This is the second article in a three part series covering soil mositure mechanics (January 24 issue), ground water and water tables (this issue) and stubble soil moisture (next issue).
In the last column, I explained that the water table is the condition where all pore spaces in the soil are filled with water. The water table is determined as much by the geology surrounding the site as it is by the site itself. To understand water tables we must understand Mother Nature’s plumbing system, and that’s geology.
Let us look at some examples of long-term water level records from observation wells in Saskatchewan. These wells were established mostly in the 60s and 70s and were placed away from active pumping wells to measure natural aquifer response to climatic conditions.
EXAMPLE 1: UNIVERSITY OF SASKATCHEWAN GOODALE FARM
The Goodale farm is located a few miles southeast of Saskatoon in silty and sandy lacustrine deposits (glacial lake). The aquifer being measured is a surficial, unconfined aquifer. The hydrograph (water level) in this observation well is a good approximation of the water table at that location.
The well was installed in early 1975, just after one of the largest recorded snow events (1973-74). In the mid-seventies the quarter section had so much surface water that the U of S looked at the feasibility of drainage, but there was nowhere to drain the water so the idea was dropped.
The water level in this well continued on a downward trend from installation to the drought of 2001-02. The surface water on that quarter disappeared all by itself. Alarmists would say that the groundwater at this site is doomed.
The downward trend in the 90s was despite some very good rainfall years and some good crops. The good rains in June and July were immediately sucked up by the crop and there was no impact on the water table. Water table rise results when significant rain/snowmelt occurs outside the growing season or when the growing season rain is way above the rate of crop use.
After the big snow years of 2005 to 2008, the water table quickly shot up 10 feet but it was still not at the starting level of 1975. Then comes 2010 with 20 inches of rain from April to freeze up. The water table rose by about seven feet in one summer/fall and is now above the level of 1975.
The lesson here is this: Climatic cycles run on a long-time clock and water table cycles can be even longer because of the time it takes for the system to respond to precipitaiton. The water table at this site will not quickly drop to 2002 levels — unless we end up with a 10 year drought — which could well happen. It has happened before.
EXAMPLE 2: UNIVERSITY OF SASKATCHEWAN KERNEN FARM
The Saskatoon well is totally different geology than the Goodale well. At the Kernen farm (Saskatoon) there is several feet of lacustrine clays and silts overlying clay loam glacial till (deposited by glaciers). The Forestry Farm aquifer (sand) comes in at about 35 feet and extends to about 90 feet. The observation well is completed at 88 feet and the water level in this well measures the piezometric (pressure) surface of the aquifer. The water level in this well is not the water table, but it will have an influence on how the water table operates.
The well was completed in 1968 and the water level rose from then to 1975. From 1975 to 2005 there was little change in the water level. But the snowy years of ’05 to ’08 brought a sharp rise, and, after a downtick in 2008- 09, rose 11 feet because of the 25 inches of rain we received in 2010. An increase of this amount in one summer/fall is unprecedented in the length of the career path of even this old fossil.
Now, in this case the water level is still 25 feet below soil surface and the water table can still drain. The Forestry Farm aquifer drains as springs along the river just north of U of S campus so impacts on water tables will likely be small.
As an aside, I use examples from the university farms because I am most familiar with them and have lived and worked though the many decades of records. Please note that myself or the U of S can take no credit for the existence of these wells. They were installed by Bill Meneley who, at that time, was with the Sask Research Council. Bill got tired of bureaucracy in the late 70s and went private consulting. I hired Bill and Earl Christiansen (a geologist) when we did the soil salinity work in the 1980s-90s. At the time my use of private consultants was questioned when I could be using geologists from the public sector. My response was simple “I hire Bill and Earl because when I ask them a question, I get an answer.”
So, what is the point of all this? The point is that there are locations in Saskatchewan and Manitoba where the water table has risen dramatically in 2010, as has the water levels in aquifers. Some of these, in part, control the water table, and the water table cycles can be longer than the rainfall/ snow cycles.
The soils most at risk are the best soils, i.e. thick, black soils. Many thick, black soils are that way because an aquifer beneath them has a water level close enough to the surface to provide sub-irrigation in times of drought. Many years of low rain they still crank out good crops. But when excess comes long they are in trouble. The excess water can not drain away any time soon because of the pressure from beneath.
On my Dundurn farm a lack of rain is the most common situation. If the soil is fully recharged with water, I elect 50 per cent coverage for crop insurance and if it has little or no soil stored water I elect 80 per cent coverage. In areas of excess rain, where the water table is close to the surface and not likely to recede quickly — perhaps 80 per cent is a good choice.
In making the Stubble Soil Moisture Map as of November 1, 2010, I am working on defining and mapping a category called “Soil Moisture Excessive — High Water Table.” Watch for it in an upcoming issue ofGrainews.
J.L.(Les)Henryisaformerprofessorandextension specialistattheUniv.ofSask.andfarmsat Dundurn,Sask.Healsorecentlyfinishedasecond printingof“Henry’sHandbookofSoiland Water,”abookthatmixesthebasicsandpractical aspectsofsoil,fertilizerandfarming.Les willcovertheshippingandGSTforGrainews readers.Simplysendachequefor$50toHenry Perspectives,143TuckerCres,Saskatoon,Sask., S7H3H7,andhewilldispatchasignedbook poste-haste.