Sat, Mar 23, 2002 - Page 8 News List

Ways to combat water shortages

By Cheng Jie-dar 鄭皆達

The drought in the Taoyuan and Hsinchu areas has led to fallow-land plans and water-use conflicts between agriculture and industry. If the drought goes on, it will also threaten household water usage. To solve the shortage, policies and measures based on factors underlying the problem have to be proposed.

Finding new sources of water and implementing conservation measures should be the basic principles of a solution to the drought. The major reasons for the drought in Taoyuan and Hsinchu are a long period of no rain and an imbalance in water supply and demand. Even though annual rainfall in Taiwan is high, it is unevenly distributed over the year. About 75 percent of the rain falls in the summer and autumn seasons.

Also, the catchment areas of a majority of rivers have high gradients and thin soil layers with low water absorption capability, and the differences in water run-off between high and low levels are therefore large.

Water reservoirs store water during high-flow periods for distribution during dry periods. Due to topographical factors, how-ever, Taiwan cannot build large-capacity reservoirs and the capa-city of existing reservoirs has been greatly diminished due to large volumes of silt.

It is also very difficult to build new reservoirs due to opposition from environmentalists and neighboring communities. If there is a long dry spell, the water in the reservoirs will be insufficient to satisfy household, industrial and agricultural needs.

Many people have recently advocated restrictions to excessive land development and construction -- in combination with increased forestation in water catchment areas -- as ways of increasing the water regulation capability in these areas.

This is a fundamental conservation measure that should have been adopted a long time ago. However, people in general don't fully understand that forests have both positive and negative effects on low-water runoff.

On the positive side, soil water permeability is higher than for other covered areas (in particular highly-disturbed areas), which means that surface runoff is rare. Most of the water will seep into the soil, with a portion going to supply water to streams, which is beneficial during periods of drought and low runoff. The soil stabilization, hillside protection and surface runoff production functions of forests also lead to decreased erosion and landslides, which is beneficial to reservoir capacities and service life.

On the negative side, evapotranspiration from forests is higher than from other types of coverage and part of the water seeping into the soil must first make up for water used for transpiration.

Further, vaporization of water intercepted by trees is higher than for other coverage types.

All in all, positive effects probably slightly outweigh the negative ones. The positive effects still have their limitations, however, and relying on forestation to protect and improve catchment areas as a solution to problems brought by droughts is not only hoping for too much, it is also too slow a method for solving urgent needs.

In a long term perspective, however, forestation can, despite limited water-regulation capabilities, guarantee a replenishment of the valuable supply of water processed by nature itself. It can also protect soil and hillsides and decrease silt buildup in reservoirs, and it is therefore a desirable long-term policy.

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