Hydrology

Hydrology is the study of the movement, distribution and quality of water over the planet. Most of us are most interested in the movement of water on the surface of the ground or surface hydrology. The hydrology of any watershed is complicated. There are so many factors that affect runoff. There are the relatively fixed factors like land use and soil types but there are many factors that are variable like soil moisture, amount of snow, rate of snow melt, vegetation, rainfall intensity etc..

The annual precipitation over the watershed averages at little more than 500 mm or about 20 inches. Roughly 25% falls as snow. The driest year since 1981 was 1988 at 288 mm about 11.4 inches and the wettest year was 1999 with 736 mm (29 inches). On average runoff from the entire watershed is about 6% of the precipitation but can vary considerably from year to year for example at Westbourne in 1989 about 1.3% of the precipitation ran off whereas in 2001 over 18% ran off. See the Big Grass Chart, Whitemud near Gladstone chart and the Whitemud near Westbourne chart. Notice how for example the runoff at Westbourne from 1988 to 1991 hardly changes despite the precipitation increasing significantly. In 2000 and 2001 the watershed upstream of Westbourne received about the same precipitation but the runoff was four times as much in 2001. This demonstrates how important intensity is in producing runoff. In other words the amount of rain and snow is only part of what needs to be considered in determining runoff, timing is also very important.

Here is a site that has maps showing recent precipitation across the prairies.

The charts forĀ Beaver and Neepawa show how variable rainfall is. For example in August of 1988 only 5 mm (virtually zero) fell in the Beaver area while in August 1985, 196.4 mm (7.7 inches) fell. In Neepawa 5.3 mm fell in June 1961 and 269 mm in June of 1971. The 1980’s were considered one of the driest decades on record yet for Beaver August of 1988 and November of 1982 are the only months among the driest and the wettest April, July, August and October were in the 1980’s. In Neepawa March of 1985 and April of 1980 were the driest on record and July of 87 and August, 1980 were the wettest. There is a slight rising trend in annual precipitation. Most months show little or no trend but the months of March and May show significant increases over the years.

It is interesting that such a small portion of what falls runs off but seems to cause us so much grief. The rest, over 90%, is lost to evaporation and is consumed by plants (evapotranspiration). A small portion soaks into the ground to the aquifer but even that eventually makes its way to the rivers and streams.

The Whitemud watershed can be divided into ten basic zones (see map) or water management areas. Each zone has its own set hydrological properties requiring different water management approaches. See the map for a more detailed description of each zone.

We often hear the complaint that floods are getting worse and it is because of the drainage works going on upstream. Generally the records don’t support this. The charts for theĀ Big Grass River and Whitemud upstream of Gladstone show a slight downward trend in flood peaks and Whitemud at Westbourne has a slight rising trend. The volume of runoff are declining for two sites but is rising for Westbourne. It is interesting that precipitation, at least in Neepawa shows an upward trend. There is little doubt that changing land use and drainage activities alter runoff patterns but it is not clear how and how much. On one hand drains may speed things up but on the other hand drains likely dry the country out reducing the runoff from the next event. Crops likely consume more water and the cultivation of soil improves infiltration of water. The charts seem to indicate there is not a strong trend one way or the other.

Every event snow melt or rainfall is different and what might increase a flood peak one year might reduce it the next. For example if we have a very wet fall the drainage system will take some of that water away before freeze up, reducing the spring runoff. Everything else being equal it is likely that man made works might increase modest flood events but likely have little affect in extreme events when water will find its way downstream regardless of our efforts to slow or speed it up.

A good deal of background information is available on the Water Stewardship site Surface Water Hydrology.