Friday, August 24, 2007

Stone terraces and water logging

Building stone terraces on sloping and hilly areas of a farm serve three main functions. One, conserve precious top soil and organic matter like decomposing and dried leaves and branches, from unmitigated erosion. Two, conserve water and minimize water run-off during strong rains, reduce flash floods, as rain water seep into the soft ground, even recharging the water table underground. And three, help beautify the farm.

Through time, with continued building of stone terraces – whether raising higher or moving backwards existing ones -- low-lying areas will be rising and uneven areas become flatter. In addition, it is possible that spring water can develop later at the bottom of high terraces as more rain water are deposited deep in the new soil created by the rising land. If the farmer is entrepreneurial enough, he can develop his farm into an eco-tourism resort with those spring waters and beautiful, high, and plentiful terraces around the farm.

So the terraces and the land become higher. One big question that will bug the farmer is whether to cement those stone terraces, or leave those stones as they are?

The advantage of cemented stone terraces is that the structures become more stable and more permanent. Like the various rice terraces in the Cordillera mountains in north Philippines, particularly the Banaue rice terraces in Ifugao. These terraces were “cemented” using special clay and strengthened by small and rough stones that fill in large spaces in-between the large stones. The terraces have been in existence for more than 1,000 years and they have withstood minor earthquakes.

There are three big disadvantages though. One, new layers of organic farm waste like rice straws, dried leaves and branches of trees, can no longer be accommodated by the terraces because the stones are no longer movables. So upland farmers just burn the rice straw every year, the same practice being done by lowland rice farmers. Two, water logging can be a real problem if there are not enough outlet. So that either the big volume of water trapped behind those terraces will destroy the structure through time, or the roots will be suffocated as water will fill in tine air spaces underneath. And three, if there are big trees near the cemented terraces, the roots show their hatred to cement by bulging wide and high, eventually destroying the structure.

Terraces that are not cemented and continuously adjusted upwards and/or backwards, have the disadvantage of being less stable, especially if they are not constructed well. But they have the advantages of avoiding the 3 pitfalls of cemented terraces mentioned above. In particular, the terraces are porous, water can come out anytime on thousands of spaces between the stones, so that probability of water logging is small. Except of course during very heavy rainfall where flash floods are occurring anywhere.

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