1. Colorado Public Meeting
Republican River Compact Administration Groundwater Model and RRCA Accounting
Colorado Public Meeting
Michael Sullivan
CDWR
May 16, 2016

2. Compact Basics Virgin Water Supply (VWS) Allocation for Each State
Annual Accounting Required

3. Accounting
Compact allows each state an amount of depletion of the Virgin Water Supply (VWS)
Surface water use, tributary GW use, and Reservoir evaporation in pre-FSS accounting
SM found all GW impacts had to be accounted (i.e. Ogallala Aquifer Use)
Updated the accounting for all GW use
Now need to calculate each State’s Calculated Beneficial Consumptive Use (CBCU)

4. Accounting Formula Sub-basin VWS = Gage +All CBCU + ΔS – IWS
Main-stem VWS = Hardy Gage – Σ Sub-basin gages + All CBCU in the Main Stem + ΔS – IWS
CWS = VWS – ΔS – FF
Allocation for state in Sub-basin and Main Stem = CWS x %
States Allocation = Σ Allocation for each state
States CBCU = Σ State’s CBCU in each Sub-basin and Main Stem
These last two are inputs to Table 3 of the accounting which determines compact compliance

5. Example Arikaree River
CBCU Colorado = 0.6 x Dc + % x Pc x M&Ic + EvNFRc + GWc
CBCU Kansas = 0.6 x Dk + % x Pk x M&Ik + EvNFRk + GWk
CBCU Nebraska = 0.6 x Dn + % x Pn x M&In + EvNFRn + GWn
VWS = Arikaree Gage at Haigler Stn. No CBCUc +
CBCUk + CBCUn – IWS
CWS = VWS - FF
Allocation Colorado = x CWS
Allocation Kansas = x CWS
Allocation Nebraska = x CWS
Unallocated = x CWS

6. Accounting (continued)
Accounting includes:
SW Diversions
Pumped Diversions
Reservoir evaporation
Federal Reservoirs
Non-federal Reservoirs
GW CBCU
Irrigation use
M&I use
Imported Water (IWC)
Augmentation
Streamflows
Flood flows

7. General Need for a Model
Need to know how much GW use impacted the VWS but we can’t ‘see’ groundwater
Must calculate it’s flow and impact
Model simply uses physics to calculate groundwater flow
Geology gives us flow parameters
Topography gives us flow directions and intercepts
Gravity and withdrawals influence flow
Climate, cropping, consumption influence returns

8. Republican Requirements
Develop and calibrate a groundwater model of the Republican River Basin in order to calculate the impact of groundwater pumping and surface water imports on compact streams
Apply the groundwater model annually to evaluate pumping and surface water imports for annual compact accounting

9. Republican Application
Run model with all three states pumping on
Run model with Colorado off and NE/KS on
Run model with Kansas off and CO/NE on
Run model with Nebraska off and CO/KS on
Run model for Nebraska Mound Credit
Result
How much CBCUgw for each state

10. Origin of the Model USGS Lincoln Field Office
Work started 1997
Open File Report June 2002
Groundwater Technical Committee
Agreement to cooperate Spring 2002
Work started July 2002
Final Settlement Stipulation Dec 2002
Model submitted to Special Master June 30, 2003

11. Conceptual Model Single aquifer to Pierre Shale
Surface water basin extended to Platte River
Major man-made stresses recharge and wells
Well Pumping
Irrigation returns
Canal leakage
Major natural stresses precipitation, ET and streams
Precipitation recharge major inflow
Only baseflow simulated
Minor stresses springs and underflow

12. Model Calibration Head at 10835 wells Baseflow at 56 gages
Selected wells given greater weight
Trends in water levels important
Baseflow at 56 gages
Base flow estimated by pilot point method
Model stream flow is only base flow
Impact defined as change in base flow

13. Model Application
Groundwater Computed Beneficial Consumptive Use (CBCU) evaluated by switching off all wells and associated recharge within the model domain in a state
Imported Water Supply (IWS) evaluated by turning off all surface water returns and canal leakage in mound area
CBCU & IWS are the difference between “historical” and “impact” run stream flow and reservoir leakage.

14. Republican River Basin Topographic

15. Model Domain
Note that the model domain is based on the geology of the Ogallala Aquifer
It is not based on the topographic divide of the Republican River Basin
The Model Domain Includes Areas in CO, KS, and NE Exceed the Topographic Basin.

17. Summary
FSS requires GW CBCU be included in the accounting as use of a states allocation
A GW model is the method adopted by the FSS and US Supreme Court to calculate GW CBCU
GW CBCU is included in the RRCA accounting
Accounting is Performed Annually