ELM Ground Water Model

Interviewee: Steve Peterson (SP)
Interviewer: Rachael Seravalli, Information Asst., Nebraska WSC (RS)

RS: Welcome to the third episode of Nebraskast, where we talk to real USGS scientists about the important water resources work they are doing all over Nebraska. My name is Rachael Seravalli. I'm with the USGS Nebraska Water Science Center. And I'm here with Steve Peterson who is a hydrologist with the Nebraska Water Science Center. We're talking today about the ELM ground-water model project. Maybe we can start by talking a little bit about what a ground-water model is.

SP: A ground-water flow model is a numerical representation of the ground-water flow system. Basically we take our concepts and understanding of the ground-water hydrology and build them into a system of equations that can then simulate the ground-water flow system. We typically then check what the simulation does against ground-water levels that have been measured in the field and estimates of stream base flow to see that the simulation works correctly. And once you have a correctly working simulation you can use it in predictive mode to analyze future water-use scenarios for future conditions or past conditions

RS: So what does that mean in the context of this project? We're looking at the Loup and Elkhorn basins, correct?

SP: Yes, the Elkhorn-Loup ground-water flow model covers about 31,000 square miles of northcentral Nebraska. It's focused on the Elkhorn River system and the Loup River system upstream of Norfolk and Columbus Nebraska, and actually includes 80 percent of the Nebraska Sand Hills. So the focus of the simulation is the major streams coming out of the Sand Hills and the Elkhorn River system adjacent to the Sand Hills.

RS: And about how many years of data are you looking at when doing your simulations?

SP: It depends on the type of data we're talking about. With regard to climate data, we use over 100 years of record. When we look at water-level data, we use everything that's available thought typically there aren't many water levels that were collected before the 1930s. With regard to streamflow data, it's probably the same length of record. Most of the larger stations go back to about the 1930s. However for water-level and streamflow data the largest body of data that's available is from more recent times. Now, with regard to geology, that stuff is not time-specific so we use everything that has ever been collected, for example by Conservation and Survey Division test holes or more recent geophysical or other investigations that help give us data and knowledge at the subsurface.

RS: This is a cooperative effort. Who's involved in this study?

SP: There are eight natural resources districts involved in this study, as well as Nebraska Department of Natural Resources, USGS, and Conservation and Survey Division (UNL). The eight natural resources districts are all in the area of the Elkhorn and Loup River systems, including the Upper Elkhorn, Lower Elkhorn, Lower Loup, Upper Loup, Lower Niobrara, Middle Niobrara, Lewis and Clark, and Lower Platte North Natural Resources Districts. Within USGS, we've had many different people involved in this study. It is a team approach. We've had people from not only USGS in Lincoln but also other offices and other states working on various parts of the project, for example runoff and recharge, geophysics, geologic data exploration, even the stream baseflow measurements, which we did in November of 2006. It has been a regional effort and continues to be so, several people within the science center and without that have been participating and working on this.

RS: So why is it important for water resources managers, water regulators, to have this kind of information?

SP: TThis kind of simulation, even the phase one simulation, provides a lot of information that water resources managers can use to analyze the effects of decisions that they might make about managing the water resources. Ground-water flow models take into account basically the entire body of known data and try to integrate that into one representation of the ground-water flow system. That's what makes them the most powerful tools that are available.

RS: So what instances do you know of so far where this ground-water model has been used?

SP: It's my understanding that the Elkhorn-Loup ground-water flow model that we published and the one that has gone through our peer-review process or a modification of it was used by the Nebraska Department of Natural Resources as one of the things they considered when they made this year's fully appropriated report.

RS: Now is the study ongoing? What's the next step?

SP: We have a second phase of the study that has gone on, and as I mentioned we had a first phase where we published a report documenting the results of the phase one ground-water flow simulation back in October 2008. We have a second phase of the study that is ongoing and reports related to that second phase are coming out right now and have been coming out. For instance, we've generated a new base of aquifer surface that was published in a Scientific Investigations Map in October 2008. We generated a Data Series report prior to that related to a series of streamflow measurements we took in November 2006. We have a runoff recharge report and a geophysical report that are going to be forthcoming and currently we are working on the second phase ground-water flow model when then incorporates a lot of this new information we have been collection. In addition, it includes some advanced approaches to model calibration and analysis that we haven't used in Nebraska previously so we're very excited to, once again, advance the science to the highest level.

RS: Was there anything particularly challenging about trying to create this model in that area of the state?

SP: One of the things that is a challenge is the Sand Hills of Nebraska are underlain by the thickest part of the High Plains Aquifer that exists, where in places it's 1,000 feet of saturated thickness. Just because of the thickness of the aquifer, there isn't nearly enough information about the geology and hydrology of that system so that is an area where we will continue to conduct studies and do investigations to help provide additional data that we can use in ground-water flow models and for other general water management decisions.

RS: And just as a reminder this report is available when you go to our Web site, correct?

SP: The phase one report which was published in October 2008 and has gone through the whole peer-review process is available from our Web site from the ELM project page.

RS: That link is ne.water.usgs.gov. Thanks for taking the time to talk to us, Steve.

SP: Thank you very much.