Assessing the major drivers of water-level declines: new insights into the future of heavily stressed aquifers

Figures & data

Figure 1. Map of the High Plains aquifer in the US and Kansas (inset). Kansas inset also shows groundwater management district (GMD – colored lines) and climatic division (dashed) boundaries (modified from Butler et al. 2013).

Figure 2. (a) Mean annual water-level changes (solid lines) in the HPA in the GMD areas and SPI 9-month October (GMDs 4, 1, and 3) and 12-month December (GMDs 2&5) values (dashed lines) during 1996–2012. See Fig. 1 for the locations of the climatic divisions and GMDs. The y-axis ranges vary among plots to accentuate the relationship between fluctuations in water-level change (left y-axis) and those in the SPI (right y-axis). Water-level data are from wells for which measurements are available throughout 1996–2013 (188, 60, 222, and 233 wells for GMDs 4, 1, 3, and 2&5, respectively). A value for a particular year represents the water-level difference between that year and the following year for a given well; the mean annual change is an unweighted arithmetic average of the values for all the wells. A SPI value of zero indicates average (historic norm) conditions, values <0 and >0 indicate dry and wet conditions, respectively. (b) Correlation plots for data displayed in (a). See Table 1 for coefficients of determination and regression equations. The point within the circle in (b) is the outlier referred to in the text.

Figure 3. (a) Mean annual water-level changes (solid lines) and reported water use (dashed lines) for the HPA in the GMD areas during 1996–2012. See Fig. 1 for the locations of the climatic divisions and GMDs; the y-axis ranges vary among plots to accentuate the relationship between fluctuations in water-level change (left y-axis) and those in water use (right y-axis). The water-level data are the same as shown in Fig. 2(a). (b) Correlation plots for data displayed in (a). See Table 2 for coefficients of determination and regression equations. The point within the circle in (b) is the outlier referred to in the text.

Table 1. Optimum^a and utilized^b (bolded) correlations of mean annual water-level changes for GMDs with climatic indices for coinciding climatic divisions during 1996–2012.

Climatic index	GMD and Climatic Division	R^2
PDSI, August	GMD4–Div. 1	0.739
Palmer Z, mean Jun.–Sep.	GMD4–Div. 1	0.822
SPI, 9-month, December	GMD4–Div. 1	0.831
SPI, 9-month, October^c	GMD4–Div. 1	0.781
Water-level change (m) = 0.1409  × SPI – 0.182
PDSI, mean Aug.–Nov.	GMD1–Div. 4	0.463
Palmer Z, mean Jun.–Nov.	GMD1–Div. 4	0.656
SPI, 9-month, October^c	GMD1–Div. 4	0.709
Water-level change (m) = 0.1524  × SPI – 0.171
PDSI, mean Jun.–Dec.	GMD3–Div. 7	0.833
Palmer Z, mean Apr.–Nov.	GMD3–Div. 7	0.830
SPI, 9-month, November	GMD3–Div. 7	0.800
SPI, 9-month, October^c	GMD3–Div. 7	0.776
Water-level change (m) = 0.3166  × SPI – 0.584
PDSI, mean Jul.–Oct.	GMD 2&5–Div. 8	0.744
Palmer Z, mean Jan.–Dec.	GMD 2&5–Div. 8	0.794
SPI, 12-month, December^d	GMD 2&5–Div. 8	0.782
Water-level change (m) = 0.4528  × SPI – 0.283

^aOptimum correlation determined for each of the climatic indices (PDSI, Palmer Z, and SPI).

^bLinear regression equations are listed for the utilized correlations. All R² (coefficient of determination) values are significant at P = 0.001 except that for GMD1 water-level change vs PDSI division 4, which is significant at P = 0.01.

^cIndex based on 9-month period ending with October.

^dIndex based on 12-month period ending with December.

Table 2. Coefficients of determination (R²) and linear regression equations for correlation of mean annual water-level changes with reported water use during 1996–2012 for the GMD areas^a.

GMD	R^2	Equation
GMD4	0.732	Water-level change (m) = −1.60 × 10^-9 × water use (m^3) + 0.664
GMD1	0.068	Water-level change (m) = −1.01 × 10^-9 × water use (m^3) + 0.131
GMD3	0.318	Water-level change (m) = −0.706 × 10^-9 × water use (m^3) + 1.17
GMDs 2&5	0.765	Water-level change (m) = −2.44 × 10^-9 × water use (m^3) + 2.02

^aCorrelations for GMD4 and GMDs 2&5 are statistically significant at P = 0.001 and for GMD3 at P = 0.02; the correlation is not statistically significant for GMD1.

Figure 4. (a) SPI 9-month October (GMDs 4, 1, and 3) and 12-month December (GMDs 2&5) values (solid lines) and reported water use (dashed lines) for the HPA in the GMD areas during 1996–2012. See Fig. 1 for the locations of the climatic divisions and GMDs; the y-axis ranges vary among plots to accentuate the relationship between fluctuations in SPI (left y-axis) and those in water use (right y-axis). The SPI data are the same as shown in Fig. 2(a) and the water use data the same as in Fig 3(a). (b) Correlation plots and R² values for data displayed in (a).

Table 3. SPI values for given year and predicted water-level declines calculated using the regression equations in Table 1 for a drought of the same length and intensity of that of the 1950s.

Year	GMD4		GMD1		GMD3		GMDs 2&5
	SPI 9-month October	Predicted decline (m)	SPI 9-month October	Predicted decline (m)	SPI 9-month October	Predicted decline (m)	SPI 12-month December	Predicted decline (m)
1952	−1.62	−0.41	−1.70	−0.43	−1.92	−1.19	−1.46	−0.94
1953	−0.53	−0.26	−1.02	−0.33	−1.14	−0.94	−1.36	−0.90
1954	−1.47	−0.39	−1.46	−0.39	−1.26	−0.98	−2.16	−1.26
1955	−1.66	−0.42	−0.78	−0.29	−0.01	−0.59	−0.62	−0.56
1956	−2.49	−0.53	−2.87	−0.61	−2.44	−1.36	−2.47	−1.40
Total		−2.01		−2.05		−5.06		−5.07

Table 4. Steps for calculating pumping reduction to obtain stable water levels (water-level change of zero) for GMD4 and GMDs 2&5 during 1996–2012 using water-level and water-use regression equations in Table 2.

Percentage reduction in water use required for stable water levels under observed climatic conditions

GMD4: Water-level change (m) = 0 = −1.60 × 10^-9 × water use (m^3) + 0.664

Water use = −0.664/(−1.60 × 10^-9) = 4.14 × 10^8 m^3

Average annual reported water use for 1996–2012 = 5.28 × 10^8 m^3

Percentage reduction in water use = ((5.28 − 4.14)/5.28) × 100 = 21.7%

GMDs 2&5: Water-level change (m) = 0 = −2.44 × 10^-9 × water use (m^3) + 2.02

Water use = −2.02/(−2.44 × 10^-9) = 8.27 × 10^8 m^3

Average annual reported water use for 1996–2012 = 8.57 × 10^8 m^3

Percentage reduction in water use = ((8.57 − 8.27)/8.57) × 100 = 3.4%

Table 5. Steps for calculating average annual water-level decline for 21.7% reduction in pumping in GMD1 based on SPI change for this reduction in GMD4 for 1996–2012.

a. SPI change corresponding to 21.7% pumping reduction in GMD4

Insert water-level change of zero for reduced pumping into regression equation in Table 1

Water-level change (m) = 0 = 0.141 × SPI – 0.182

SPI = 0.182/0.141 = 1.29

Average SPI (9-month October) for 1996–2012 = −0.011

SPI change corresponding to pumping reduction = 1.29−(−0.011) = 1.30

b. Compute average annual water-level change for GMD1 for SPI change of 1.30

Use regression equation in Table 1 for GMD1

Water-level change (m) = 0.152 × SPI – 0.171

Average SPI (9-month, October) for climatic division 4 (GMD1) = 0.105

SPI for change of 1.303 = 0.105 + 1.303 = 1.408

Water-level change (m) = 0.152 × 1.41 − 0.171 = 0.044 m

Average annual water-level change observed for 1996–2012 = −0.155 m

Reduction in average annual water-level decline = 0.044 − (−0.155) = 0.199 m

Butler, J.J., Jr., et al., 2013. Interpretation of water-level changes in the High Plains aquifer in western Kansas. Groundwater, 51 (2), 180–190. doi:10.1111/j.1745-6584.2012.00988.x

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