How to Calculate Aquifer Recharge

Aquifer recharge is the result of water coming into and water leaving an underground basin. Data that is necessary to calculate aquifer recharge revolves around precipitation, surface water, irrigation, seasonal trends, evapotranspiration, depth to aquifer, topography, soils, geology, vegetation, storm events and snowmelt as well as underground slope decreases. Aquifers can be recharge can occur indirectly from water flowing down through unsaturated and saturated soil zones or directly from where rivers and lakes meet the aquifer water table. There are various methods that can be employed to calculate recharge. Computer models, the Darcian Method, isotopic or chemical tracers or geophysical techniques are all possible methods.

Calculate Recharge

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  Roots prevent water from reaching the aquifer through evapotranspiration. roots image by cassiusjb from Fotolia.com

  Calculate recharge by adding the difference between ground water flows into and out of the aquifer, the base flow, evapotranspiration rates and the change in water storage (surface water, ground water, unsaturated zones and snow). The recharge rate is expresses at L/T which is the volume/unit square of an area per unit time. Evapotranspiration is an important factor to consider accurately because a lot of water is soaked up in the vegetation root zone and thus does not reach the aquifer.

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  Use computer models to estimate recharge. Watershed, surface water flow and ground water flow models can all be used to predict recharge rates and are very useful in predicting the effects of climate and land use changes on recharge rates. Data assumptions regarding things like irrigation or pumping rates along with atmospheric temperature changes over time, can all be tweaked before running the computer models to predict aquifer rates in light of human influenced factors.

  These models are best suited to unconfined aquifers within shallow water tables, with varying groundwater fluctuations over time. Water table height is a critical data input. The models are most accurate over very short term periods.

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  Soil characteristics, such as permeability, are necessary inputs for recharge calculations. tilled field image by Niki from Fotolia.com

  Employ the Darcian method to calculate recharge. This method relies on the hydraulic conductivity of soil to calculate recharge rates and is useful if pressure gradients are small. Below a certain depth, water flow is considered to be steady, driven by gravity alone. To employ the Darcian method, you need the hydraulic conductivity of soil zones as measured in the field along with core samples from deep in the unsaturated zone. In general, you evaluate the steadiness of flow in the field, obtain a core sample from the steady flow areas, measure the field water content precisely and then measure the hydraulic conductivity at or near the field water content sample point. This calculation determines the recharge rate.

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  Use isotopic or chemical tracers, such as dyes or agricultural chemicals to prepare chemical mass balance equations. Match the chemical patterns to infiltrated or aged water within the aquifer. Sampling and analysis must be used to identify the chemical patterns. Chloride is the most common chemical tracer. The equation used to calculate recharge is the chloride concentrations in precipitation divided by the chloride concentrations in pore water.

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  Use geophysical techniques to refine recharge estimates. Geophysical techniques can provide accurate representations on boundary conditions, constraints on percolation and most importantly can identify spatial and temporal variations. Techniques such as ground penetrating radar, reflectometry and nuclear magnetic resonance provide a clearer picture of underground formations and flows that refine data used directly in equations or in computer modeling efforts.