Clean freshwater is a vitally important natural resource that is essential for life. Although water covers approximately three-quarters of the Earth’s surface, only 2.5% of the global water stock is freshwater, and over two-thirds of this freshwater is locked up in the form of ice and snow in the Arctic, Antarctic and mountainous regions. This leaves less than 1% of the global water resources as freshwater (mainly groundwater and surface water) that is accessible to meet human and freshwater ecosystem needs. Although desalination can enable an increase in the amount of freshwater at a local level, the volume of water generated will never significantly impact on freshwater availability.
Although freshwater is a renewable resource, which is replenished by the global water cycle, that does not mean that the supply is unlimited, as the availability of freshwater is primarily limited by the rate of replenishment, not by existing stocks. Furthermore, the availability varies as the distribution of freshwater is uneven, with huge contrasts in availability in different parts of the world and wide variations in seasonal and annual precipitation in many places. Even within areas that have sufficient per capita water availability, there can be large mismatches between water resources and population. Variations in the amount of rainfall over the course of a year can significantly impact on water availability, but availability not only depends on rainfall but also on the capacity for storage and the degree to which groundwater and river flows are replenished.
In the future, the world’s water supplies are undoubtedly going to be subject to increased stress due to a number of demographic, environmental, economic and social factors, including:
- Population growth – most likely to occur in locations already experiencing water stress.
- Agriculture – to feed the growing population.
- Urbanisation – which will require cities to draw in water resources from wider areas, in increasing competition with agriculture and lead to an increase in impervious surfaces which can disrupt the replenishment of water sources.
- Industry – to meet the needs for energy, goods and services of the growing population.
- Lifestyles – rising living standards tending to result in higher per capita water usage through increasing consumption of goods and services and more water-intensive food production (e.g. meat).
- Ecosystems – over-abstraction of water can lead to the collapse of wetland, river, lake and estuary ecosystems, which reduces their resilience and ability to directly provide flood attenuation, waste assimilation and food production.
- Water quality – pollution from human activities is increasingly impacting on surface and groundwater quality and posing serious threats to humans and ecosystems.
- Energy requirements – significant quantities of energy are required to transport, treat and abstract water and, where the energy is from fossil fuels, leads to increasing greenhouse gas emissions.
- Climate change is expected to have a significant impact on water supplies. The predicted impacts of climate change on freshwater systems are mainly due to the projected increases in temperature, sea level and precipitation variability. Decreased glacier extent or snow water storage will also affect river flows, whilst rising sea levels could cause saline intrusion into groundwater and estuaries resulting in decreased freshwater availability in coastal areas.
For centuries, humans have secured water in various ways. Where they are available, natural springs and rivers have provided freshwater, whilst in other places surface water must be collected in tanks and dams, extracted from near-surface groundwater shallow wells, or by tapping deeper aquifers by boreholes. The control and storage of irregular water flow through dams and harvesting systems has long been used to regulate seasonal flows, limit flooding and overcome dry spells. However, because water is heavy relative to its value and requires significant quantities of energy to abstract and transport, it is usually not economically feasible to move it in bulk quantities over long distances. This means that local access to water infrastructure is also an important factor in freshwater availability.
Despite the dramatic growth in demand for water from industrial and domestic activities over the last century, agriculture remains the most significant global use. Humans have a minimum basic water requirement of around 50 liters per day to meet the needs of hydration, cooking, personal hygiene and washing, but around 50 times this amount is needed to produce enough food for a day.
Although water usage in buildings only accounts for a small proportion of our total water needs, reducing water use in buildings can play an important role in alleviating some of the pressure on water resources. As buildings are often remote from water sources, in addition to the energy needed to abstract the water, infrastructure is required to transport the water to where it is needed. As well as the additional energy for transport, there will be significant embodied environmental impacts associated with the infrastructure. Furthermore, in water-stressed areas where rates of water abstraction exceed rates of replenishment, this will cause negative ecological impacts such river systems that no longer reach the sea, shrinking lakes and sinking groundwater tables.
Concerns over water scarcity and the associated need for water conservation have led to an increased focus on the potential to save water in buildings. This can be achieved by reducing the demand for water, by using water more efficiently and through better management of water resources. The measurement of water use in a building plays a crucial role in these processes.
The first step to reducing water use is to monitor consumption. For the purpose of reducing water use in buildings, the most straightforward and obvious performance metric would be the total volume of water consumption in a building, measured in liters (L) or cubic meters (m3), i.e. absolute performance. [Note: 1,000 L = 1 m3]. However, buildings vary widely in size and functions. For example, a residential building that consumes 100 m3 of water per year is not necessarily performing better than a building that consumes 150 m3 per year, as the number of people living in each of them may be different. Therefore, water performance of a building has to be defined by a more precise metric, and the total water consumption correlated with another parameter that influences water use, i.e. normalized performance.
When measuring water use it is important to differentiate between treated (potable i.e., drinkable) water that is provided by network supplies and untreated water that is collected and stored locally for use by the building (e.g. harvested rainwater).
It is also important to understand the different terms used to measure water use. Water withdrawal is the amount of water extracted from any source in the natural environment for human purposes, whilst water demand is the volume of water required for a given activity. These quantities may differ due to supply losses which may occur in the supply network or across the site boundary of the building. For buildings the term water consumption is commonly applied to the volume of network supply water entering a building whilst the total water used by the building may also include locally collected and recycled water. The amount of water that is restored to the water system is the return flow, which may be as groundwater infiltrated through permeable soils or directly drained into rivers or other freshwater bodies.
Measuring the amount of network supply water entering the building is the most relevant quantity to measure for buildings as this reflects most of the environmental impacts associated with water use.
In general, water performance metrics follow the form of either ‘water volume consumed’ per ‘individual using building’ per ‘unit time’ or ‘water volume consumed’ per ‘floor area of building’ per ‘unit time’. These two metrics can be combined to give a ‘water volume consumed’ per ‘occupant density’ per ‘unit time’ metric, where occupant density is the amount of floor area per person. On the whole, metrics measuring daily water consumption use liters as the measure of volume, whereas annual metrics use cubic meters.
Measurement is fundamental to reducing the negative impacts of water consumption in buildings. Across its suite of standards for all life cycle stages of buildings, BREEAM recognises buildings that specify water meters and submeters that allow the management and monitoring of water use, encouraging reductions in water use through the identification of areas of high usage and investigation of the potential causes. BREEAM is seeking to increase comparability of measurement across all building life cycle stages through the use of consistent metrics. Measurement of actual water consumption is only applicable to the BREEAM In-Use scheme. However, the current BREEAM UK New Construction scheme recognises projects that commit to measuring water consumption once the building is occupied, with the actual water performance being rewarded under the new Post-occupancy Stage assessment. These requirements aim to help focus attention on the gap between the predicted and actual performance of buildings.
This article was written by Christine Pout, Principal Consultant at BRE