The irrigation of 19,000 hectares of farmland in Taoyuan, Hsinchu and Miaoli has been suspended and the land now lies fallow. In the south, Mudan Reservoir (牡丹水庫) in Pingtung County’s Mudan Township (牡丹) released water in preparation for the onslaught of Typhoon Atsani, although only at 10m3 per second.
Seeing precious fresh water disappear into the sea highlights how different the hydrological conditions can be across even a small geographical area like Taiwan, and the importance of having water resources staff ever ready to address shortages as well as flood prevention.
Taiwan receives abundant rainfall, but as the Water Resources Agency’s annual report shows, measures have to be taken almost every year to prevent shortages. Rainfall across the nation is uneven, and when there is less rain, artificial rain, increased irrigation, water allocation and other measures are implemented.
In severe cases, relief wells are opened, irrigation is suspended and land is left fallow. This year, desalination measures have also been adopted.
Why does all this happen? One factor — in addition to the uneven distribution of rainfall — is that there is not enough space to accumulate water in this small country. These geographical restrictions mean that reservoir volumes are simply insufficient.
The effective capacity at the nation’s biggest reservoir — Zengwen Reservoir (曾文水庫), in Chiayi County and Tainan — is about 508 million cubic meters. Taiwan’s tallest dam — the 180m tall Deji Reservoir (德基水庫) in Taichung — can only hold about 187 million cubic meters.
The total capacity of all 95 reservoirs across Taiwan is less than 2 billion cubic meters, but last year total water supplied to agriculture, daily necessities and industry exceeded 7.04 billion cubic meters. This gives an idea of the pressure on the nation’s reservoirs.
All these values are averages. Including Jiji Weir (集集堰) in Nantou County, Shigang Dam (石岡壩) in Taichung and other such weirs — small storage volumes, large supply volumes, mainly used to raise water levels to facilitate water use — is not entirely fair.
As irrigation has been suspended in the Taoyuan, Hsinchu and Miaoli areas, I analyzed supply and effective capacity ratios — water utilization rates — for first-class reservoirs in these areas last year.
The utilization rate at Shihmen Reservoir (石門水庫) in Taoyuan’s Dasi District (大溪) was 3.4; in Hsinchu County’s Baoshan Township (寶山) the rate at Baoshan Reservoir (寶山水庫) and Baoshan Second Reservoir (寶二水庫) was 6.78 and 2.4 respectively; at Yongheshan Reservoir (永和山) in Miaoli County’s Sanwan Township (三灣鄉), 2.0; at Mingde Reservoir (明德水庫) in Miaoli County’s Touwu Township (頭屋), 2.59; and at Liyutan (鯉魚潭) in Miaoli County’s Sanyi Township (三義), 2.7.
Even if Shihmen Reservoir is full, that would only guarantee water supply for the next three-and-a-half months. This is an average, and the period would be even shorter during a drought, as water use would go up. If follow-up measures are insufficient, that could lead to shortages.
Of the 18 first-class reservoirs, only Sinshan Reservoir (新山水庫) in Keelung’s Anle District (安樂) and Feicuei Reservoir (翡翠水庫) in New Taipei City’s Shihding District (石碇) had a utilization rate of less than 1 last year.
Increasing water storage capacity is fundamental to addressing drought: If the nation’s reservoirs could have the same capacity as those in other countries, so that a reservoir at capacity would last several years, there would be no need to worry about droughts.
The problem is that it has become difficult to build new reservoirs and artificial lakes, and this makes it even more important to dredge existing reservoirs. Although current dredging cannot match sedimentation, dredging at reservoirs with a high utilization rate should be given priority.
For example, dredging at Shihmen Reservoir so far this year has removed 2.6 million cubic meters of sediment. Multiplied by the utilization rate, that means it can supply an additional nearly 9 million tonnes of water next year.
Increasing water resources comes at a price. Developing subsurface water, increasing rainwater harvesting, improving detention pond water management, recycling wastewater, reducing tap water and irrigation canal water waste are all ways of fighting shortages that should be implemented, although they all carry a price tag.
At the Kaoping Great Lake (高屏大湖) project in Kaohsiung’s Meinong District (美濃區) — a lowland reservoir similar to Chengcing Lake (澄清湖) in the city’s Niaosong District (鳥松) — the cost is land resources.
We must not reject the construction of reservoirs and artificial lakes — fighting water shortages requires a multipronged approach.
Chang Yen-ming is a former director of the Water Resources Agency.
Translated by Perry Svensson
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