Where Does Irrigation Water Come From? Part 2: Groundwater

See Part 1

Ground Water

Groundwater refers to natural stores of water below ground. The “water table” refers to the minimum depth at which groundwater sits in any area. Below the water table there may be large bodies of porous rock, sediment, or other voids where potentially large amounts of water collect. These aquifers can be enormous natural structures which store millions of gallons of water and stretch hundreds of miles, or they can be relatively small areas that span only a few hundred feet.

Natural springs, lakes, rivers and streams are all supplied by groundwater. Wells are one of the ways that humans have traditionally tapped significant quantities of fresh water. Note that aquifers are not present everywhere, which explains why wells can’t be dug at random -- they must access an aquifer that holds enough water to meet the needs of the well-digger. Modern wells use powerful pumps that can rapidly withdraw very large amounts of water for irrigation, municipal, and industrial uses.

The water that flows out from aquifers is recharged over time through precipitation that soaks through the ground to the water table. When rain is in short supply, especially over a long period of time, aquifers may run dry. Aggressive withdrawals through wells or by over-utilizing surface water sources only exacerbate the problem.

Since aquifers are often interconnected, it can be difficult to judge the volume of any given location. Underground water flows at different rates, especially through different types of rock that usually vary in porosity. A drop in the water table can cut off some travel routes between aquifers, which means significant stores of water may become inaccessible without digging new wells.

Groundwater depletion is an increasingly important issue throughout the US and around the world. Since groundwater supplies a significant portion of agricultural water as well as industrial and municipal needs for major cities, depletion threatens the production of food and the viability of some major population centers.

The costs of locating aquifers, drilling wells, pumping and even transporting the water to irrigation mechanisms can make groundwater fairly expensive, but the effects of surface water depletion are even more apparent. Occasionally, water obtained through depleted lakes and rivers becomes such a low quality that it’s not suitable even for agriculture.

Depletion of groundwater triggers widespread changes that may not appear until it’s too late to fix. Some examples are

  • Wells dry up
  • Water levels fall in streams, lakes and wetlands
  • Water quality deteriorates, often becoming more salty
  • Pumping costs increase
  • Land subsidence

Decreasing water levels in streams and wetlands are directly related to groundwater pumping. When water levels fall below the depth needed to keep those biomes alive, significant loss of riparian vegetation and wildlife habitat are inevitable. Loss of the protective buffer provided by wetlands means that flood control and natural filtering mechanisms are lost as well.

Water in aquifers is not 100% fresh water. Not quite half of it is saline, which normally rests below the level of freshwater, but excessive pumping can bring the level inland and upward, which means that water obtained from nearby wells will be salty, not suitable for human, animal, or agricultural use.

Land subsidence is a shocking result for those not familiar with groundwater issues. Basically, the water stored in aquifers act as support for the land above it. When that water is withdrawn, the space it occupies becomes an empty cavity and the soil itself will collapse into the cavity. Not only does this cause damage to surface structures such as buildings, roads and sewer systems, but a collapsed aquifer will never recover. That’s a permanent loss of natural water storage and a permanent loss of natural water management for local habitats.

Since over-use of groundwater supplies presents such dire consequences, it’s imperative that farmers ensure they’re using all available methods to collect, store, and use agricultural water in the most efficient way. Water that’s lost through an inefficient system may eventually return to the water table through precipitation or seepage through the soil, but the travel time is long and farmers will have to pump even more to make up for the loss.

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