Frequently Asked Questions - Definitions/Terminology

Q. What is a drainage basin?

A drainage basin, also often referred to as a catchment area or watershed, is the surface area which drains to a lake or contributes to the flow at a particular point in a stream or river.

Q. What are quartiles and how are they related to flows, water storage and snow water equivalent?

Quartiles are three values which divide sorted data sets into four (approximately) equal parts. The lower quartile (1st quartile) is the value that separates the bottom ¼ of a dataset from the remaining top ¾ of the dataset. The second quartile (median) is that value that divides the dataset into two equal parts such that 50% of the values are lower and 50% of the values are higher than the median (2nd quartile). The upper quartile (3rd quartile) is that value that separates the upper ¼ of a dataset from the bottom ¾ of a dataset.

Quartiles are used to show the "normal range" of data (band between the lower and upper quartiles) for flows, water storage and snow water equivalent (pillow data). See also Water Supply Outlook and how normal is applied in the Water Supply Outlook.

Note that the median for our flow and pillow datasets is termed the average. The median and average for most of our datasets are very close given the large numbers of data points within our datasets.

Q. What is snow water equivalent? How is this different from snow depth?
A. Snow depth is the measurement from the top of the snow to ground level. The amount of water produced by a snowpack of a given depth varies depending on the density (water content) of the snowpack. Snow water equivalent is the amount of water that would be obtained by melting a depth of snow.

Q. What is a snow pillow?
A. A snow pillow is an a automated set of apparatus that measures, records and transmits snow water equivalent. from a remote site to an office or laboratory equipped to receive such information.

Precipitation and temperature gauges are usually part of the total installation.

Q. What is a snow course?
A. A snow course is a series of approximately 10 snow sampled points spaced at approximately 30 metre intervals. At each sample point the snow water equivalent is determined manually on site. All manually determined snow water equivalents from each sample point are then averaged to arrive at one snow water equivalent value for the whole snow course.

Q. What is the difference between snow pillow and snow course data?
A. Snow water equivalent values on the snow pillow may or may not match the snow course value at a particular location. While snow pillow data is very valuable information, the quantity of snow on the pillow is only representative of the accumulation at that specific spot. A snow course survey is measured at numerous spots and provides a more representative value of snow in the area. In some locations, there can be considerable difference between the snow pillow and snow course values. Factors such as wind and exposure of the site can cause the snow pillow values to be significantly different from the snow course survey. The snow pillow graphs on our Web/site show the daily average snow water equivalent. The monthly snow survey is the average of all measurements conducted within five days of the end of the month. Also, where snow pillow and snow course measurements are available for the same site, snow pillow records tend to be much shorter (10-15 years) in length compared to the snow course sites. As a result, the difference in the average value between the snow pillow and the snow course can be attributed to snow water equivalent being derived two different ways (physically measured compared to an instrument reading), site location and length of data record. In some cases, the values can deviate by 10-20%. Therefore, while snow pillows are excellent for analyzing trends and for monitoring accumulation between snow surveys, snow course values should always be used when considering the quantity of snow at a particular location as they best represent that area.

Q. What is meant by water year?
A. Water year is the time period from November 1 of one year through October 31 of the following year. The water year number comes from the calendar year for the January 1 to October 31 period. For example, Water Year 2000 would begin on November 1, 1999 and continue through to October 31, 2000. The water year instead of the calendar year is used in the Water Supply Outlook since Alberta's hydrologic cycle essentially begins November 1 with winter snow accumulation and precipitation followed by spring runoff which replenishes the water supply.

Q. What is meant by Water Mastering?
A. Water Mastering is the monitoring and enforcement of the Water Act's "first in time, first in right" priority allocation system undertaken by Regional staff (most notably staff responsible for water management in the southern region) to limit water withdrawals when diversions exceed the water supply during low water periods. Depending on the availability of water, water licencees with low (most recent) priority numbers (date at which a water licence is issued) are requested to either limit their water withdrawals to a certain fraction of their licenced allocation or refrain from withdrawals of any amount. Regional staff base their water allocation decisions on information such as the Monthly Water Supply Report, natural flow reports, current flow information, etc. Water management orders may be issued to selected licencees and withdrawals are monitored by the Water Master.