As the demand for farm products becomes more intense, crops are planted more densely, and multiple plantings are cycled through in a single growing system. Natural irrigation from rainfall is rarely sufficient even in areas that have generally been considered “wet.” Even though the annual rainfall may be sufficient, the timing may not suit the needs of agriculture, with rainy winters and dry summers, for example. In these situations, farmers need to obtain and distribute supplemental water through artificial means.
Generally, irrigation systems are designed to access water from finite groundwater supplies and often over-appropriated surface-water supplies. In some cases, farmers have made provisions to collect and store precipitation during periods of ample rainfall in man-made reservoirs. As agricultural demand intensifies, each of these sources are being pressed to their limits and it’s necessary to balance or even reduce dependencies in many places.
Surface water refers to water that can be accessed directly, without the need to dig under the ground. Examples are rivers, streams, ponds and lakes. Precipitation is classified as surface water as well, especially in the case of snow, which, at high elevations, is naturally stored as snowpack. Meltwater runoff from snowpack can replenish rivers and recharge groundwater supplies. In California, for example, the Sierra Nevada snowpack was able to provide up to 60% of the state’s total water needs before being seriously depleted by recent drought.
Groundwater is a popular source for irrigation water because it is generally easy to access and less expensive to draw from. The Colorado River is a direct source of irrigation water for farmers across 7 states and into Mexico, while still more withdrawals are made to support irrigation needs in states not directly connected to the river.
It’s easy to assume that withdrawing water from lakes and streams will only affect those individual bodies of water. However, inadequate flow of surface water impacts aquatic animals and may damage or kill aquatic plants and other elements of a habitat. Wetlands that dry up when streams and springs cease to flow are no longer effective at controlling floodwater when intense rainfall events follow a sustained drought. Lakes play a critical role in maintaining healthy ecosystems, so when levels fall or one or more dry up completely, the health of the entire biome is at risk.
Still, even surface water sources are ultimately supported by groundwater, which naturally releases from underground stores when rivers and lakes fall below a minimum level. This can help maintain critical surface water volume, but when coupled with direct groundwater withdrawals, the pressure on aquifers can be intense. Unfortunately, groundwater moves slowly, so in the case of large aquifers like those supporting the Great Lakes, problems arising from overuse are slower to appear and generally are less likely to raise alarm.
