Groundwater Flow Models of Upper Solomon Subbasin
In addition to developing the Northwest Kansas Model, in 2006 S.S. Papadopulos and Associates (SSPA) was contracted to begin constructing an Upper North Fork model and an Upper South Fork model to help us better understand these subbasins and to promote more informed, effective water resource management. The Solomon Groundwater Models Final Report was released on June 30, 2009. These models are extensions of the Northwest Kansas Model and are effective tools for analyzing the complex interactions between the alluvial and High-Plains aquifer systems.
The active model cells are shown in Figure 1. Grid cells were extended in the Solomon models to match the size and extent of the Republican River Compact Administration (RRCA) Groundwater Model grid cells to allow for relatively seamless integration between the models. The models were calibrated to simulate conditions from 1948 to 2005. They were then used to simulate scenarios 50 years into the future to predict the effects of irrigation pumping and streamflow depletion to the Solomon River (SSPA, 2009).
The Solomon Working Group submitted the following six baseline scenarios to run in both the Upper North and Upper South Fork models. All scenarios were run to simulate 50 years into the future.
- Continued status quo pumping (using 2005 pumping rates) held constant for 50 years
- Ogallala versus alluvial pumping — manipulate so one is on while the other is off and vice versa
- Turn off all pumping — stop all irrigation pumping during the 50-year period
- Lower evapotranspiration to see the outcome of eliminating or reducing phreatophytes (deep-rooted trees and plants that obtain water from the aquifer) to 50 percent of the original potential evapotranspiration for the 50-year period
- Turn off pumping in marginal soils
- Eliminate from the model any years with anomalously high precipitation
Figure 1: Upper Solomon Models and GMD 4 Priority Areas
The results from running the six scenarios were similar for both the Upper North and Upper South Fork models. For both subbasins, the status quo scenario predicted storage depletion in the western portions of the model domains and baseflow and streamflow decline in the eastern portions. In both subbasins, alluvial pumping was, on average, higher than Ogallala pumping within the model domain. It is important to note that Ogallala pumping that is outside of the model domain is not taken into account within the models or model results.
In the Upper North Fork, model simulations show that both alluvial and Ogallala pumping cause storage decline in the western portion of the model domain. However, in the Upper South Fork, the storage decline in the west part of the model domain is attributed to Ogallala pumping only. The model also showed that, due to the higher volume of alluvial pumping within the Upper North Fork model domain, curtailing alluvial pumping would be more effective in stabilizing the system than curtailing Ogallala pumping.
For both the Upper North and South Forks, the model scenarios showed storage decline, more pronounced in the west and diminishing to the east. This trend reflects groundwater development and the fact that the eastern parts of the subbasins are beyond the eastern extents of the Ogallala aquifer (Figure 1). Toward the east of the subbasin, evapotranspiration and groundwater pumping has led to decreased streamflow over time. These conditions are especially noticeable during drier years and times of drought.
Both the Upper North Fork and Upper South Fork models show that scenario 4, reducing evapotranspiration by 50 percent, had a significantly positive effect. There are 7,937 acres of phreatophytes within the Upper North Fork Subbasin and 4,574 acres within the Upper South Fork Subbasin. Phreatophytes in this region are from native and nonnative species. The effect of evapotranspiration is greatest on baseflows and does not have a significant effect on storage.
The scenario that eliminates anomalously high precipitation from the model had a significant negative effect in the Upper North Fork, dropping the recharge rate from 22,040 acre-feet to 18,454 acre-feet. This indicates the importance of high precipitation events to recharge; although it is noted that within a few years this effect dissipates (SSPA, 2009). Boundary flux conditions were found to be more sensitive to precipitation in the Upper South Fork than in the Upper North Fork. The scenario to curtail pumping in marginal soils did not have a significant effect, as there are relatively few acres in the model domain with such soils.
In addition to being beneficial to developing future management strategies, the Upper North Fork and Upper South Fork models will also be helpful to determine efficient uses of water within the subbasin. The models' water budgets allow for a fairly intuitive visualization of how water moves through the subbasin and where losses are occurring.
Visit the Solomon River Basin page for more information on the basin.
