What is a common consequence of agricultural drainage with respect to water quality and crop productivity?

• A. Drainage lowers the water table and can transport salts, contributing to salinity issues.
• B. Drainage increases water table and reduces salt transport.
• C. Soil salinity enhances crop yields.
• D. Drainage has no impact on water quality.

Answer: (C)

The key idea is that agricultural drainage lowers the water table to improve soil aeration, but in salts-containing soils or with saline irrigation, that drainage can move dissolved salts through the root zone and into drainage waters. As water is removed by drainage and then evaporates at the surface, salts accumulate in the soil, creating salinity. This salinity raises the osmotic potential in the soil solution and can lead to ion toxicities, which reduces a plant’s ability to take up water and nutrients, ultimately lowering crop productivity. Drainage can also export salts and nutrients to streams or groundwater, impacting water quality downstream. In contrast, the notion that salinity would enhance yields is not correct, since salinity generally reduces yields. And drainage does affect water quality, not leave it unchanged.