Comparison of Selected Methods for Estimating Groundwater Recharge In Humid Regions

Method:	Category:	Spatial Scale:	Temporal Scale:	Quantity Estimated:	Ease of Use:	Data Needs:	Cost:
Darcian unit-gradient	Unsaturated Zone	Point	Long-Term Average	Potential Recharge	Moderate	Low	High
The Darcian unit-gradient method can produce point estimates of potential recharge from measurements of hydraulic conductivity and water content in the unsaturated zone. If flow under field conditions is steady and driven by gravity alone, then, according to Darcy’s Law, downward percolation rate will be numerically equal to the hydraulic conductivity of the material at the measured in-situ water content. The Darcian method does not necessarily indicate total recharge as it only accounts for diffuse, matrix flow. Recharge due to preferential flow is inherently non-Darcian and, if significant, must be determined separately. The method is conceptually simple, but accurate measurements of water content and hydraulic conductivity can be difficult and expensive.