The Gnangara Mound pines use more water than the production of FOUR Kwinana desalination plants
It took the Water Corporation 4 years to release a report (Xu, 2008) which authenticate that pine plantations are the major cause of falling water levels in the Gnangara Mound. Modelling results indicates that about 38% of the decline is due to pine plantations, 32% is due to rainfall reduction, and 30% due to the increase in abstraction by the Water Corporation from the superficial and confined aquifers (Xu, 2008). (The decline due to rainfall should be much lower, as it is most probably calculated as percentage of rainfall which is not applicable to the Gnangara hydrogeological environment.).
On the other hand the detailed study of water balance of the pine plantations on the Gnangara Mound which is being conducted by CSIRO for the last ten years is yet to be released. Silberstein (2007) gave a power point presentation which seems to be the summation of the ongoing (never ending) study. The results showed that there is no recharge to the aquifers under the pines and that there is no water under the pines newer than 1990. This study also showed that there are upward gradients at the shallower sites and higher transpiration with shallow water tables. The most important conclusion of the study is that pines use over 100mm of groundwater when the water table is 15m or less (Silberstein 2007).
It can be safely concluded from CSIRO studies that there is ZERO recharge under the 225 square kilometers of pine plantations in the Gnangara mound and that the water use by the pines which is around 180 GL/yr is more than 16% of the total water use and is equivalent to the production of four desalination plants similar to the one constructed in Kwinana. At the same time the pines are using more than 22 Gl/yr of groundwater which is recharged somewhere else outside the pine areas.
From both studies it can be hypothesized that the main cause of the declining water levels in the central areas of the Gnangara Mound is due to the pine plantations as there is absolutely no recharge under the pines since 1990 and that more than 100mm of groundwater recharged elsewhere are used by the pines. It can also be stated that the declining water levels which are caused by the water uptake by the pines is also exasperated by the increasing groundwater abstraction by the Water Corporation, the continuously increasing private users and uncontrolled home bores.
Although there is a plan to harvest the pines within the next 25 years, yet the slow rate by which this is happening and the increasing draught condition is not decreasing the groundwater consumption by the pines but is actually increasing groundwater uptake by the pines due to the fact that as rainfall is decreasing; the pines are extending their roots to get more groundwater to compensate for the surface water deficit, also the mature pines (which are being culled) were established on better rainfall conditions and were more rainfall dependant, and are adapting to a new water regime by compensating from groundwater, whereas the younger pines started on lower rainfall conditions and were established on a groundwater regime and are already adapted to the receding groundwater conditions and do not follow the rain/dry natural cycle.
Pines now follow a continuous groundwater diet and the rainfall water is the dessert.