In hydrogeology, the representative elementary volume (REV) is the smallest volume over which a measurement can be made that will yield a value representative of the whole. Below REV, the parameter is not defined and the material can not be treated as a continuum.
In order to establish a given porous medium's properties, we are going to have to measure samples of the porous medium. If the sample is too small, the readings tend to oscillate. As we increase the sample size, the oscillations begin to dampen out. Eventually the sample size will become large enough that we begin to get consistent readings. This sample size is referred to as the representative elementary volume. If we continue to increase our sample size, measurement will remain stable until the sample size gets large enough that we begin to include other hydrostratigraphic layers. This is referred to as the maximum elementary volume (MEV).
Groundwater flow equation has to be defined in an REV
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