I've been reading through the information here, and I have a question about how water tables work on a slope. I am intrigued by underground water, and I'm always trying to figure out what it's doing that I can't see.
I live at the base of a small mountain in the Pacific Northwest where we have very wet winters and dry summers.
During the first week in September, in the driest part of the year, I had a 20 by 20 pond excavated on a gentle slope at the base of this mountain and on the lower side I built a dam of clay on good clay subsoil. The pond will be at most about 6 feet deep.
In the upper corner of the slope where this pond is now located I had already dug a 3 by 5 foot hole that was about 2 feet deep. I have watched this for years and even in the driest part of summer there was always 6 inches of water in this hole and most of the summer there was 1 1/2 to 2 feet. I used it to irrigate a garden downhill and the water always replenished itself.
The top of the newly built dam on the lower part of the slope is slightly below where the top of the water table in the old watering hole usually was, and the old water hole is now incorporated into the upper corner of the new pond.
Although it was excavated at the driest time of year the clay was slick with water and the excavation uncovered a couple spots where small trickles of water ran out of the bank on the side that is backed by the mountain.
I'm not sure if it is correct to call these springs or if they are something else. One is located about 3 feet up from the bottom of the hole and seems to put out about 2 cups of water every 5 minutes and the other is about 4 feet up from the bottom and puts out maybe a cup every 10 minutes.
There may have been other springs lower down that got covered with water before their existence could be noted. The bottom spring at 3 feet seemed very consistent in it's out put during September. It is just a little hole about the size of a drinking straw. The water rose from this like the water in a drinking fountain about 1/4 inch above the hole.
The top spring is much more puzzling as it waxes and wanes for no apparent reason and with no rainfall to influence it. It ran for a week after the initial excavation, dried up, and then started again. As the dry season continued the top spring just maintained a small pool of water at the 4 foot level which never overflowed and would quickly refill if I bailed it out. If I muddied the water in the top spring's pool, I could see a tiny trickle of clear water still flowing into the pool through a crack in the clay but it never overflowed the pool. I wondered how the water could be flowing in and disappearing like that. The bottom spring that was a foot beneath it continued to run clear, even when the top spring was muddied, suggesting if the upper spring was also somehow sucking the water back in, both these little springs were independent of each other.
September was dry with no significant rainfall but by the end of the month the water from this bottom spring had filled up the first 3 feet of the pond and covered the hole with 6 inches of water.
It seems the pond stopped coming up at about the point where the water was trying to rise above the natural undisturbed clay subsoil to where it first met the new clay dam . I'm not sure yet if it is leaking or wicking. When we got a couple inches of rain the pond came up about 4 inches and then slowly went down to that same level. We just had about 6 inches of rain and the pond came up a foot for the first time covering the top spring. Now the heavy rain has stopped the water level is dropping at a rate of an inch a day.
The aquifer hasn't fully recharged yet as a seasonal creek which is not connected to the pond is still not running, so I expect the general water table is going to get quite a bit higher.
If over the next year I continue to loose water down to the level of where the dam begins I will suspect there is a leak in the dam, and if I continue to loose water down to the level of the top spring I will suspect that spring is sucking water and I will try and block it with clay.
About 50 feet and slightly downhill from the lower edge of this pond is a 1 1/2 foot deep drainage trench running along the back of my garden. I need this trench to get rid of excess water, just as much as I need the pond for when it is too dry later in the season. As soon as things get wet in the winter water trickles into this trench from numerous underground seepages. I don't know if this is connected with the water table that feeds my pond, but as I already have 4 feet of water in the pond well above this trench it seems it isn't.
I have been reading in other places here about springs and the water table and how these can give water and take it away, and although I understand the idea that the water in a pond that is connected to a spring or water table will be the same as the level of the groundwater or aquifer, what I can't get my head around is how that works on a slope when the underground water is in aquifers above and below the pond.
As I watch the pond fill I worry that it might not fill enough due to getting drained by a lower water table downslope from the pond, and then I worry that I may have uncorked the bottom of the mountain and find myself contending with a lot more water than I want.
Is it possible to uncork the bottom of a mountain full of water?
I'm also not sure what happens in a pond where the usual water table at the high end of a slope where a pond has been excavated, would be about 4 feet higher than at the low end?
Which end of the ponds water table usually decides the level of the pond?
Last edited by Littlestfish; 11/12/08 07:37 PM.
