Long-term trends in reservoir water quality and quantity in two major river basins of the southern Great Plains

Figures & data

Figure 1 Brazos and Colorado river basins showing locations of reservoirs and weather stations (+). Inset indicates location of the basins within the state of Texas.

Table 1 Water-quality and storage variables analyzed in reservoirs of the Brazos River basin for monotonic trends in their monthly values using Seasonal Kendall Tau test. Shown for each variable are the specific period of record (POR) for each analysis, tau, Sen slope (change per year), and minimum, median, and maximum values. Trends found to be significant after q-value correction (q < 0.05) are highlighted in bold, italic type. NA: either data were not available or available data did not meet minimum density criteria (see text); NC: Sen slope not calculated.

Variable	Statistics	Buffalo Springs Lake	Hubbard Creek Reservoir	Possum Kingdom Lake	Lake Granbury	Whitney Lake	Waco Lake
Chloride (mg/L)	Tau	0.127	0.133	0.049	0.026	0.064	−0.309
	Sen Slope	2.56	1.522	2.769	0.762	1.185	−0.147
	Min, Median, Max	143, 313, 539	41, 260, 1253	124, 766, 1643	81, 547, 1803	88, 347, 1079	8, 14, 29
	POR	1972–2009	1965–2009	1973–2010	1970–2010	1970–2009	1973–2009
Dissolved Oxygen	Tau	NA	−0.072	0.086	−0.065	0.099	−0.028
(mg/L)	Sen Slope	NC	−0.009	0.013	−0.008	0.011	−0.008
	Min, Median, Max	NA, NA, NA	4.7, 8, 12.3	5.1, 8.5, 12.6	4.2, 8.1, 12.9	4.9, 8.4, 15.8	4.4, 8.4, 12.2
	POR	NA	1965–2009	1973–2010	1970–2010	1970–2009	1973–2009
Fluoride (mg/L)	Tau	NA	NA	NA	NA	0.063	−0.375
	Sen Slope	NC	NC	NC	NC	0.001	>−0.001
	Min, Median, Max	NA, NA, NA	NA, NA, NA	NA, NA, NA	NA, NA, NA	0.23, 0.27, 0.5	0.19, 0.25, 0.27
	POR	NA	NA	NA	NA	2001–2009	2001–2009
Hardness (mg/L of	Tau	NA	0.099	−0.109	−0.064	−0.129	NA
CaCO3)	Sen Slope	NC	0.704	−3.222	−0.852	−1.507	NC
	Min, Median, Max	NA, NA, NA	175, 280, 344	279, 416, 598	202, 364, 494	210, 259, 331	NA, NA, NA
	POR	NA	1965–2009	1975–2010	1970–2010	1970–2004	NA
Nitrate+Nitrite	Tau	NA	0.162	0.24	0.3	NA	NA
(mg/L)	Sen Slope	NC	0.002	0.003	0.003	NC	NC
	Min, Median, Max	NA, NA, NA	0.05, 0.11, 0.28	0.02, 0.05, 0.73	0.02, 0.05, 0.41	NA, NA, NA	NA, NA, NA
	POR	NA	1965–1992	1975–1994	1970–1994	NA	NA
Nitrogen	Tau	NA	NA	NA	NA	-0.053	NA
(mg/L)-Dissolved	Sen Slope	NC	NC	NC	NC	−0.05	NC
	Min, Median, Max	NA, NA, NA	NA, NA, NA	NA, NA, NA	NA, NA, NA	0.34, 0.45, 0.77	NA, NA, NA
	POR	NA	NA	NA	NA	1999–2003	NA
Nitrogen	Tau	NA	NA	−0.136	NA	NA	NA
(mg/L)-Total	Sen Slope	NC	NC	−0.017	NC	NC	NC
	Min, Median, Max	NA, NA, NA	NA, NA, NA	0.5, 0.72, 1.59	NA, NA, NA	NA, NA, NA	NA, NA, NA
	POR	NA	NA	1980–1992	NA	NA	NA
pH	Tau	−0.169	0.164	0.023	0.058	0.193	0.056
	Sen Slope	−0.009	0.005	< 0.001	0.002	0.004	0.003
	Min, Median, Max	7.8, 8.4, 9.1	6.3, 8.1, 8.4	7.5, 8.3, 8.7	7.3, 8.2, 8.7	7.6, 8.2, 8.7	7.5, 8.1, 8.7
	POR	1972–2009	1965–2009	1973–2010	1970–2010	1970–2009	1973–2009
Phosphorus	Tau	NA	0.202	NA	−0.09	−0.778	NA
(mg/L)-Dissolved	Sen Slope	NC	<0.001	NC	>−0.001	−0.017	NC
	Min, Median, Max	NA, NA, NA	0.01, 0.04, 0.12	NA, NA, NA	0.01, 0.01, 0.04	0, 0.01, 0.06	NA, NA, NA
	POR	NA	1993–2009	NA	1993–1997	2001–2004	NA
Phosphorus	Tau	NA	0.337	0.188	0.326	0.231	0.202
(mg/L)-Total	Sen Slope	NC	0.001	0.001	0.001	0.001	0.001
	Min, Median, Max	NA, NA, NA	0.02, 0.02, 0.07	0.02, 0.05, 0.2	0.01, 0.06, 0.26	0.01, 0.07, 0.11	0.02, 0.08, 0.31
	POR	NA	1966–1992	1973–2010	1970–2010	1970–2009	1973–2009
Potassium	Tau	NA	0.157	−0.356	−0.093	NA	NA
(mg/L)-Dissolved	Sen Slope	NC	0.035	−0.004	−0.024	NC	NC
	Min, Median, Max	NA, NA, NA	5.2, 8.12, 54.5	2.29, 7.52, 16.18	4.93, 6.34, 7.84	NA, NA, NA	NA, NA, NA
	POR	NA	1966–2009	1975–2003	1974–1997	NA	NA
(Continued on next page)

Table NaN Water-quality and storage variables analyzed in reservoirs of the Brazos River basin for monotonic trends in their monthly values using Seasonal Kendall Tau test. Shown for each variable are the specific period of record (POR) for each analysis, tau, Sen slope (change per year), and minimum, median, and maximum values. Trends found to be significant after q-value correction (q < 0.05) are highlighted in bold, italic type. NA: either data were not available or available data did not meet minimum density criteria (see text); NC: Sen slope not calculated. (Continued)

Variable	Statistics	Buffalo Springs Lake	Hubbard Creek Reservoir	Possum Kingdom Lake	Lake Granbury	Whitney Lake	Waco Lake
Potassium	Tau	NA	NA	NA	NA	0.333	NA
(mg/L)-Total	Sen Slope	NC	NC	NC	NC	1.056	NC
	Min, Median, Max	NA, NA, NA	NA, NA, NA	NA, NA,NA	NA NA, NA	5.98, 7.84, 11.63	NA, NA, NA
	POR	NA	NA	NA	NA	2002–2009	NA
Specific	Tau	NA	0.22	0.031	−0.052	−0.164	−0.008
Conductance	Sen Slope	NC	9.451	3.494	−4.988	−11.125	−0.022
(μS/cm)	Min, Median, Max	NA, NA, NA	223, 1068, 3561	776, 3037, 4107	659, 2287, 4692	620, 1411, 3630	263, 362, 445
	POR	NA	1965–2009	1973–2010	1970–2010	1970–2006	1973–2008
Storage (millions of	Tau	NA	−0.184	−0.185	−0.378	−0.015	−0.054
m^3)	Sen Slope	NC	−1.949	−1.243	−0.659	−0.178	−0.08
	Min, Median, Max	NA, NA, NA	141, 289, 391	452, 626, 877	135, 179, 186	364, 667, 1752	110, 182, 440
	POR	NA	1970–2009	1970–2009	1972–2009	1970–2009	1970–2009
Sulfate (mg/L)	Tau	−0.214	0.348	−0.193	−0.175	−0.131	−0.252
	Sen Slope	−1.885	1.123	−2.936	−2.102	−0.962	−0.196
	Min, Median, Max	105, 224, 320	14, 53, 346	56, 339, 595	55, 232, 607	46, 151, 420	10, 24, 112
	POR	1972–2009	1965–2009	1973–2010	1970–2010	1970–2009	1973–2009
Temperature (C)	Tau	0.182	0.008	0.084	0.036	0.077	0.122
	Sen Slope	0.034	0.001	0.019	0.007	0.018	0.048
	Min, Median, Max	5.6, 24.4, 27.4	6.4, 19, 30	7.1, 21.5, 30.9	5.8, 22, 31.8	4.4, 22.4, 31.6	5.5, 20.5, 31.3
	POR	1972–2009	1965–2009	1973–2010	1970–2010	1970–2009	1973–2009
Coldest	Tau	0.27	0.192	0.057	−0.071	0.021	0.153
Temperature (C)	Sen Slope	0.084	0.07	0.014	−0.019	0.009	0.054
	Min, Median, Max	5.6, 6.4, 13.6	6, 11.1, 14.5	6.4, 11.1, 14.5	6.4, 9.6, 15.8	4.4, 9.2, 12.9	5.5, 10.5, 13
Warmest	Tau	0.088	0.13	0.164	0.32	0.291	0.14
Temperature (C)	Sen Slope	0.012	0.031	0.023	0.035	0.034	0.036
	Min, Median, Max	24.6, 26.5, 27.4	24.8, 28.2, 29.5	24.8, 28.2, 29.5	29.1, 30.6, 31.8	29.2, 30.2, 31.6	27.4, 29.6, 31.3

Table 2 Water quality and storage variables analyzed in reservoirs of the Colorado River basin for monotonic trends in their monthly values using Seasonal Kendall Tau test. Shown for each variable are the specific period of record (POR) for each analysis, tau, Sen slope (change per year), and minimum, median, and maximum values. Trends found to be significant after q-value correction (q < 0.05) are highlighted in bold, italic type. NA: either data were not available or available data did not meet minimum density criteria (see text); NC: Sen slope not calculated.

Variable	Statistics	Lake Colorado City	E. V. Spence Reservoir	Twin Buttes Reservoir	Lake Buchanan	Lake Travis
Chloride (mg/L)	Tau	0.508	0.139	0.11	−0.519	−0.439
	Sen Slope	24.109	6.148	0.857	−2.8	−1.242
	Min, Median, Max	265, 891, 1120	95, 1030, 13294	52, 113, 311	30, 62, 159	23, 43, 134
	POR	1969–2003	1968–2010	1973–2010	1973–2010	1973–2010
Dissolved Oxygen	Tau	0.258	−0.017	−0.229	−0.002	−0.088
(mg/L)	Sen Slope	0.05	−0.003	−0.029	>−0.001	−0.009
	Min, Median, Max	5.4, 9, 12.6	4, 8.9, 16	4.9, 8.4, 10.5	6.7, 8.8, 12.3	3.5, 8.2, 10.7
	POR	1972–2003	1968–2010	1973–2010	1973–2010	1973–2010
Fluoride (mg/L)	Tau	NA	NA	NA	NA	NA
	Sen Slope	NC	NC	NC	NC	NC
	Min, Median, Max	NA, NA, NA	NA, NA, NA	NA, NA, NA	NA, NA, NA	NA, NA, NA
	POR	NA	NA	NA	NA	NA
Hardness (mg/L of	Tau	NA	0.367	NA	NA	NA
CaCO3)	Sen Slope	NC	8.545	NC	NC	NC
	Min, Median, Max	NA, NA, NA	440, 767, 1260	NA, NA, NA	NA, NA, NA	NA, NA, NA
	POR	NA	1969–2010	NA	NA	NA
Nitrate+Nitrite (mg/L)	Tau	NA	NA	NA	−0.025	0.071
	Sen Slope	NC	NC	NC	>−0.001	<0.001
	Min, Median, Max	NA, NA, NA	NA, NA, NA	NA, NA, NA	0.01, 0.06, 0.62	0.01, 0.06, 0.49
	POR	NA	NA	NA	1982–2010	1982–2010
Nitrogen	Tau	NA	NA	NA	NA	NA
(mg/L)-Dissolved	Sen Slope	NC	NC	NC	NC	NC
	Min, Median, Max	NA, NA, NA	NA, NA, NA	NA, NA, NA	NA, NA, NA	NA, NA, NA
	POR	NA	NA	NA	NA	NA
Nitrogen (mg/L)-Total	Tau	NA	NA	NA	NA	NA
	Sen Slope	NC	NC	NC	NC	NC
	Min, Median, Max	NA, NA, NA	NA, NA, NA	NA, NA, NA	NA, NA, NA	NA, NA, NA
	POR	NA	NA	NA	NA	NA
pH	Tau	0.349	0.392	0.051	0.041	−0.053
	Sen Slope	0.013	0.013	<0.001	0.001	-0.001
	Min, Median, Max	7.4, 8.6, 9	7.8, 8.4, 8.7	7.5, 8.3, 8.8	7.7, 8.3, 8.7	7.6, 8.2, 8.6
	POR	1969–2003	1969–2010	1973–2010	1973–2010	1973–2010
Phosphorus (mg/L)-	Tau	NA	NA	NA	NA	NA
Dissolved	Sen Slope	NC	NC	NC	NC	NC
	Min, Median, Max	NA, NA, NA	NA, NA, NA	NA, NA, NA	NA, NA, NA	NA, NA, NA
	POR	NA	NA	NA	NA	NA
Phosphorus	Tau	NA	−0.126	NA	0.288	0.367
(mg/L)-Total	Sen Slope	NC	>−0.001	NC	0.001	0.001
	Min, Median, Max	NA, NA, NA	0.01, 0.06, 0.33	NA, NA, NA	0.01, 0.06, 0.52	0, 0.04, 0.45
	POR	1969–2003	1972–2010	NA	1973–2010	1973–2010
Potassium	Tau	NA	NA	NA	NA	NA
(mg/L)-Dissolved	Sen Slope	NC	NC	NC	NC	NC
	Min, Median, Max	NA, NA, NA	NA, NA, NA	NA, NA, NA	NA, NA, NA	NA, NA, NA
	POR	NA	NA	NA	NA	NA
Potassium (mg/L)-Total	Tau	NA	NA	NA	NA	NA
	Sen Slope	NC	NC	NC	NC	NC
	Min, Median, Max	NA, NA, NA	NA, NA, NA	NA, NA, NA	NA, NA, NA	NA, NA, NA
	POR	NA	NA	NA	NA	NA
(Continued on next page)

Table NaN Water quality and storage variables analyzed in reservoirs of the Colorado River basin for monotonic trends in their monthly values using Seasonal Kendall Tau test. Shown for each variable are the specific period of record (POR) for each analysis, tau, Sen slope (change per year), and minimum, median, and maximum values. Trends found to be significant after q-value correction (q < 0.05) are highlighted in bold, italic type. NA: either data were not available or available data did not meet minimum density criteria (see text); NC: Sen slope not calculated. (Continued)

Variable	Statistics	Lake Colorado City	E. V. Spence Reservoir	Twin Buttes Reservoir	Lake Buchanan	Lake Travis
Specific Conductance	Tau	0.566	−0.08	0.058	−0.507	−0.445
(μS/cm)	Sen Slope	117.167	−14.929	0.5	−12.742	−5.066
	Min, Median, Max	492, 3120, 11400	1707, 3031, 5337	435, 742, 1520	377, 550, 964	357, 471, 912
	POR	1969–2003	1968–2010	1973–2010	1973–2010	1973–2010
Storage (millions of m^3)	Tau	0.215	−0.222	−0.358	NA	−0.126
	Sen Slope	0.163	−3.624	−2.988	NC	−2.127
	Min, Median, Max	15, 25, 40	1, 167, 436	0, 95, 233	NA, NA, NA	518, 1702, 1284
	POR	1970–2009	1970–2009	1970–2009	NA	1970–2009
Sulfate (mg/L)	Tau	0.404	0.231	−0.021	−0.505	−0.449
	Sen Slope	29.25	8.056	−0.094	−1.923	−0.813
	Min, Median, Max	47, 460, 1710	116, 711, 2677	10, 51, 198	5, 38, 106	14, 25, 103
	POR	1969–2003	1968–2010	1973–2010	1973–2010	1973–2010
Temperature (C)	Tau	−0.286	0.037	−0.014	0.146	0.167
	Sen Slope	−0.086	0.008	>−0.001	0.033	0.037
	Min, Median, Max	8, 21.9, 30.8	3.4, 17.6, 29.5	9.2, 20.8, 29.7	8.7, 21.1, 30.2	11.5, 23.9, 31.5
	POR	1969–2003	1968–2010	1973–2010	1973–2010	1973–2010
Coldest Temperature (C)	Tau	−0.294	0.061	0.265	0.133	0.264
	Sen Slope	−0.096	0.018	0.111	0.031	0.046
	Min, Median, Max	8, 9.2, 10.6	3.4, 7.6, 10.8	9.2, 11.5, 13.4	8.7, 10.9, 12.1	11.5, 13.2, 16.1
Warmest Temperature	Tau	−0.259	0.01	−0.154	0.248	0.216
(C)	Sen Slope	−0.085	<0.001	−0.03	0.034	0.029
	Min, Median, Max	28.2, 29.7, 30.8	26.8, 27.8, 29.5	24.9, 26.4, 29.7	27, 29, 30.2	29, 30.1, 31.5

Figure 2 Scatterplots of total (ground + surface) water withdrawals by year for the Brazos River (open circles) and Colorado River (solid circles) basins. The association was significant at P < 0.05 for the Colorado and marginally significant at P < 0.1 for the Brazos (discussed in text). A regression line is superimposed on the scatterplot for the Colorado.

Table 3 Results of quantile regression for detection of trends in annual reservoir inflow in the Brazos and Colorado river basins. Flow rate data were obtained from USGS gages on major streams upstream of reservoirs for the period of record (POR), which varied by gage. Distributions of daily average flow rates were constructed for each year of the POR, and annual changes (10⁻⁶ m³/s per year) in the 10th, 50th, and 90th quantiles were estimated (Estimated annual change). The 10th, 50th, and 90th quantiles (10⁻⁶ m³/s) were also estimated for the entire POR (Estimated for POR), and the estimated annual changes were also expressed as a percentage of this value (Percent annual change). Trends found to be significant after q-value correction (q < 0.05) are highlighted in bold type.

					10th quantile				50th quantile				90th quantile
Basin and reservoir	Inflowing stream	Drainage area (ha)	Period of Record	N	Estimated for POR	Estimated annual change (SE)	Percent annual change	Q-value	Estimated for POR	Estimated annual change (SE)	Percent annual change	Q-value	Estimated for POR	Estimated annual change (SE)	Percent annual change	Q-value
Brazos River
Hubbard Creek Res.	Hubbard Cr.	158767	1970-2010	14975	0	0	0	.	666	−26 (3)	−3.83	<0.001	28069	−235 (105)	−0.84	<0.001
	Big Sandy Cr.	72520	1970–2010	14975	0	0	0	.	40	0 (0)	0	.	8020	28 (45)	0.35	0.046
Possum Kingdom L.	Brazos Riv.	3394713	1970–2010	14975	10425	82  (28)	0.79	<0.001	105862	−473 (121)	−0.45	< 0.001	922118	−3280 (2230)	−0.36	0.01
Lake Granbury	Brazos Riv.	4058789	1970–2010	14975	41688	731  (26)	1.75	<0.001	198074	−609 (269)	−0.31	< 0.001	1499575	−6560 (2710)	−0.44	<0.001
Whitney L.	Brazos Riv.	4209268	1970–2010	14975	19247	−113  (34)	−0.59	<0.001	136335	−3780 (311)	−2.77	< 0.001	1732087	−529 (3520)	−0.03	0.07
	Paluxy Riv.	106190	1970–2010	14975	882	5.7  (5.7)	0.64	0.03	13634	170 (14)	1.25	<0.001	108270	2340 (244)	2.16	<0.001
Waco Lake	No. Bosque Riv.	296814	1970–2005	13149	5052	79  (14)	1.57	<0.001	35287	872 (51)	2.47	<0.001	410649	13600 (943)	3.31	<0.001
	Mid. Bosque Riv.	47138	1970–1985	5844	24	−8.5  (0)	35.3	<0.001	7938	−971 (93)	−12.2	<0.001	97848	−3940 (762)	−4.03	<0.001
	Hog Creek	20254	1970–1985	5844	64	−14  (0)	−22.8	<0.001	2486	−317 (23)	−12.8	<0.001	35287	−1900 (255)	−5.38	<0.001
Colorado River
E. V. Spence Res.	Colorado Riv.	1204350	1991–2010	7305	56	−14  (3)	−25.1	<0.001	2727	−266 (11)	−9.76	<0.001	37695	−2890 (311)	−7.67	<0.001
Twin Buttes Res.	Mid. Concho Riv.	289044	1970–1995	9496	0	0  (0)	0	.	2486	−11 (14)	−0.45	0.04	18445	801 (94)	4.34	<0.001
Lake Buchanan	Colorado Riv.	5133121	1970–2010	14975	39309	−728  (77)	−1.9	<0.001	141943	−3060 (94)	−2.16	<0.001	890116	−12700 (1760)	−1.43	<0.001
Lake Travis	Pedernales Riv.	233358	1970–2010	14975	339847	−10620  (1312)	−3.1	<0.001	2039082	1089 (1927)	0.049	0.049	10082130	28320 (15972)	0.28	0.01

Table 4 Results of quantile regression for detection of trends in annual precipitation in the Brazos and Colorado river basins. Monthly precipitation totals were calculated from daily precipitation data obtained for the period of record (POR, 1965–2009) from the National Climatic Data Center for selected weather stations. Monthly totals were grouped by year, and annual changes (monthly sum in mm/yr) in the 10th, 50th, and 90th quantiles were estimated (Estimated annual change). The 10th, 50th, and 90th quantiles (monthly sum in mm) were also estimated for the entire period of record (Estimated for POR), and the estimated annual changes were also expressed as a percentage of this value (Percent annual change). Trends found to be significant after q-value correction (q < 0.05) are highlighted in bold type.

		10th quantile				50th quantile				90th quantile
Basin and reservoir	N	Estimated for POR	Estimated annual change (SE)	Percent annual change	Q-value	Estimated for POR	Estimated annual change (SE)	Percent annual change	Q-value	Estimated for POR	Estimated annual change (SE)	Percent annual change	Q-value
Brazos River
Buffalo Springs Lake	1045	1.5	0.006  (0.025)	0.39	1.00	26.2	0.046  (0.106)	0.18	1.00	104.9	−0.164  (0.348)	−0.16	1.00
Hubbard Creek Reservoir	1480	6.4	−0.009  (0.057)	−0.13	1.00	45.5	0.127  (0.106)	0.28	0.96	129.3	0.150  (0.233)	0.12	1.00
Possum Kingdom Reservoir	2048	4.6	−0.082  (0.049)	−1.78	0.74	41.4	0.091  (0.094)	0.22	0.99	128.3	0.191  (0.274)	0.15	1.00
Lake Granbury	1000	10.9	0.031  (0.077)	0.28	1.00	54.9	0.026  (0.128)	0.05	1.00	148.1	−0.272  (0.354)	−0.18	1.00
Lake Whitney	1070	14.0	0.183  (0.093)	1.31	0.74	60.2	0.159  (0.149)	0.26	0.96	154.4	0.300  (0.612)	0.19	1.00
Lake Waco	2120	12.2	0.075  (0.045)	0.62	0.74	61.0	0.127  (0.116)	0.21	0.96	151.6	0.530  (0.374)	0.35	0.96
Colorado River
Lake Colorado City	1333	1.3	−0.035  (0.033)	−2.78	0.96	28.4	0.029  (0.089)	0.10	1.00	110.5	−0.175  (0.244)	−0.16	1.00
Lake E.V. Spence	1868	1.0	−0.011  (0.027)	−1.04	1.00	31.2	−0.104  (0.093)	−0.33	0.96	105.9	−0.029  (0.187)	−0.03	1.00
Twin Buttes Reservoir	2067	1.3	−0.013  (0.018)	−1.00	1.00	29.2	−0.046  (0.095)	−0.16	1.00	101.3	0.102  (0.189)	0.10	1.00
Lake Buchanan	1868	6.4	0.085  (0.045)	1.33	0.74	45.2	−0.009  (0.121)	−0.02	1.00	129.5	−0.009  (0.247)	−0.007	1.00
Lake Travis	2097	9.1	0  (0.049)	0	1.00	52.8	0.009  (0.123)	0.02	1.00	144.3	−0.017  (0.317)	−0.01	1.00

Table 5 Principal component analysis of monthly water quality data. Loading factors are shown for each variable in the first three principle components (PC) retained by the analysis. Variables with loadings ≥ |0.4| are highlighted in bold type. Eigenvalues, and proportional and cumulative variance are shown for each PC.

	Loading factors
Variables included in analysis	PC1	PC2	PC3
Chloride	0.564	−0.105	0.064
Dissolved oxygen	0.122	0.698	−0.017
pH	0.025	0.179	0.727
Total phosphorus	0.086	−0.090	−0.634
Specific conductance	0.570	−0.096	−0.010
Sulfate	0.564	−0.093	0.069
Temperature	0.126	−0.667	0.244
Eigenvalues	1.688	1.326	1.039
Proportion of total variance	0.407	0.251	0.154
Cumulative variance	0.407	0.658	0.812

Figure 3 Biplots of principal components 1 and 2 from PCA of monthly water quality data for selected reservoirs in the Brazos River and Colorado River, Texas. The maximum period of record was 1965–2010. Data are grouped according to 3 different association criteria: (A) basin, (B) individual reservoir, and (C) season (winter, Dec–Feb; spring, Mar–May; summer, Jun–Aug; fall, Sep–Nov). To facilitate visual identification of patterns, 95% confidence ellipses (SE) were constructed around the respective groups. Vectors shown in inset represent variables with factor loadings >|0.4|. DO = dissolved oxygen; SAL = salinity-related variables (chloride, specific conductance, and sulfate); TEMP = temperature.

Figure 4 Biplots of principal components 1 and 3 from PCA of monthly water quality data for selected reservoirs in the Brazos River and Colorado River, Texas. The maximum period of record was 1965–2010. Data are grouped according to 2 different association criteria: (A) season (winter, Dec–Feb; spring, Mar–May; summer, Jun–Aug; fall, Sep–Nov), and (B) individual reservoir. To facilitate visual identification of patterns, 95% confidence ellipses (SE) were constructed around the respective groups. Vectors shown in inset represent variables with factor loadings >|0.4|. SAL = salinity-related variables (chloride, specific conductance, and sulfate); TP = total phosphorus; pH, standard units.

Table

Year	Month	Time	Value
1973	9	1973.75	26.7
1973	10	1973.833	21.95
1973	11	1973.917	17.5
1973	12	1974	13.6
1974	1	1974.083	11.542
1974	2	1974.167	NA
continued