Supply and Use on Multiple Scales: Units of Water

Units of measurement: volumes, fluxes, and concentrations

The uses of water for human activity vary immensely, and as a result, water resource management covers a wide range of temporal and spatial scales. In some cases, the timescales are short and volumes relatively small (i.e. domestic pumping of several gallons per minute, over timescales of minutes or hours). At the other extreme, water allocations for states or municipalities are often considered in the context of average annual flows in the billions of gallons. Because so many different scales of measurement are used to describe water flux or discharge (volumes of water) and flow rates (the velocity of flow), it is important to have some facility with the various units of measurement and get a sense for their relative magnitudes.

As one example, the total fluxes of water through river systems – commonly used to define allocations of water for states or nations - are measured and reported in acre-feet. This is a unit of water volume equal to the amount of water that covers an area of one acre, one foot deep. One acre-foot is equivalent to 325,851 gallons (see summary of unit conversions from the U.S. Geological Survey), and is often considered as the amount of water needed for a family of four for about one year.

As we'll discuss in Module 3, over shorter timescales, river discharges are reported in units of cubic feet per second (cfs), cubic meters per second (m3/s), or gallons per minute (gpm). As one example, on average, Spring Creek carries about 50 cfs at Houserville, PA; this increases downstream to about 90-100 cfs at Axemann as the creek is fed by springs and small tributaries. Short-lived peak discharge may exceed 500 cfs after storm events. For comparison, the flow of the Mississippi River at St. Louis, MO is typically about 400,000-600,000 cfs; in major floods the discharge is over 1,000,000 cfs. The flow rates of rivers and groundwater, as we will see in Modules 3-4 and 6, are reported as a velocity - units of length per time. These measures represent the velocity of the water itself, or of an object (stick, boat, person, etc...) carried by the river or stream.

Yet other key quantities in hydrology are reported in units of an equivalent depth (or length) per time. For example, rainfall rates are described in units of inches, cm, or mm per hour (for individual storm events) or per year (i.e. annual average precipitation). Evaporation rates are reported in the same way – but of course, represent water transport in the opposite direction (up!). The total volume of water these represent depends on the area over which they occur.