Supply and Demand
The Cowichan Watershed can be characterised as having high precipitation/recharge, high storage, and low population/demand in the western and upper half of the watershed, contrasted with low precipitation, low storage, and high population and high demand in the eastern and lower half. Supply and demand are not matched regionally and the supply-demand gap is pushed to extremes in the late summer and early fall.
A watershed is not just the visible surface features either. A high percentage of the water that originates in the watershed as precipitation - rain and snow - is retained as groundwater.
It is estimated that 95% of the fresh water in the world excluding ice caps and glaciers, is retained as groundwater. Groundwater moves more slowly than surface runoff, but it is in motion. Moving even more slowly is groundwater contained in aquifers, underground areas covered by low-porosity caps.
The hydrological interactions between water on the surface - in streams and lakes, groundwater, and aquifers are complex. They are understood in principle, but data collection and mapping of subsurface hydrological features and vectors are difficult. We are fortunate that groundwater features of the lower Cowichan Watershed are better mapped than in many areas. However more information is required so we can understand the relationships between groundwater and surface water.
Aquifers tend to be repositories of high-quality water, consequently, they are the target of domestic, municipal, and agricultural wells. With modest demand, aquifers will recharge seasonally. With growing demand and reduced precipitation many of BC's aquifers are unable to meet demand and have become stressed. Population growth in all regions, including Cowichan, results in an increasing demand for water, particularly in summer. Exacerbated by the reduced summer precipitation which is one of the manifestations of climate change, the Cowichan River, and the groundwater in the lower Cowichan Watershed are experiencing demand for more water than they can deliver in late summer. Data for two monitoring wells in the lower watershed show a declining trend and more baseline information is clealy required.