The Cooperative Hydrology Study (COHYST) is a cooperative effort to improve the understanding of the hydrological and geological conditions in the Platte Basin. COHYST came about as a result of concerns by various Nebraska interests about what was being proposed by the three-state Platte River Cooperative Agreement and the quality of the information and data that was available to make the decisions that the U.S. Fish & Wildlife Service and the Cooperative Agreement Governance Committee were proposing to make.

The Platte River Cooperative Agreement was entered into by Nebraska, Colorado, Wyoming, and the U.S. Department of Interior (represented by U.S. Fish & Wildlife Service and U.S. Bureau of Reclamation) in July of 1997. The purpose was to develop and implement a Recovery Implementation Program on the Central Platte River in order to protect the whooping cranes, the least terns, and the piping plovers, each a threatened or endangered species that have been known to utilize the Platte River. It was proposed in order to prevent the long delays and major costs that were being experienced by the many projects up and down the river.

As project sponsors and owners attempted to get licenses, permits, approvals, or financial assistance from the federal government, they ran into reviews by the U.S. Fish & Wildlife Service because of the Endangered Species Act (ESA). A prime example of the delays and cost is the 15 years and over $30 million spent by Nebraska Public Power District and Central Nebraska Public Power & Irrigation District in trying to get their hydro-electric operations re-licensed through the Federal Energy Regulatory Commission (FERC). Other Nebraska projects as well as projects in Colorado and Wyoming were facing similar delays, costs, and loss of water. (For an example of the potential magnitude of these ESA-related impacts, we can examine recent impacts to the Klamath River Project in Oregon and California lost all their water for one year).

The program proposed by the Platte River Cooperative Agreement consists of three major components:
1)   Land for wildlife habitat on the Platte River
2)   Increased flows in the river during times when U.S. Fish & Wildlife Service feels flows are too low for the benefit of those         threatened and endangered species and
3)   No new depletions to Platte River flows or flows to tributary streams.

This last item, “no new depletions,” was one that caught the attention of many Nebraskans because the Fish & Wildlife Service were proposing not only “no new depletions” caused by diversions from the Platte or it’s tributaries, but also “no new depletions” from groundwater pumping! What they proposed was to restrict or regulate the installation of new wells that, when pumped over time, would deplete the flows of the Platte River or any tributary flowing into the Central Platte.

Many Nebraskans were concerned with how the Cooperative Agreement participants (and ultimately the federal and state administrations) would likely decide what wells would “deplete” the river and how much or how little “depletion” would cause a new well to be subject to regulation. The proposed program would require any new well drilled after July 1, 1997 to “replace” any depletion to the U.S. Fish & Wildlife Service’s desired flows. These “offsets” would have to be made up by acquiring, by purchase or otherwise, an amount of water equal to the “depletion” amount and provide it back to the river at the appropriate time. With that kind of requirement, the decision on what wells would deplete flows and how much a well might deplete the U.S. Fish & Wildlife Service’s flows is a decision that would dictate whether or not a farmer could put in a new irrigation well to develop more land or whether he would be prohibited because of cost for “offset” water, as well as dictate the additional expense for municipal wells and industrial wells and whether or not a new industry would be discouraged from coming to the area. The only “tool” available to the U.S. Fish & Wildlife Service and the Platte River Cooperative Agreement’s Governance Committee to make those decisions and determine the level of impact from various wells was a simplistic formula developed by U.S. Geological Survey (USGS) called SDF which stands for Streamflow Depletion Factor.

SDF only considers three factors:
1)   Distance a well is from a river or stream
2)   Average storage capacity of the aquifer and
3)   The average transmissivity (how fast water can move or pass through) of the aquifer.

SDF can be used to roughly estimate the magnitude of impact a pumping well will have on a river or stream in a given amount of time (example: A well located x miles from the river and pumped for 15,000 days (41 years) will have depleted the flow of the river by an amount equal to 28% of the total amount that well pumped during the 15,000 days, or in simpler words, 28% of the volume of water that well pumps in 15,000 days will show up as a depletion to the river in that 15,000 days).

While SDF is used extensively in Colorado and has been their basis for groundwater regulation, many Nebraskans were concerned that it was too simplistic and not accurate enough if it was going to be the basis for regulating new wells. It was also considered by many to be a method that would over-estimate the impact, and thereby over regulate wells and require offset water from new wells that shouldn’t be regulated and require more offset water from regulated wells than was necessary.

Because of those general and widespread concerns a group of Nebraska interests joined together in early 1998 for the purpose of developing hydrologic and geologic databases that could later be used for groundwater modeling that would be more accurate than the SDF method and less likely to “overstate” impacts. The effort, called COHYST, was started to ensure that the decisions regarding the Platte River Cooperative Agreement were based on supportable science and data.

To assure that the proper databases were gathered and constructed in a sound and scientific manner, the COHYST sponsors sought the assistance of independent outside consultants to advise them on managing the project and the best technical approach to use.

Request for proposals were sent to a dozen or so firms and two firms, Parsons Engineering & Science and Hemmenway Groundwater Engineers were hired in 1998. U.S. Geological Survey (USGS) was hired in 1999 to provide additional technical advice to assist with the construction of the models.

* Parsons is a worldwide engineering and science consulting firm with one of their offices in Denver, Colorado.

* Hemmenway Groundwater Engineers is a Colorado firm specializing in groundwater investigations and having extensive experience in the mid-west.

The proposed Platte River Program was originally scheduled to be ready to implement in 2000 but as it drug on it became evident that there may be sufficient time to not only develop good databases but also to develop the more complex groundwater model prior to implementation of the Platte River Cooperative Agreement Program, and so in mid-1999 the participants interviewed and later hired three individuals to actually start building the groundwater models.

Where SDF only considers distance, storage and transmissivity, the COHYST model is based on the much more complex “mod-flow”groundwater model that was also developed by USGS, but was developed to provide a much more detailed analytical tool than the more simplistic SDF method.

All of these and a host of other data like rainfall, water consumption of difference crops, runoff, and growing season; are all data used to “describe” each node. While each “node” has it’s own independent set of data and characteristics, the model ties the nodes together so they can interact with all the nodes adjacent to them. So, if you pump out a lot of water from one node, it can effect not only that node, but also the node(s) next to it.

The end goal of a properly calibrated groundwater model is to be able to simulate or predict groundwater elevations and groundwater interactions with rivers, streams and drains under a variety of conditions and over time. One of the primary ways of determining whether a model is properly calibrated is to see if it can replicate measured field conditions as they change over time.

By starting the model with conditions that existed in 1940 and running the model, adding the changes that occurred between 1940 and 1970, and 1980 and 2000, we determine if the model can replicate 1970, 1980 and 2000 groundwater conditions. Making the necessary adjustments so the model can replicate those historic conditions is a major part of the calibration process. That “calibration” stage is where COHYST is currently.

The final stage of calibration is projected for October-December 2004. Once properly calibrated, whenever it might be, the model will be a valuable tool to assist Central Platte NRD Board of Directors as well as six other NRDs and the State of Nebraska in analyzing impacts on groundwater levels and impacts on stream flows of any proposed “changes” or proposed “regulations” to the surface and groundwater systems.

A far better tool than one that only looks at “depletion to a river” and only considers distance from a river and an average storage and transmissivity of an aquifer.

Partners to the Sponsors include:
*Three cities-Grand Island, North Platte, and Scottsbluff
*Three statewide farm & irrigation organizations-Nebraska Farm Bureau, Nebraska Water Resources Association, Nebraska Water Users, Inc. *Two environmental organizations-Nebraska Audubon Society and Platte River Whooping Crane Trust

Assisting and Participating in the efforts are:
*US Geological Survey
*University of Nebraska
*U.S.D.A.- Natural Resources Conservation Service

* Parsons Engineering & Science - A worldwide Engineering & Science Consulting Firm. Hired in 1998 by COHYST.
* Hemmenway Groundwater Engineers - A Colorado consulting firm specializing in groundwater. Hired in 1998 by COHYST.
* U.S. Geological Survey - Hired to provide additional technical advise on modeling. Hired in 1999.

Total cost to Nebraska will be approximately $7,000,000.00 with the Nebraska Environmental Trust providing $2,895,000.00 or 41% through a grant. While the cost is a lot of money, it’s a lot less than the cost of losing industrial development or the prevention of developing new land because of an overstated impact.

Central Platte NRD’s share will be $504,000.00 or approximately 7%, with $32,500.00 supplied in cash. The other $471,500.00 supplied in the form of in-kind-services (mostly staff time). This works out to be an average of $84,000.00/year ($5,500.00 cash and $78,500.00 in-kind services).