The intermediate zone may be absent or may be several hundred feet thick, depending on the local geology, topography, and climate. In Reno County the intermediate zone is absent in some areas and is nowhere more than 60 feet thick. The capillary fringe lies directly below the intermediate belt and over the water table and is formed of water held up from the zone of saturation by capillary force. The water in this zone is not available to wells, which must be deepened to the zone of saturation to obtain water. The porosity of a rock aggregate is its property of containing interstices.
Porosity is expressed as the percentage of the total volume occupied by the interstices.
To convert this figure to percentage of volume, the moisture equivalent is multiplied by the apparent specific gravity of the dry material. The specific yield is equal to the porosity minus the specific retention. The specific yield of a formation is needed to estimate the quantity of water available to wells and to estimate the quantity of water represented by a rise or decline in the water table during periods of recharge or discharge. The quantity of water an aquifer will yield to wells depends upon the physical and hydrologic properties of the materials composing the aquifer. Geologic descriptions of the materials penetrated by test holes and wells are useful in making estimates of the quantity of water an aquifer will yield.