Thursday, December 23, 2010

Drought and Taiwan

A spider guards her eggs.

When most people think of natural disasters, the ones that spring to mind are events such as earthquakes, typhoons, or floods. Yet perhaps the most influential, and destructive, of natural disasters is one that few immediately think of: drought.

The impact of drought on history is profound. It may have been long-term drought that sent Homo sapiens out of Africa in search of new lands to colonize. In Taiwan drought has been a regular feature of the island's history. For example, the massive drought of 1638-9 was part of a larger pattern of drought across Asia that may have contributed to the end of the Ming Dynasty.

Though we often think of Taiwan as a wet, green island, droughts of varying severity are quite common in Taiwan. In the last century, southern Taiwan has undergone a long-term decline in rainfall and water shortages and droughts have become more common. In the north rainfall has increased, but in both places the number of rain-days has fallen and dry days increased. Overall, between 1945 and 1960, the island's rain patterns began to shift, with drought worsening in the south and the center but declining in frequency for all but the worse droughts in the north.1,3 Global warming, driven by human activities, is one obvious culprit: Taiwan's years with the most rain-days all occurred before 1953.2 Southern Taiwan, the island's agricultural heartland, is feeling the biggest impact.

Since 1895 Taiwan has been hit by major droughts from 1899-1902, 1906-1910, 1913-1916, 1960-1965, 1978-1980, and 1993-1997. Other important droughts occurred in 1946, 1954-55, and 2002. In several cases drought length and magnitude has apparently been influenced by volcanic eruptions, which produce years with significantly fewer rain-days.2 Sulfur-rich eruptions impact Taiwan in two ways: first, they pump aerosols into the atmosphere that inhibit precipitation; and second, they cool the surface of the ocean, reducing evaporation and moisture in the atmosphere. The El Nino-Southern Oscillation (ENSO) also affects Taiwan's droughts but the overall effect is generally much lower than that of volcanoes.4

Drought actually comes in several flavors. Meteorological drought, discussed in the paragraphs above, refers to the conventional idea of no rain. Hydrological drought is concerned with how meteorological drought plays out in local water basin systems.
It takes longer for precipitation deficiencies to show up in components of the hydrological system such as soil moisture, streamflow, and ground water and reservoir levels. As a result, these impacts are out of phase with impacts in other economic sectors.
Agricultural drought links the above two types of drought to their impact on agriculture. For example, the combination of drying from climate change and lower reservoir levels means that rice fields are often left fallow in the most productive part of the year in southern Taiwan because there is not enough water for them. Curiously, as southern Taiwan dries out, droughts appear to be starting earlier and terminating earlier.5

The changes in precipitation have had complex impacts on Taiwan. Wang6 writes:
The rapid declines both in precipitation amount and days in southwestern Taiwan for the past 60 years have resulted serious difficulty in the water resources management. For example, the extraction of surface water for the water usage in Taiwan has been diminished from 150x108 m3 in late 1970 to about 120x108m3 in 1990s, a 17% shortage of average annual demand (~180x108m3). The major water deficiency occurs in the southwestern region that has been suffered the heavy reduction of precipitation for the past decades. Groundwater has been forced to significantly increase to meet the overall water demand every year. Consequently, over-exploitation of groundwater since 1980 caused chain-series effects, such as groundwater level waning, land subsidence, seawater intrusion along the coast and groundwater quality deterioration (Wang et al., 2004). River pollution is another critical problem for the southwestern Taiwan partly due to the drop of precipitation of the past decades.
The disastrous effects of both drying and drought show how much more expensive drought is than other natural disasters -- its effects are long-term, subtle, and profound. It is sobering to imagine the south in its global context -- rainfall in southern Taiwan is three times the world average, yet the area is constantly afflicted by drought and water shortages.

The negative effects of drought -- increases in groundwater pumping, polluted water, subsidence, saltwater encroachment, and so forth -- interact with other problems generated by Taiwan's unique topography and its breakneck development. One article7 points out:
Pressure to move people inland has led to road construction and deforestation, both of which have contributed to an already high denudation rate of topsoil. As a consequence of this, thirteen rivers in Taiwan are now ranked among the top 20 worldwide in terms of sediment yield.
[Rapid run-off of water resulting in water shortages] is especially serious for Taiwan because the island is so highly dissected geographically. Many places in the central mountain ranges are geographically classified as extremely dissected with a roughness of >160 m km-1... Only 0.4% of all land on Earth is dissected as much.
Luckily in 1993 Taiwan began shifting to a more conservation-oriented forest management system which has impacted this situation positively. Still, as the article above observes, inter-basin management in Taiwan is difficult because the mountains are so steep. Exacerbating the various problems are the island's water management policies, including the low price of water which encourages overuse, the lack of support for conservation, the refusal to enforce groundwater pumping rules, and the preference for large, water-intensive industrial projects, especially in the south.

What does this mean? Sometime in the next few decades these trends -- overuse of water, frenetic groundwater pumping, subsidence, seawater intrusion, drought and long-term drying -- are going to come to a head in southern and south-central Taiwan and force major changes in agricultural production and water sourcing. Perhaps someday resources such as deep-rock aquifers in Taiwan may be exploited, but at present, much of the impact of long-term warming could be mitigated by sensible water policies, enforcement of water regulations, and shifts in crop preferences and production methods. Yet there seems little momentum for that at the central government level. Hopefully one of the political parties will take get out ahead of the curve on this key issue.

1. Historical trends and variability of meteorological droughts in Taiwan. Chen Shien-tsung; Kuo Chun-chao, Yu Pao-shan. Hydrological Sciences Journal, 54: 3, 430 — 441
2. This on volcanoes
3. The Influence of Climate Change on meteorological drought characteristic in Taiwan, BALWOIS conference paper
4. The Impact of Worldwide Volcanic Activities on Local Precipitation-Taiwan as an Example Li, et al. Journal of the Geological Society of China
5. The Impact of Climate Change on Agriculture Water Resources for Paddy Rice over Southern Taiwan
6. Precipitation Changes and Their Impacts on the Water Resources Management in Taiwan. Chung-Ho Wang
7. Island-based catchment—The Taiwan example. Chen, A. et al
8. Analysis and Evaluation of Taiwan Water Shortage Factors and Solution Strategies
Wang, CY, Wang, JB

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Robert Scott Kelly said...

Thank you for this Michael. Greatly informative.

Jenna said...

Very informative!

I just wanted to add: I've always found it sadly funny that Taiwan is such a heavily rained-on country, and yet there's always a "water shortage". I know the reasons why, of course, though I do think some of it could be solved with better infrastructure. It's just so twisted to be washing your hands while it's *pouring* outside, *for the fourth time that week*, while looking at a sign that says "Water Shortage: Please Conserve Your Water. Thank you your cooperation!!"

Okami said...

It's a shame too because it can easily be remedied with some foresight and maintenance. I don't really see either party looking at this as an issue they should.

Dredging reservoirs, building desalination plants, water treatment plants, fixing broken pipes and increasing the cost of water(it's subsidized); These are all unsexy things that your normal legislator or county chairman can't get their head around. They only seem to understand "turn on nozzle and water comes out". Other things that come to mind are lowering the subsidy for rice and increasing it for a drought resistant grain. The training infrastructure is already there. The problem is that the rice harvesting infrastructure is there, but the drought resistant grain harvesting infrastructure isn't. Farmers also get a subsidy no matter what. Farmers also get subsidized electricity for pumping groundwater. With the US grain crop increasingly being needed for ethanol production and the third world getting richer, food prices are going to do nothing but go up for the next 5-10 years before they crash hard.