Kansas Geological Survey, Open-file Report 2012-3
Gaisheng Liu, Brownie Wilson, Donald O. Whittemore, and James J. Butler, Jr.
KGS Open File Report 2012-3
Ground-water levels have been declining during the last few decades in the Ogallala-High Plains aquifer (HPA) in western Kansas, including within Southwest Kansas Groundwater Management District No. 3 (GMD3). The water-level declines have decreased ground-water discharge to the Arkansas and Cimarron rivers, thereby causing decreasing streamflow. As a part of planning and management activities, the Kansas Water Office (under a cooperative agreement with the U.S. Bureau of Reclamation) and GMD3 contracted with the Kansas Geological Survey (KGS) to develop a computer model of the HPA in the GMD3 area to further characterize the hydrologic system and water availability (Liu et al., 2010, KGS Open-file Report 2010-18).
Previous model results indicated that ground-water pumping has caused substantial decreases in aquifer storage (Liu et al., 2010). The storage decline rate started to increase in the 1950s, accelerated in the 1960s to mid-1970s, and then approximately leveled from the late 1970s to 2007, although it varied substantially each year depending on pumping. The accumulated decline in ground-water storage simulated for the entire model area for 1947-2007 is 66,409,000 acre-ft (AF), which comprises 29.3% of the simulated predevelopment (approximated as 1944-1946) storage. The storage decreases have been accompanied by a decrease in streamflow out of the model. Water-level declines in the HPA have resulted in the "capture" of ground water that otherwise would have discharged to streams; without this capture, the aquifer storage loss would have been approximately 12% greater than simulated. The total storage volumes simulated for the portion of the HPA within the GMD3 area for predevelopment and the end of 2007 are 193,454,000 and 133,622,000, respectively, giving a storage decline of 59,832,000 AF, which is 30.9% of the predevelopment value. The average water-level decline simulated for all the model cells within the GMD3 area is 69.89 ft between the predevelopment period and 2007.
In this work, the model developed by Liu et al. (2010) was applied to simulate aquifer responses to different possible future conditions and management scenarios. These conditions and scenarios were chosen by the KWO and GMD3. Specifically, the model was used to predict how the HPA aquifer within the GMD3 area will respond during 2008-2068 under nine different future scenarios: 1a) No change in water use policy with normal climate, 1b) No change in water use policy with drier climate, 2a) All pumping shut off in the model with normal climate, 2b) All pumping shut off in the model with drier climate, 3a) Applying the conceptual GMD3 allocation model--matching ground-water extraction to target volume of 40% of current storage used in 25 years, with the reallocation amount regressed against normal climate, 3b) Conceptual GMD3 allocation model--matching ground-water extraction to target volume of 40% of current storage used in 25 years, with the entire reallocation amount used up each year, 3c) Conceptual GMD3 allocation model applied every 10 years, with the dynamical reallocation amount used up each year, 4a) Conservation Reserve Enhancement Program (CREP) with current enrollment, and 4b) CREP with maximum enrollment.
In the first scenario, future ground-water pumping was estimated based on the present-day water rights (2008) and a repeat of 1947 to 2007 climate conditions (annual precipitation and Palmer Drought Severity Index (PDSI)). The regression equations established from the previous model calibration (Liu et al., 2010) were used to compute the ratio of water use/authorized quantity. Scenario 1b is similar to the first scenario, except that the climate is drier with precipitation decreased by 25% and PDSI decreased by 2 points with a lower limit of -6. In scenarios 3a and 3b, the GMD3 conceptual allocation model was applied to estimate future ground-water pumping. The allocation model, which is based on a 2-mile-radius circle, reallocates annual authorized quantities to match 40% of current aquifer storage while taking into account precipitation recharge and water right seniority. In scenario 3a, the quantity determined from the allocation model was assumed to be the maximum allowable water use, and future ground-water pumping was obtained by regression with the reallocated quantity treated as the authorized quantity in the water-right database. On the other hand, in scenario 3b the quantity determined from the allocation model was assumed to be the actual ground-water pumping for all future years. In scenario 3c, considering the continuous decline of water level and aquifer storage, the allocation model was applied every 10 years to provide a dynamical adjustment of reallocated pumping to match future storage projected by the ground-water model. Scenarios 4a and 4b were performed to assess the impacts of the CREP program on the HPA. The model settings for scenarios 4a and 4b are similar to those in the first scenario with continuous pumping under historic climate except that pumping is reduced in the CREP area. In scenario 4a, the current enrollment of water right retirement (as of October 2010) was taken into account, while in scenario 4b the total possible enrollment in the CREP program was simulated to assess its maximum potential impact.
In all pumping scenarios, the aquifer can become dry in certain areas due to continuous pumping, forcing the associated wells to operate at a reduced pumping rate or shut off completely. To dynamically adjust well pumping, future scenario models are divided into 61 one-year step models. At the beginning of each future year, transmissivity is calculated for every active model cell based on the simulated water level at the end of the previous year and the detailed lithology information from the KGS PST+ (practical saturated thickness plus) program. When transmissivity is less than 5,000 ft2/d, well pumping starts to reduce by a log function. When transmissivity is equal to or less than 1,000 ft2/d, the aquifer is assumed incapable of supporting any significant pumping and the wells in the model cell are therefore shut off completely.
Two types of boundary conditions were employed for future scenario simulations in the following manner: a) for specified-head boundaries, the average water-level change during the past 25 years was used to project future water levels on the head boundaries, until 10 ft of minimum saturated thickness was reached and the water levels would then remain 10 ft above bedrock throughout the rest of future years; b) for specified-flux boundaries, the average flux over the last 25 years was used for all future years. For ditch diversions along the Arkansas River, the average diversion rates between 1989 and 2007 were used for all future years. For the drier climate scenarios, in addition to the drop of precipitation by 25% and PDSI by 2 points, the stream input flows were reduced by 25% for both the Arkansas and Cimarron rivers.
Future scenario simulation results indicate that unless a significant reduction in water use occurs, ground-water pumping will continue to exceed natural aquifer recharge and produce a decline in water level and aquifer storage. An increasing number of wells will be forced to reduce pumping rate or completely shut off in the future as a result of reduced water availability from the aquifer. In the scenario for no change in water use policy with normal climate, aquifer storage will decrease by 55 million AF between 2008 and 2068, slightly less than the storage loss between predevelopment and 2007 (60 million AF). In 2068, due to the significant decrease in aquifer transmissivity, 42% of the projected pumping in GMD3 will not be met and 31% of the wells will be forced to reduce pumping by more than 75% of the projected amount. In 2068, only three counties have significant aquifer storage left (Stevens, Seward, and Meade). As expected, drier climatic conditions will further worsen the overall situation, producing more water-level decline and storage loss, and causing more wells to reduce pumping or shut off.
In the no-pumping scenario, the recovery of water level and aquifer storage is very slow because the precipitation recharge is small. Under normal climate, the storage gain from natural recharge processes between 2008 and 2068 is 13 million AF, only 22% of the storage loss between predevelopment and 2007. Therefore, if ground-water pumping was completely shut off in 2008, it would take more than two hundred years for the aquifer system to fully return to predevelopment conditions.
In the GMD3 management model scenarios, if the reallocated amount of water is treated as the authorized quantity, regressed future pumping is significantly lower than that in the scenario for no change in water use policy. This will significantly slow down the rate of water-level decline and aquifer storage loss. If the reallocated water is assumed to be fully consumed each future year and without the dynamical adjustment of reallocation in future years, the overall water-level decline and storage loss are slightly greater than the scenario for no change in water use policy. When the reallocation is dynamically adjusted every 10 years, the overall water-level decline and storage loss become slightly smaller than the scenario for no change in water use policy. At the county level, the storage decline is much more different between GMD3 management model scenarios and the scenario for no change in water use, because the allocation model produces change in how the pumping is distributed spatially. The remaining storage in 2068 for a particular county can be higher or lower than that in the scenario for no change in water use.
Simulations of water right retirement through CREP show that CREP will have the most significant impact on the local aquifer system in the vicinity of the project area. In 2018 and 2068, the aquifer storage increases from current CREP enrollment are 0.4% and 1.7%, respectively, of the GMD3 storage in the no change in water use policy scenario. In the Kearny County CREP area, the current CREP water right retirement reduces local water-level decline by 23 ft in 2068; in the Gray County CREP area, the reduction in water level decline is 9 ft. Additional reduction in the rate of water-level decline and storage loss will occur if the total possible enrollment in CREP is reached.
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Kansas Geological Survey, Geohydrology
Placed online May 3, 2012
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