Located in the temperate zone, Uruguay is a humid subtropical country where the rainfall makes it well suited for growing tree plantations. The annual rainfall is 1,200 to 1,500 mm and it is well distributed during the year without a regular dry season.
On average, water reaching the water courses, which finally ends up going to six major water bodies (e.g. the rivers Río Uruguay and Río Negro), represents around 35% of the total rainfall. This means that water is an abundant resource in Uruguay.
The level of the water courses in Uruguay depends primarily on the rains, much more than on land use, and is therefore very fluctuating. This fluctuation reflects the amount of rain fallen in previous days, ranging rapidly from a minimal flow to a flooding situation. The impact on a plantation block is normally temporary, being reset in the next rainfall event.
For plantations in general the distribution, regularity and availability of sufficient water is essential. In addition to the amount of annual rainfall and its variability over the years, it is important to analyse the monthly distribution of rainfall.
Water usage of eucalyptus plantations has been studied and monitored for decades. In comparison to most tree species, eucalyptus uses less water. Planted eucalyptus species are very efficient in terms of biomass production per water unit usage. Eucalyptus does not use groundwater or reduce groundwater recharge. Tree plantations reduce rapid run-off through the soil surface, thus minimising soil erosion.
During periods of severe drought, not common in Uruguay, eucalyptus trees stop growing and recycle the water inside their own biomass. They can even start dropping leaves to keep their need for water for survival at a very minimum level if there was a more severe drought.
Soils in Uruguay are relatively shallow, either because of a clay-rich layer (B horizon) that prevents root development or because of a rocky layer (C horizon). In most cases the effective rooting depth does not exceed the first metre. Therefore eucalyptus growing in Uruguay does not use groundwater as it never reach the aquifers.
Changes in precipitation and water availability have always been one of the key considerations in planning of any industrial operations including plantations grown by UPM.
In 2019 UPM engaged with the Finnish Meteorological Institute to further study the physical impacts of climate change, considering aspects such as water availability and weather extremes. Read the study
Impacts to water are taken into account in forest management:
- Management measures to minimise the risk of impacting water resources by planting design. Most of the unplanted areas are low lying and natural drainage areas, which have the highest relevance for the water cycle.
- The planting lines are cultivated against the maximum slope, favouring the infiltration of water into the soil, which minimises soil erosion and sedimentation in water bodies and promotes a sustained subsuperficial (i.e. through the soil) water flow towards the water streams.
- Harvesting is done in a mosaic of different age classes within each catchment to minimise possible fluctuations in water streams derived from the changing land cover.
Water monitoring studies have been carried out by the National University of Uruguay, and other institutions together with other forestry companies, to monitor the effect of plantations on water balances over 25 years. The main results so far indicate that the plantations’ land use, in comparison to grasslands under cattle grazing:
- reduce the water yield by around 20%, and the reduction coincides with the amount of water that is intercepted by the forest canopy and evaporated to the atmosphere
- the effect of this reduction becomes less noticeable as the size of the catchments increases
- groundwater recharge is not affected
- water quality is not affected.