Nick Smith has suggested to farmers that they start offsetting their greenhouse gas emissions by planting trees. A risk with this is that planting trees may compromise water supply.
Compared with pasture, trees tend to reduce the amount of rainfall that reaches rivers and aquifers. A larger canopy traps more rain as it falls, so it evaporates directly from the leaves before even seeing the soil. Deeper rooted trees are also able to tap deeper soil water and groundwater stores, supplying more water for plant transpiration. On the other hand, a row of shelter trees slows the drying of pastures during strong, warm winds, such as during Canterbury nor’westers.
If there is ample water to begin with, the effect may be inconsequential. If the region already experiences seasonal water shortages, planting trees may be a risky proposition.
A rule of thumb I use to delineate at-risk regions is a threshold of 600-700 mm of annual rainfall. Any less, and planting forests where they were not previously may translate to drier streams and lower water tables. These numbers are rough, and would need more attention for any given region, but the message is simple.
The effect on water supply would increase with the area of forest planted. So would the amount of carbon sequestered. This leads to a trade-off between carbon sequestration and water supply. Farmers should consider carefully where they plant their trees so as not too compromise their irrigation needs.