Water Catchments.

Drinking Water Catchments, Transpiration and Sap Flow Technology.

Drinking water is a precious resource, and the management of drinking water catchments is of utmost importance to ensure a sustainable supply for communities around the world. While various factors contribute to the quality and quantity of water in catchments, the role of trees in this ecosystem is often underestimated. Trees play a pivotal role in maintaining the health and quality of drinking water catchments through their process of transpiration, which significantly affects the overall water balance. In this article, we will explore the significance of tree water use in drinking water catchments, citing statistics on how much trees transpire.

Tree Transpiration: A Fundamental Process.

Sap flow in water catchments.

Transpiration is the process by which water is absorbed through a plant's roots, travels up to the leaves, and is then released into the atmosphere as water vapor through tiny openings called stomata. This process not only helps cool the surrounding environment but also contributes to the overall water cycle by releasing moisture into the atmosphere. Trees, with their extensive root systems and large leaf surfaces, are particularly efficient at transpiring water.

The Relationship Between Transpiration and Sap Flow.

Sap flow is the movement of fluid in the xylem (sapwood) of plants. Although sap flow occurs within plants, it is assumed that most of the fluid moves from the roots, stem and leaves along the transpirational pathway. That is, sap flow eventuates into transpiration as water vapour leaves the plants via stomata. Therefore, even though sap flow and transpiration are different physiological processes, they are often considered synonymous and highly correlated.

Sap Flow and Tree Transpiration in Water Catchments.

The following are some examples of scientific research on sap flow in water catchments. These examples are not exhaustive but provide an overview of the extensive literature.

Annual tree transpiration: According to research conducted by the United States Geological Survey (USGS), a single large oak tree can transpire on average approximately 420 litres (1100 gallons) of water per day. Over the course of a year, this amounts to several thousand gallons, or 150,000 litres, of water for a single tree.

Slope and drainage conditions: sap flow and transpiration are not uniform across water catchments and can vary significantly between trees over a short distance. For example, a study from south-east Australia showed that sap flow varied according to slope orientation, drainage and hillslope position. Therefore, research aiming to quantify water catchment transpiration need to consider spatial variation.

Drought, climate change and interspecies variation: a study in northern Europe of four tree species, Scots pine (Pinus sylvestris), Norway spruce (Picea abies L. H. Karst.) and birch (silver birch: Betula pendula Roth. and downy birch: B. pubescens Ehrh.), demonstrated interspecies variation to seasonal dryness and a short-term drought. For example, the birch trees had higher sap flow during the drought period compared with Scots pine. The researchers concluded that “birch may compete with Scots pine trees at the study sites in terms of water consumption and growth in future”.

Ecological disturbance, tree functional type and sap flow: fire is a major disturbance in drinking water catchments around the world including California, Canada, Australia and more. Studies have shown that the dominant species functional type can have a significant effect on catchment water use and yield following fire. For example, Buckley et al (2012) measured a significant increase in water use in reseeder functional type (Alpine Ash) 7 years following a fire disturbance and that this water use resulted in a decrease water catchment yield . In contrast, Gharun et al (2013) found no difference in water use 3 years after a fire disturbance in a mixed species functional type (resprouters and resee
Tree water use contributes significantly to the hydrologic cycle and water dynamics in drinking water catchments.

ders) . Therefore, the dominant species post fire disturbance can influence hydrology and water catchment dynamics.

Implications for Drinking Water Catchments.

Tree water use contributes significantly to the hydrologic cycle and water dynamics in drinking water catchments. The dynamics, though, are complex and depend on species composition, functional type, location on hillslopes and drainage lines, as well as disturbance regimes from fire and other agents. Furthermore, the effects of climate change, changing temperature and precipitation regimes, as well as atmospheric carbon dioxide concentration, will influence water catchment dynamics in the future.

Given the significance of drinking water to the health of society and the economic functioning, a thorough understanding of the dynamics is required. Instrumenting trees with sap flow sensors will provide valuable information for researchers, managers, and policy makers.