Water Yield of a Well

After development of well, it should be evaluated to determine the yield and drawn down to how much water is flowing in to a well. Flow of water in to gravity well. If the well is not disturbed for sufficient time, the water level in the well will stabilize at the level of groundwater in the aquifer. Few holes, called observation wells, are drilled in the ground surrounding the well. The water in these observation wells is also at the same level as in the well before pumping is started. After water is pumped from the well for sometime, the water in the observation wells will stand at different levels. The water in observation well near will be at lower level than that in observation well farther away from the well. If the level in these observation wells is joined, it will form a free surface curve. The free surface curve is the boundary of the cone of depression. This is the region from where water flows in to the well and is pumped out. If the pumping is continued at a uniform rate, the water level in the well fall for some time and then it will become stationary at one level. At this level, the rate of pumping will equal the rate of flow in to well from the aquifer. If the rate of pumping is increased, the water level in the well will fall down further and region influenced by cone of depression also increases. The draw down is the difference in static ground water level and stationary level of water in well. Yield is nothing but the rate of pumping and is generally specified as liters per minute. As the pumping rate increases, the velocity of inflow in to the well also increases. As the velocity of inflow increases, the soil particles get disturbed and begin to flow in to the well. This velocity is called critical velocity. The yield, available before this velocity is reached, is the maximum permissible yield from the well. The draw down, at which the critical velocity is reached, is defined as critical head. Normally wells are pumped at working heads much lower than critical head.

Flow into a Gravity Well