Estimates of Ground-Water Recharge in Minnesota Dave Lorenz and Geoffrey Delin USGS Water Science Center of Minnesota Research supported by the USGS, Office of Ground Water and DNR Waters

Study Objectives Quantify recharge to unconfined sand and gravel aquifers in Minnesota using multiple methods representing different time and spatial scales. Compare and contrast the results.

Estimation Methods Used Multiple regression analysis relating recharge to precipitation, ET, and soils data (Regional Regression Recharge) Ground-water level fluctuation (water- table fluctuation) Unsaturated-zone water balance (zero- flux plane) Ground-water age dating

Regional Regression Recharge Method Recharge based on the Rorabaugh method that estimates average recharge in a drainage basin from streamflow records.

Rorabaugh MethodTheory

Rorabaugh MethodComputation

Stream Gaging Station Selection Criteria reviewed: length of record, common periods of record, missing data, size of watershed, (maximum of 3,000 mi 2 ), and existence of control structures (dams or diversions). 40 stations selected based on these criteria

Stream Gaging Stations Used in RORA Baseflow Recharge Analyses

Landscape Characteristics Several landscape characteristics were considered originally: Soil characteristics; Percent sand, percent clay, porosity, bulk density, permeability, and specific yield. Other landscape characteristics: percent various classes of geologic deposits in basin, basin slope, stream slope, and percent lake area in basin.

Landscape CharacteristicsFinal Decided to use specific yield (SY) as the landscape characteristic in the model: Direct measure of the capacity of the material to hold and release water under gravity. This is a linear property. That makes it possible to project back to the land surface. Highly correlated with other properties that affect rechargepermeability and hydraulic conductivity.

Specific Yield

Several methods to estimate SY were used. The method described in Rawls (1982) was used in the final regression equation. It uses percent sand, clay and organic matter. Data from STATSGO.

Shown is average precip Regression used decadal average going back through Precipitation inches

Shown is average ET Regression used decadal average of growing degree days. Evapotran- spiration (ET)

Regression Equation Decadal averages for recharge and precipitation were used reduces serial correlation between precipitation and recharge and smoothes out the variability in precipitation and recharge. Generalized least squares regression was used to account for the correlation between decadal data for each basin. Recharge = (SY) (P) (GDD*) GDD* is the minimum of GDD or 1350 degree days above 10 degrees celsius.

Average Recharge through soils in Minnesota

Water-Table Fluctuation (WTF) Method Data from 38 wells equipped with data loggers at five different sites Temporal variability in recharge

Water-Table Fluctuation Method ΔhΔh Recharge = SY Δh

Multiple WTF Methods Utilized Graphical method RISE program (Rutledge, 2003) Master Recession Curve

Correlation Between Graphical WTF Recharge and UZ Thickness 2003 data from 23 wells at 3 different sites Anomalously high recharge for UZ thicknesses > 3.5 m Bemidji

Effects of Measurement Interval on WTF Recharge Estimates 1993 datalogger data from MSEA well R2 near Princeton, MN Hourly / daily No change in estimated recharge going from hourly to daily measure Recharge estimates based on WTF method (RISE program) 0-54 % under- estimation of the recharge: from daily to weekly measurement Weekly (- 23%) % under- estimation of the recharge: from daily to monthly measurement Monthly (- 48%) Measurement interval, days Estimated recharge, cm/yr

Unsaturated-Zone Water Balance (zero-flux plane) Method Bemidji, Williams Lake, and Princeton MSEA sites Temporal variability in recharge

Unsaturated Zone Water Balance Method Lowland SitesUpland Sites Bemidji well 981 Bemidji well 9015 MSEA well R1 MSEA well R2 Bemidji well 9014 Williams Lake site Recharge, percent of precipitation

Ground-Water Age Dating Method Average recharge, spatial variability

Ground-Water Age Dating Method From Delin et al. (2000) Example from Princeton MSEA site using CFC data Recharge = vertical GW velocity x porosity x porosity SF 6 and 3 H- 3 He techniques can also be used; min. time resolution of ~1 year BP

Method Comparison

Comparison of Average Recharge Rate Computed at Each Site Shallow depth to water table results in WTF recharge rates being too large for Glacial Ridge, Des Moines River, and Williams Lake sites Pretty good agreement between regional estimates at most sites WTF Method Other site-specific Methods Regional Methods Similarity in recharge rates for some methods at some sites Of the WTF approaches, MRC estimates generally are the greatest; RISE program lowest Methods are scale dependent

Almost the end

Statewide Analysis WTF Methods 45 wells with weekly data available from DNR database Datalogger site (36 wells total) Bemidji Williams Lake Des Moines River MSEA Glacial Ridge

Graphical Method Manual method for estimating recharge. Developed in the late 1950s. Baseline recession that would have occurred in the absence of recharge projected to the time of peak in the hydrograph. The value of Δh determined manually.

Graphical Calculation for WTF Method From Delin (1990)

RISE Program Simple program that calculates the daily rise of water level in an observation well. The program makes no allowance for the baseline recession that would have occurred in the absence of recharge. The input data can be read right out of NWIS Web or can be created from data logger files. Rutledge (2003) electronic communication

RISE Calculation for WTF Method From Delin (1990)

Master Recession Curve Method First step is to define a Master Recession Curve from typical recessions for a well. This is accomplished by a nonlinear regression that estimates the recession rate and recession asymptote. Other methods for estimating a master recession curve have also been developed. Program calculates the daily recession of water level in an observation well and the rise from the difference between the theoretical recession and the actual water level.

MRC Calculation for WTF Method From Delin (1990)

Recharge Estimates - WTF Method Williams Lake examples: Precipitation and recharge in cm/yr UZ thickness: 5 m 9 m 2 m 16% 18% 11 % 13% 9% 10% 91% 120% 101%

Unsaturated Zone Water Balance From Delin and Herkelrath (in press) Time Zero-flux plane

Wells Sampled for SF 6 GW age dating. Also used CFCs for dating 22 wells sampled for SF6, including 2 nests SF 6 sample site (18 this study) Other GW age- dating site (6) Bemidji Williams Lake Des Moines River EXPLANATION MSEA Glacial Ridge Rock River Perham Prairie Island