Ministry of FLNRO

Glossary of Terms

Automatic Snow Weather Station or ASWS: A station where snow water equivalent and other parameters are measured automatically. The data collected are transmitted by satellite (generally every 1 hours). These sites are normally very remote with access by helicopter only. For a more complete description, see "What is an ASWS?"

Cumulative Precipitation: The total precipitation in a region since the previous November 1. Usually expressed as a percentage of normal.

Drought: Drought is a recurrent feature of climate involving a deficiency of precipitation over an extended period of time, resulting in a water shortage for activities, communities or aquatic ecosystems. In British Columbia, drought may be caused by combinations of insufficient snow accumulation, hot and dry weather or a delay in rainfall.

Drought may be defined by several methods depending on the particular cause of the dry conditions. This definition is intended to be inclusive and is taken from the BC Drought Response Plan

Freshet: The substantial rise in water level of a stream or river caused by melting snow in the spring.

Groundwater Observation Well: A well that is used exclusively for measuring and recording the level of the groundwater at that location.

Hydrograph: A plot of the level or flow of a river over a period of time.

Normal: is the average value of a parameter over a fixed, usually 30-year, period. At present the normal period is 1981-2010. Thus the normal water equivalent of a snow course is the mean or average value for the 1981-2010 period, for that sampling date.

Peak Flow: The greatest flow reached by a river at a point in a set time period. Normally expressed in cubic metres per second (m3/s.)

Regional Snowpack Index: The sum of the snow water equivalents at selected representative snow courses in the region. Often expressed as a percentage of normal.

Return Periods: Return Period Estimates are discharge rates that are associated with the statistical probability of a given discharge occurring in any year. Return Periods are expressed in years, for example, a 2-year return period event has a 50% chance of occurring in a given year, a 20-year return period event has a 5% chance of occurring in a given year, and a 100-year return period event has a 1% chance of occurring in a given year.  Therefore, a flood event of any given magnitude could occur in any given year. However, the annual likelihood of a given flood magnitude is associated with its’ statistical probability based on the long-term record of the river.

Sampling Period: A sampling period is a window around the nominal sampling date in which snow surveys can be made. e.g. A February 1 sampling can be made as early as January 26 or as late as February 6. This is necessary because weather can make snow survey access impossible on certain days. Measurements outside these limits are flagged as early or late.

Snow Course: A marked location, free from encroachment, where snow depth and snow water equivalent are measured on a regular basis with standard snow sampling tubes.

Snow Pillow: A large rubber/neoprene bladder containing anti-freeze laid on the ground prior to snowfall. The pressure of the fluid in the bladder is measured and this enables the determination of the snow water equivalent that location. See ASP, above.

Snow Survey: A manual measurement of the snow depth and snow water equivalent at a site. Samples are taken at several pre-determined points at a snow course or snow pillow site. The data are most frequently used to compare the current readings with other readings taken at the same site at other times. For a more complete description see: "What is a Snow Survey?"

Snow Water Equivalent: The water content of a snowpack at a point, expressed as the depth of water that would result from melting the snow. Normally measured in millimetres (mm).

Stage: The level of a river or lake at a point. Usually expressed in metres (m), often to an arbitrary datum.

Volume Forecast: A forecast of the volume of water expected to pass a given point on a river (or flow into a lake) in a set time period. This is based on current and antecedent conditions, but assumes normal weather patterns through the forecast period. Units are usually thousands of cubic decametres (kdam3), which is the same as millions of cubic metres (m3*106)