A) Westenbroek et al. (2009) describe a soil-water budget model (SWB) designed for generating recharge values for input to groundwater flow models. Download a copy of the USGS report Techniques and Methods 6-A31 and read it. Describe the assumptions that are inherent in the model and assess their importance and limitations.

B) Download the file set for SWB swbDownload that contains the executable version of the code and the input files required to run the example problem for the Black Earth Creek watershed. (Directory structure is described on p. 9 of the documentation). The example calculates recharge rates on a daily basis for a 340 by 390 grid (each grid point represents a 100m by 100m area). We will not examine individual grid estimates, instead we will look at summary data over the entire modeled area for a single year, 1992. The control file name is recharge.ctl; open that file in a text editor and place comment signs (#) in the first column of all SOLVE statements except the SOLVE statement for 1992. Then run the model by opening a script window in the new folder and typing:

swb recharge.ctl

Daily ouput values statistics are contained in the file: recharge_daily_statistics.csv. The statistics include mean, minimum, and maximum values for all water-budget components in units of inches.

Generate cumulative plots for the entire year for mean values of total precipitation, recharge, actual evapotranspiration, total change in storage (the sum of change in snow cover and change in soil moisture), and total surface runoff (as defined in this model, total surface runoff = outflow - inflow + interception +rejected recharge + runoff outside).

Discuss seasonal patterns for all of the water budget components.

Rerun the model for 1992 by using the alternative options for estimating ET. How much change is there in estimated recharge rates?

Problem 3.2

The computer program VS2DI is a software package for simulating water solute, and heat transport within variably saturated porous media. Water movement is assumed to be governed by the Richards equation. VS2DI was used to simulate water movement from land surface to the water table for a 1-dimensional soil column with properties of a generic silt loam for 1992 with precipitation and potential evapotranspiration data for Black Earth Creek. The water table was assumed to be at a constant depth of 5 m. Results in terms of cumulative values of water budget components are given in the file BlackEarthCreek.xls for 3 consecutive years. Each year used the same precipitation and PET data. Plot the results for each year. Why are there differences among the different years? These simulations predicted no surface runoff because all of the precipitation could be abosorbed by the soil. Compare these results to those obtained with the SWB model in Problem 3.1.

Problem 3.3

Download the VS2DI software and Examples and Tutorials at: http://wwwbrr.cr.usgs.gov/projects/GW_Unsat/vs2di1.2 The graphical user package is described in USGS WRI Report 99-4130 . Those who are not familiar with models of water movement through the unsaturated zone should read through USGS WRI Report 83-4099 .

This problem will be solved with VS2DTI. Before running this problem, the user should first work through the VS2DTI tutorial problem to gain familiarity with the program. Click the File menu, select Open, and navigate to the directory where the tuturial is found. Once you are familiar with VS2DTI, you can input the current problem. The simulation file to be run for this problem is problem3.3.vs2 . As with problems 3.1 and 3.2, the simulated domain consists of a vertical column of soil 5 m in depth. The top boundary is land surface and the bottom boundary is the water table. The top boundary is a flux boundary and uses the 1992 daily precipitation data for the Black Earth Creek site. Evapotranspiration is simulated by an empirical method that uses several parameters, including potential evapotranspiration (PET). Instead of using daily estimates of PET (as was done for the simulation run in Problem 3.2), monthly averages (calculated from the daily values determined in problem 3.1) are used as input. The simulation is set up to run for 3 consecutive years. We will want to look at results for only the final year.

The file balance.out will contain the results of interest:

TOTAL FLOW IN (TOTAL) - Infiltration from precipitation

TOTAL FLOW OUT (TOTAL) - Recharge to the water table

TRANSPIRATION (TOTAL) - Total water lost from the column to evapotranspiration.

Generate cumulative plots for the year for these three variables. Discuss differences among these results and those of problem 3.1. Do results differ depending on whether daily PET (Problem 3.2) or Monthly PET (this problem) estimates are used?

Review the empirical approach used to determine evapotranspiration in USGS WRI Report 83-4099 . Conduct a sensitivity analysis by systematically altering values of parameters that are used to estimate ET. How sensitive are estimates of actual ET and recharge to changes in parameter values?

(NOTE: This problem is designed for advanced students. Learning to use VS2DTI requires a substantial time commitment by the student. The model is a useful tool for investigating recharge processes and will be used in problem sets for other chapters.)

Problem 3.4

Flint and Flint (2007) report on use of the Basin Characteristic Model (BCM) to estimate in-place recharge and runoff potential in the Basin and Range carobomate-rock aquifer system in Nevada and Utah. Down load a copy of the USGS report SIR 2007-5099 and read it. Describe the authors' conceptual model of recharge processes in the region. How does the BCM calculate recharge? What is meant by "in-place" recharge? What are some of the important assumptions inherent in application of the BCM to this region? What data are required to run the BCM? Describe the datasets that the authors prepared for this application. Discuss advantages and limitations of this application.

References

Flint, A.L., and Flint, L.E., 2007, Application of the Basin Characteristic Model to estimate in-place recharge and runoff potential in the Basin and Range Carbonate-Rock Aquifer System, White Pine County, Nevada, and adjacent areas in Nevada and Utah, U.S. Geological Survey Scientific Investigations Report 2007-5099.

Westenbroek, S.M., Kelson, V.A., Dripps, W.R., Hunt, R.J., and Bradbury, K.R., 2010, SWB - A modified Thronthwaite-Mather soil-water-balance code for estimating groundwater recharge, U.S. Geological Survey Techniques and Methods 6-A31.

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