the water table, it is not saturated and does not contribute to the transmissivity. When the soil layer is entirely below the water table, its saturated thickness corresponds to the thickness of the soil layer itself. When the water table is inside a soil layer, the saturated thickness corresponds to the distance of the water table to the bottom of the layer. As the water table may behave dynamically, this thickness may change from place to place or from time to time, so that the transmissivity may vary accordingly. In a semi-confined aquifer, the water table is found within a soil layer with a negligibly small transmissivity, so that changes of the total transmissivity () resulting from changes in the level of the water table are negligibly small. When pumping water from an unconfined aquifer, where the water table is inside a soil layer with a significant transmissivity, the water table may be drawn down whereby the transmissivity reduces and the flow of water to the well diminishes. Resistance The resistance to vertical flow () of the th soil layer with a saturated thickness and vertical hydraulic conductivity is: Expressing in m/day and in m, the resistance () is expressed in days. The total resistance () of the aquifer is the sum of each layer's resistance: The apparent vertical hydraulic conductivity () of the aquifer is: where is the total thickness of the aquifer: The resistance plays a role in aquifers where a sequence of layers occurs with varying horizontal permeability so that horizontal flow is found mainly in the layers with high horizontal permeability while the layers with low horizontal permeability transmit the water mainly in a vertical sense. Anisotropy When the horizontal and vertical hydraulic conductivity ( and ) of the soil layer differ considerably, the layer is said to be anisotropic with respect to hydraulic conductivity. When the apparent horizontal and vertical hydraulic conductivity ( and ) differ considerably, the aquifer is said to be anisotropic with respect to hydraulic conductivity. An aquifer is called semi-confined when a saturated layer with a relatively small horizontal hydraulic conductivity (the semi-confining layer or aquitard) overlies a layer with a relatively high horizontal hydraulic conductivity so that the flow of groundwater in the first layer is mainly vertical and in the second layer mainly horizontal. The resistance of a semi-confining top layer of an aquifer can be determined from pumping