cation exchange capacity (CEC) The sum of exchangeable bases plus total soil acidity at a specific pH, values, usually 7.0 or 8.0. When acidity is expressed as salt extractable acidity, the cation exchange capacity is called the effective cation exchange capacity (ECEC) because this is considered to be the CEC of the exchanger at the native pH value. It is usually expressed in centimoles of charge per kilogram of exchanger (cmol_ckg^-1) or millimoles of charge per kilogram of exchanger. See also acidity, total.

hydrology The science dealing with the distribution and movement of water:

• agrohydrology - The science dealing with the distribution and movement of rainfall and/or irrigation water and soil solution to and from the root zone in agricultural land, and with the distribution and movement of irrigation and surface water in conveyance systems on agricultural land.
• ground-water hydrology - The science dealing with the movement of the soil solution in the saturated zone of the soil profile.
• soil hydrology - The science dealing with the distribution and movement of the soil solution in the soil profile.
• surface hydrology - The science dealing with the distribution and conveyance of water on the soil surface.
• wetland hydrology - The science dealing with water depth, flow patterns and duration, and frequency of flooding that define and delineate wetlands.