If I take water from the water table using a local well on my property and use this water for irrigating crops, Will this cause the water table to rise? If anything would this cause the water table to drop due to evaporation during the irrigation process. I understand how bringing water from distance can cause water tables to rise. I don't understand how this happens when taking and returning water from a local source. Please note this is an arid climate with little annual rainfall.
Much of the water you pump out and use for irrigation will be lost to evapotranspiration. This will be even more if you use profligate irrigation methods like flood, furrow, or sprinklers. So quite a bit less will be percolating back down to the water table than you took out, if any. Of course on a small site the effect on the regional water table will be pretty minimal, but you can create a cone-shaped area of depressed water table right around your well, and might eventually run the danger of running it dry. We keep tabs on the level of water in our well through the year by lowering a weighted string down it once a month and then measuring the distance from the top to where the string gets wet.....
So if irrigation water is local then we deplete the water table. And if irrigation water is brought in from a distance we raise the water table. Each have issues.
The short answer is that Yes it will lower the water table, due to massive water loss.
You are effectively taking cool protected water water deep in the soil and putting it on the surface for the wind to blow it away, the sun to evaporate it and for animals, plants, etc to sweat/transpire it away. You might even export the 90% water orange/watermelon/fruit/leave away for a small profit.
However you might be able to increase the water table in a few specific circumstances:
Instead of buying your produce from a wasteful farmer nextdoor you grow your own in a very water efficient way (drip irrigation, cultivar selection (almond vs pecans))
You only use water to establish your water efficient mycelium-root network and from then on the plants survive only on natural rainfall, and with careful earthwork some of the rainfall even soak in and reach the water-table.
To complicate things even more if you are 'importing/trucking in' water and it is from the same aquifer it will lower your water table.
If it is upstream less ground water is left to flow down to you, if it is down stream, the water will flow down there faster leaving you with less.
You might also have more than one water table:
There is the subsurface water table during the wet season,
then there might be a polluted ground water about 15ft below
then there might be a drinking water-clean aquifer 150ft below
aaron hirsch wrote:So if irrigation water is local then we deplete the water table. And if irrigation water is brought in from a distance we raise the water table. Each have issues.
Quite so. Either you are using the water supplies of your area or you are using the water supplies of another area.
Something I think you might want to consider trying is a technique called groundwater banking. This works by using water when it is necessary in times of drought from an aquifer, and then recharging the aquifer when the rains are plentiful. These cycles can be seasonal, yearly, biannual, or much much longer depending on the cycles of your location.
aaron hirsch wrote: Please note this is an arid climate with little annual rainfall
Considering that you are in an arid climate and you made this post in the greening the desert subforum, I think it may be worth considering ways of altering your climate into one that is more productive.
aaron hirsch wrote: Any ideas would be great...
I will take this as an invitation to start talking about altering the climate so water is no longer a shortage issue, or at least less of one. Here are some techniques that can be used to start recharging your aquifer and raising the water table:
-Swales are on-contour "ditches" that catch and slow rainwater so that it has time to sink into the ground. In the process, this also decreases erosion and runoff.
-Air wells work on the principle of thermal inertia which states that objects with thermal mass can capture and store heat from the day and release that heat during the night. Air wells are typically made of rocks, concrete, or stone which have high thermal mass. When dawn starts, the well is still cold and water condenses onto the surface and trickles into the storage container.
-Rock Mulch works on a similar principle and can be used to catch water from the air for your plants.
-Gabions can be used to slow water down, too, in case of flood events.
-The Desert Oasis and Greening the Desert videos discuss similar concepts and more.
A certain amount of water will percolate down, just as rain does. If the incoming water is from a distant source, any water that moves down, adds to that aquifer.
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