Groundwater Recharge Calculator
Groundwater recharge is the fraction of precipitation that infiltrates through the soil profile and enters the water table, replenishing aquifers. The water balance method is the simplest way to estimate recharge: Recharge = Precipitation - Actual Evapotranspiration - Surface Runoff. This assumes all water not lost to evapotranspiration or surface runoff becomes groundwater recharge, which is a simplification (some water is stored in the unsaturated zone). Enter annual values to estimate recharge as a depth of water and as a percentage of precipitation.
Water balance recharge formula
Recharge (in/yr) = Precipitation (in/yr) - ET (in/yr) - Runoff (in/yr)
Recharge (mm/yr) = Recharge (in/yr) * 25.4
Recharge (% of P) = Recharge / Precipitation * 100
[If result is negative, ET + Runoff exceeds Precipitation: return 0.00]
Negative values indicate that evapotranspiration plus runoff exceed precipitation, implying water storage depletion. This can occur seasonally in arid climates but is not physically sustainable over long periods. Annual averages should balance to zero or positive recharge.
US groundwater resources context
- The High Plains (Ogallala) Aquifer underlies 174,000 square miles and provides 30 percent of all US groundwater used for irrigation (USGS).
- The Ogallala Aquifer receives only 0.5 to 1 inch of recharge per year in many areas but is pumped 1 to 3 feet annually, creating a significant deficit.
- USGS estimates that about 12,000 km3 of fresh groundwater is withdrawn globally per year, with the US being one of the largest users.
- Managed aquifer recharge (MAR) projects in California, Arizona, and Texas artificially recharge aquifers using treated wastewater and stormwater to offset overdraft.
- USGS's National Groundwater Monitoring Network (NGWMN) provides water level data from thousands of wells across the US.
Frequently asked questions
What is groundwater recharge?
Groundwater recharge is the process by which precipitation or surface water infiltrates the soil and percolates down to replenish aquifers. It is the net inflow to the saturated zone of the subsurface. The simple water balance approach estimates recharge as precipitation minus evapotranspiration minus surface runoff.
How much of precipitation becomes groundwater recharge?
In humid regions, recharge may be 20 to 40 percent of annual precipitation. In arid regions it may be less than 1 percent. In highly urbanized areas with impervious cover, recharge can drop to near zero. USGS estimates US-average groundwater recharge at about 10 to 15 percent of precipitation.
Why is groundwater recharge important?
Groundwater recharge replenishes aquifers that supply about 30 percent of US streamflow and 40 percent of irrigated agricultural water (USGS). If pumping exceeds recharge, aquifer levels decline, causing wells to run dry and land to subside. Many major US aquifers are currently overdrafted.
What factors affect groundwater recharge?
Recharge depends on soil type (sand recharges faster than clay), land use (urban areas have less recharge), vegetation (deep-rooted plants intercept more water), topography (steep slopes produce more runoff), and precipitation intensity (intense storms produce more runoff than gentle rainfall).
Where can I find evapotranspiration data for my area?
USGS provides ET estimates through the National Evapotranspiration datasets. PRISM Climate Group provides gridded precipitation and temperature data for the US. State water resource agencies and the NRCS publish county-level water balance data. The FAO GAEZ database provides global ET estimates.
Official sources
- USGS: USGS Groundwater Resources.
- USGS: National Groundwater Monitoring Network.
- EPA: EPA Ground Water and Drinking Water.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.