with a single set of parameters. The classic example is the unit hydrograph: it shows how runoff varies in time, but not where in space.
Distributed model: the watershed is split into many cells, each with its own parameters. The dynamic wave model, for example, describes flow varying in both time and space.
The cost: distributed models need many spatially varying parameters and heavy computation, which still limits them in practice.
1.4 Applications of hydrology
Hydrologic equations + modeling methods → simulate hydrologic processes → solve water-resources engineering problems. Four typical examples:
Pollution control, ecological conservation and erosion control also draw on hydrology; the textbook leaves these to specialized books.
1.5 Taiwan's land and water
Taiwan lies in the western Pacific between Japan and the Philippines; the Tropic of Cancer crosses its southern half. Total area is about 36,000 km².
Adding up the cells above 0 m in this satellite terrain data gives about 36,600 km², close to the textbook figure.
On the right is the real terrain, with heights exaggerated 3× so the relief stands out.
The Central Range runs down the middle
| Elevation | Textbook | Ours | |
|---|---|---|---|
| Mountains | above 1000 m | 32% | 32% |
| Hills & terraces | 100–1000 m | 31% | 39% |
| Alluvial plains | below 100 m | 37% | 29% |
The mountain share matches. The 100 m hill–plain boundary is sensitive to data resolution and to what counts as a "terrace", hence the gap of a few points.
The plains are where people and farming are concentrated.
What the mountains are made of
Taiwan's mountains are mostly sedimentary and metamorphic rock: weak, easily fractured and deeply weathered.
Add intense rainfall and fast flow, and erosion is severe; frequent earthquakes also destabilize hillslopes.
So Taiwan's rivers carry heavy sediment loads during floods; this will come up again.
Rain
Rivers
Taiwan has 129 rivers. All are short with small basins, steep and fast-flowing; most show a sharp contrast between flood flow and low flow.
Specific discharge: discharge divided by drainage area, in m³/s/km². It lets rivers of different sizes be compared fairly.
Zhuoshui River: 7.7 m³/s/km² (textbook), about 25 times the Shinano and 450 times the Yangtze; the other two are derived from these ratios. Log scale.
Putting it together
Weak rock, steep fast rivers, and rain packed into a few months: floods are frequent in the wet season, yet water often runs short in the dry season.
So we must regulate rivers to control floods and store wet-season water for the dry season.
How large must a reservoir be? The tool used in Exercises 7 and 8 of Chapter 1 is the mass curve.
Supplement: mass curve (Rippl method)
Interactive lab
Using the monthly flows of Exercise 8, adjust the demand D. Bottom left is the mass curve; the 3D reservoir runs month by month for two years. Try to:
Mass curve (two years)
Cumulative flowDemand lineRequired storage K
Self-check
01To see "where and when" a flood inundates a watershed, which is more suitable?
02In the dry season, the slope of the mass curve becomes…
03Exercise 8: demand is 40 m³/s; flows from March to June are 35, 25, 15 and 22. What is the cumulative deficit over these four months, in (m³/s)·month?
Summary
Next chapter
Chapter 2 Watershed Characteristics
How is a watershed boundary drawn? What does a river network look like? What components make up the hydrograph at the outlet?
Coming soon