erosion
While irrigation has provided a number of important benefits to society in the form of higher yields of food and fiber, there are a number of potential drawbacks to the use of this technology.

Irrigation assumes water will be distributed over the soil, often by sprinklers, and sometimes by drip systems or surface flows. Water traveling over soil will result in erosion. The displacement of the top soil has impacts of various natures. When soil is displaced but remains in the field, the impact is low. But when the soil is widely displaced, the soil cannot be used for the intended crop and also clogs drainage ditches and streams, harms aquatic habitats, fouls waters used for recreational activities, and increases the need for water treatments. Irrigation should be wisely managed so erosion and its eventual impacts are minimized.

Some soils have high salt levels because decomposition of salty parent materials. In addition, plants consume water by evaportranspiration which leaves residues of salt in the soil. Residual salts are leached into deep soil. They may eventually leach into underground aquifers. Although the farm using irrigation to increase yield and quality may not suffer the effects of increased salinity, the properties downstream may. Soils in arid and semiarid areas often have residual salts. The soil can be used for production if salts are leached from the root zones of plants but the salts appear somewhere else in the hydrolic cycle.

Plants need over 20 nutrients to thrive but excess nutrients are harmful. Too much of a good thing can be just as damaging as insufficiency. Algae growing in a pond may reflect the abundance of available nutrients but too much algae can deplete the oxygen needed by fish. Nitrates are commonly built up in irrigated soils. They pose a threat to public and animal health.

Flowing water sweeps other things along as it travels. Pesticides properly used in agricultural production can be moved to other locations with irrigation. The use of irrigation and pesticides should be harmonious. Besides pesticides, pathogens (harmful agents) can be carried and spread during irrigation.

Another problem with irrigation centers around the increasing demand for water use by urban dwellers. Farmers are often more efficient and thrifty with water than the average family but the farmers used more water at a time. Urban families expect to have water at the ready to wash the car, clean the driveway, clean the gutters, wash the dog, and a variety of weekly chores. Often these water use activities are luxuries. Some may argue that farmers waste water, and irrigation is based on greed: force the land to produce more. A balance between individual desires and efficient utilization of a resource is difficult to reach.

Many of the problems associated with irrigation can be minimized with wise use but wise use requires knowledge, skills, commitment, and understanding.

Links:

http://www.ars.usda.gov/main/site_main.htm?modecode=53-10-20-00
http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/sag3267
http://water.unl.edu/web/crops/soil