SECTION 2 HYDROLOGY AND FLOW REGIMES

Transcription

1 SECTION 2 HYDROLOGY AND FLOW REGIMES In this section historical streamflow data from permanent USGS gaging stations will be presented and discussed to document long-term flow regime trends within the Cache-Bayou DeView watershed. Subsequently, the results of stage and streamflow monitoring at 29 additional stations established for this study (see Figure 1-1) are summarized and discussed. 2.1 Sources of Long-Term Flow Data There are four active USGS gage stations within the Cache-Bayou DeView watershed. Three are located on the Cache River and one is located on the Bayou DeView (see Table 2-1). Streamflow data (as daily mean discharges in cubic feet per second [cfs]) for the USGS gaging stations were downloaded from the webpage of the USGS Arkansas District Office ( Additionally, there are two gaging station maintained by the Memphis District (MVM) of the U. S. Army Corps of Engineers (USACE) located at Brasfield AR and near Pitts AR on the Cache River. Data on river stage for the Brasfield AR locality are currently available at ( A summary of the gaging stations and the type(s) of data available for each station is presented in Table 2-1. Because of the lack of flow data from the USACE stations, they were not considered in the flow regime analysis summarized in Sections 2.2 and 2.3. Cache River 319 Study 2-1 Section 2

2 2.2 General Streamflow Conditions Cache River Hydrographs for the Cache River USGS gaging stations covering the time interval of the study are illustrated in Figures 2-1 through 2-3. The hydrograph from the Egypt locality exhibits a spikiness, or relatively rapid rise and fall of flow levels in response to precipitation events. Such a pattern is typical of most headwater portions of watersheds, and is consistent with the location of the Egypt gaging station within the upper portion of the Cache River watershed. A portion of the spikiness is likely reflective of the channelized and altered nature of the Cache River system, upstream of the Egypt gaging station. In contrast, the hydrograph from the Cotton Plant locality exhibits relatively broad flow peaks typical of many low-gradient rivers found in the lower portions of their watersheds. Hydrograph response to precipitation events at the Cotton Plant gaging station is much slower and the flood pulses generated at this location in the watershed are much longer lived than those typical of the upper portions, such as the Egypt locality. The Indicators of Hydrologic Alteration (IHA) software package (Richter et. al., 1996) was used to determine the median value for and variability of median monthly flows for the USGS gaging stations on the Cache River. Results are illustrated in Figures 2-4 through 2-6. The interquartile spread between the 25 th and 75 th percentiles (bottom and top of the boxes ) and the 10 th and 90 th percentiles (bottom and top t-bars extending from the box ) of the monthly flows for each month are also presented. The magnitude of the interquartile spreads provides measure of the variability typical of each median monthly flow value. For all localities, interquartile variability is greatest for the wet season months (December through April or, in most cases May) and least during the dry season months of July through September. Flow-duration curves for the period of record for mean daily flow values for the USGS stations on the Cache River are presented in Figures 2-7 through 2-9. Annual median daily flow values for the Cache River range from 298 cfs at the Egypt, 448 cfs at Patterson, and 798 cfs at Cotton Plant gaging stations. For the gaging station at Egypt and Patterson, daily median flow values for the Cache River approach 0 cfs as the 100% exceedence limit is approached, while for the station at Cotton Plant, the streamflow values remain greater than zero at the 100% exceedence level. Such a situation does not correspond to a dry channel at the Egypt and Patterson gaging stations, but rather reflects stagnant flow conditions during periods of very low stage. Annual, peak-streamflow flood frequency analyses for USGS gaging stations on the Cache River are presented in Figures 2-10 through A recurrence interval of 1.5 years for bank full floods was assumed to estimate the magnitude of bank full floods at gaging stations on the Cache River. Based on studies summarized by Leopold (1994) and Trush et.al. (2000) such an assumption is a reasonable default value for river systems in which actual determinations have not been made. Recent investigations elsewhere within the lower White River basin (Haase, 2005) suggest that the bankfull flood recurrence interval may be as low as 1.15 to 1.2 years for some localities on the lower White River, but no recurrence interval determinations have been completed on the Cache River itself. Cache River 319 Study 2-2 Section 2

3 With the assumption of a 1.5 year recurrence interval, the streamflows corresponding to bankfull floods for the three USGS gaging stations ranged from 3,693 cfs at Egypt AR, to 5,184 cfs at Patterson AR, to 4,077 cfs at Cotton Plant AR. The decrease in the magnitude of the 1.5-yr flood between the Patterson and the Cotton Plant gaging stations may be due to the shorter period of record for the Cotton Plant station (1987 to present) compared to the period of record for the Patterson gage (1938 to present, with minor gaps). The decrease may also partially reflect the impacts of channel alteration and flow diversion in the segment of the Cache River downstream of the Patterson gaging station, though such impacts do not appear to influence the median daily flow values for the two stations in a similar manner. Frequency-of-occurrence relationships for the annual 7-day, low-flow events are presented in Figures 2-13 and For the Egypt locality streamflow values for the annual 7-day low-flow intervals with return frequencies greater than ~ 3 years are 0 cfs. At the Patterson gage the 7- Q10 flow value is 5cfs, while at the Cotton Plant gaging station the 7-Q10 value is ~29 cfs Bayou DeView A hydrograph of mean daily streamflows for Bayou DeView at the USGS gaging station at Morton AR is illustrated in Figure The hydrograph pattern is moderately spiky in that the flood peaks exhibit rapid rising and falling limbs with a pattern characteristic to that observed at the gaging station at Patterson AR on the Cache River. Median monthly flows calculated using the IHA software (Richter, et. al., 1996) for the Bayou DeView at the USGS gaging station near Morton AR, are presented in Figure The interquartile spread between the 25 th and 75 th percentiles and the 10 th and 90 th percentiles are also illustrated as a measure of the variability typical of each median monthly flow value. As with the results for the Cache River, the greatest variability in the median monthly flows occurs during the wet months (winter) and the small range of variability occurs in the dry or summer months. It is noteworthy that the median flows for August through September approach 0 cfs for each month. The flow-duration curve for mean daily streamflow values at the Morton gage, and frequency analyses for the annual, peak-flow flood, and the annual 7-day, low-flow event are illustrated in Figure 2-17 and The median flow value for the Bayou DeView at the Morton gaging station is 116 cfs. The 1.5 year bankfull flood streamflow value is 2, 697 cfs and, similar to the Cache River at the Egypt gaging locality, streamflow values for the annual 7-day low-flow intervals with return frequencies greater than ~ 3 years are 0 cfs. 2.3 Analysis of Long-Term Flow Regime Variability Based on the length of the period of record for the USGS gaging stations at Patterson AR and Egypt AR (Cache River), and Morton AR (Bayou DeView), the complete flow records from these gaging stations were selected for more detailed analysis with the IHA software package. The IHA software calculates the statistical parameters for a suite of 48 hydrologic variables that are used to characterize the magnitude, timing, duration, and frequency of baseflow, and highand low-flow events that are of importance to both the water resource utilization and ecological Cache River 319 Study 2-3 Section 2

4 sustainable management within the watershed (Richter et. al., 1996; 1997). Results from IHA analysis were used in the analysis of median monthly flows discussed in Section and In this section, additional IHA results for monthly median flows, annual maximum and minimum streamflow events, high- and low-flow pulses, zero-streamflow days, hydrograph rise and fall rates, and the timing of annual extreme streamflow events will be presented. The IHA results will be summarized and interpreted with regard to identification and characterization of longterm hydrologic alteration of the Cache River watershed flow regime. Summary statistical results for median monthly flows for all USGS gaging stations within the Cache River watershed were presented in Figures 2-4 through 2-6, and Statistical output from IHA analyses can also be used to examine temporal trends in median monthly flows in greater detail. April and August median monthly flows for the Cache River at Patterson AR (Figures 2-20 and 2-21) can be used to illustrate additional details of the inter-annual variability such as the year-to-year interquartile spreads discussed in Section and summarized in Figure 2.4. April and August were selected for discussion because they exhibit the most pronounced temporal variability observed and the trend represents the two major types of variability noted for the monthly median streamflow data sets. The April median flow data exhibit a slight, but decreasing trend in magnitude and year-to-year variability with time. After 1980, the inter-annual variability such as the difference in value between year-to-year data points has significantly decreased in comparison to pre-1980 period. April median streamflow magnitudes for the Cache River at Egypt AR, and the Bayou DeView at Morton AR (Appendix A, Figures A-1 and A-2) exhibit similar characteristics to the data from the Patterson AR gaging station. The patterns observed for April median monthly flows are also representative of those observed for May median monthly flows. Year-to-year August monthly median streamflow values for the Cache River at the Patterson AR, gaging station are presented in Figure 2-21, and since 1970 illustrate an increasing tend for median monthly values. Data for the Cache River at Egypt AR and the Bayou DeView at Morton AR (Appendix A, Figures A-3 and A-4) also exhibit such a trend of increasing monthly median flow values, which is also observed in the September monthly median streamflow data and at all three gaging localities. Inter-annual results from IHA analyses for 1-, 3-, 7-, 30-, and 90-day minimum and maximum streamflow values for the Cache River at the Patterson AR, gaging station are presented in Figures 2-22 and The parameters plotted represent the maximum and minimum streamflows observed within a water year for consecutive 1-, 3-, 7-, 30-, and 90-day time intervals within the year. The maximum streamflow events do not exhibit significant change in magnitudes or variability with time. Such a situation is also true for the maximum-streamflow events on the Cache River at the Egypt gaging station (Appendix A, Figure A-5). The maximum streamflow events for the Bayou DeView at the Morton gaging station (Appendix A, Figure A-6) do exhibit a somewhat of a decrease in the magnitude of the events and in there variability that begins at Minimum 1-, 3-, 7-, 30-, and 90-day stream flow events for the Cache River at the Patterson gaging stations are illustrated in Figure Between 1975 and 1980, the magnitudes of all but the 90-day minimum streamflow exhibit a decreasing trend as compared to a pre-1975 base. A Cache River 319 Study 2-4 Section 2

5 similar decreasing trend for magnitudes of all but the 90-day minimum stream flow events is noted for the Cache River at the Egypt gaging station (Appendix A, Figure A-7). The Cache River at Egypt, the 90-day minimum streamflow exhibits a general increasing trend that begins at 1980, similar to the situation noted for the Cache River at the Patterson gaging station. Most minimum streamflows for the Bayou DeView at Morton AR, with the exception of the 30- and 90-day minimums typically approach 0 cfs with sporadic excursions to greater values. The 30- day minimum streamflow does not exhibit a consistent increasing or decreasing trend, while the 90-day minimum streamflow exhibits a slightly increasing trend, similar to that noted for the gaging stations on the Cache River. Non-flood, high-streamflow pulses, and non-drought, low-streamflow pulses typically have important ecological functions in most river systems (Richter et. al., 1997). The parameters have water resource management significance in that they provide monitor of the quantity of water within a channel that is available for potential anthropogenic uses. High-stream flow pulses are defined as when stream is equal to, or greater than the streamflow corresponding to the 25 th percentile value on the flow duration curve calculated for the river at the gaging station under consideration. A low-streamflow pulse is defined as when the streamflow is equal to or less than the magnitude corresponding to the 75th percentile value on the flow duration curve for the river at the relevant gaging station. This parameter monitors general high-streamflow conditions that includes not only floods and maximum events, but also high-flow pulses that remain confined within channel, or are well below the bankfull flood magnitude defined in Sections and The annual number and mean duration of high-streamflow events for the Cache River at the Egypt gaging station are illustrated in Figure The temporal pattern for the number of highstreamflow events at this locality exhibits a pronounced upward trend, while the pattern for the mean duration of individual events exhibits a decreasing tend. Both trends are pronounced within the period beginning in Temporal trends noted for high-streamflow events on the Cache River - Egypt locality are more pronounced that any trends exhibited within the highstreamflow data from the Cache River - Patterson AR, or from the Bayou DeView - Morton AR locality (Appendix A, Figures A-9 and A-10). The Cache River - Patterson data exhibit similar trends to for data at the Egypt locality, though the trends are significantly less pronounced. The high-streamflow data for the Bayou DeView - Morton AR exhibit very muted trends similar to those identified for the Cache River - Egypt. Lack of data collection during portions of the 1980s and 1990s decades makes trend analysis complicated from the Patterson and Morton AR localities Trends similar to those described in the preceding paragraph are noted for the low-streamflow pulses on the Cache River at Egypt AR (Figure 2-25). The annual number of low-flow pulses increases with time. However, the annual mean duration of individual low-streamflow pulses does not exhibit a significant decrease with time. Similar, though less pronounced patterns are exhibited by the low-streamflow pulse data for the Cache River at Patterson, and the Bayou DeView at Morton AR (Appendix A, Figures A-11 and A-12). The number of zero-flow days within an individual water year for the Cache River at the Egypt gaging station is illustrated in Figure The data illustrate a marked increase in the Cache River 319 Study 2-5 Section 2

6 occurrence and number of annual zero-flow days at this locality after mid 1990s. Such a trend is not noted within the available data for the Cache River at the Paterson AR locality (Appendix A, Figure A-13), though the occurrence of a data gap in the 1990s hinders interpretation. Data for the Bayou DeView at Morton AR (Appendix A, Figure A-14) exhibit a highly variable pattern for the occurrence of zero-flow days. The Bayou DeView at the Morton locality experiences significantly greater numbers of zero-flow days in a water year than does the Cache River at either gaging stations considered in this study. Mean annual rise and fall rates are annual averaged rates of hydrograph increase or decrease. Data for the Cache River at Egypt AR (Figure 2-27) suggest that rates of change for both rising and falling hydrograph limbs increase after the mid 1980s. Similar, but less pronounced trends are exhibited by data from the Cache River at Patterson AR (Appendix A, Figure A-15). Hydrograph rise and fall rates for the Bayou DeView at Morton AR (Appendix A, Figure A-16) exhibit opposite and more complex trends. During the early 1940s, the rise rate abruptly increased and then began a decline back toward rates typical of the early 1940s throughout the period 1944 to1970. Subsequent to 1970, the annual rise rate appears to have remained relatively constant with a fixed range of variability. The annual hydrograph fall rate exhibited an increase that corresponds to the abrupt change in the rise rate noted previously, and subsequently the fall rate has remained constant within a rather limited range of values. The date of occurrence of annual 1-day maximum high- and low-flow events for the Cache River at the gaging station at Egypt AR is illustrated in Figure Data for the Cache River at Patterson AR and the Bayou DeView at Morton AR are presented in Appendix A, Figures A-17 and A-18. The data from all localities does not exhibit significant or prominent temporal trends, with the potential exception of the Cache River at Patterson AR. The dates for the occurrence of both maximum and minimum annual streamflows exhibit a baseline shift within the period 1950 to 1960 and an increase in the number of times the maximum streamflow event occurs within the interval from late November to December (Figure A-17). 2.4 Project Findings Pressure transducers were installed at 29 localities (localities illustrated in Figure 1-1 and described in Table A-1 in Appendix A) throughout the Cache-Bayou DeView watershed at the beginning of the study period. The transducers recorded stage and water temperature at 1-hr intervals and were left in place for 12 to 14 months. In addition to the pressure transducers placed within stream channels, four baro transducers were placed throughout the Cache-Bayou DeView watershed to record atmospheric pressure trends throughout the study period. Data from the baro loggers is used to compensate the stream stage data record obtained from the inchannel transducers. Hydrographs of stage data obtained at the 29 localities are presented in Appendix A (see Figures A-19 through A-44). The stage data were collected so that stream discharge values could be determined for use in calculation of total suspended solid fluxes for the sampling localities. Results of the flow and flux calculations are presented in Section 6. Cache River 319 Study 2-6 Section 2

7 2.5 Summary and Conclusions Indicators of Hydrologic Alteration (IHA) results for the Cache-Bayou DeView watershed suggest that the hydrologic flow regime within the watershed continues to experience ongoing hydrologic alteration. Such alteration appears to be complex and impact differing facets of the regime to varying degrees. The hydrologic alteration manifests itself only when a range of hydrologic parameters, such as those investigated by the IHA software package, are examined. Analysis of monthly median streamflow values suggests that trends observed are a manifestation of ongoing hydrologic alteration of the flow regime within the watershed. In the case of these parameters, the alteration impacts flows in the spring and fall of the year. Likely causes could be natural climatic variability and changes in the nature and intensity of agricultural practices within the watershed. The increasing trend in the number of zero-flow days on the Cache River at Egypt AR likely is also associated with this aspect of flow alteration on the Cache River Another aspect of flow alteration within the Cache-Bayou DeView is illustrated by the trends and changes noted within the maximum and minimum extreme streamflow events, high- and low-streamflow pulses, and changes for the hydrograph rise and fall rates. In the case of these parameters, the effect is not seasonal in character, but is inter-annual in nature. The overall impact of such changes is to increase the spikiness of the flow regime, within the middle and lower reaches of the watershed. 2.6 References Haase, C. S., Analysis of natural flow regime alteration within the lower White River Basin of Arkansas: results and implications for Ecologically Sustainable Water Management. Abstracts with Program for 2005 Regional Wetlands Technical Conference, Corpus Christi TX, May 17-19, Leopold, L. B., A View of the River. Harvard University Press, Cambridge, MA. 298p. Richter, B. D., Baumgartner J. V., Powell, J., and Braun, D. P., A Method for Assessing Hydrologic Alteration within Ecosystems. Conservation Biology, vol. 10, Richter, B. D., J. V. Baumgartner, R. Wigington, and D. P. Braun How much water does a river need? Freshwater Biology, 37, pp Trush, W. J., McBain, S. M., and Leopold, L. B., Attributes of an alluvial river and their relation to water policy and management. Proceedings of the National Academy of Sciences, vol. 97, no. 22, pp Cache River 319 Study 2-7 Section 2

8 Table 2-1. Permanent hydrologic monitoring stations within the study area. Cache River 319 Study 2-8 Section 2

9 Figure 2-1. Hydrograph for the Cache River at the USGS gaging station at Egypt AR covering the time period of the Cache River 319 study. Figure 2-2. Hydrograph for the Cache River at the USGS gaging station at Patterson AR covering the time period of the Cache River 319 study. Cache River 319 Study 2-9 Section 2

10 Figure 2-3. Hydrograph for the Cache River at the USGS gaging station at Cotton Plant AR covering the time period of the Cache River 319 study. Figure 2-4. Median monthly flows for the Cache River at the USGS gaging station at Egypt AR. Box tops and bottoms represent the interquartile spread between the 25 th and 75 th percentiles, and T-bars represent the 10 th and 90 th percentiles of median monthly flows. Cache River 319 Study 2-10 Section 2

11 Figure 2-5. Median monthly flows for the Cache River at the USGS gaging station at Patterson AR. Box tops and bottoms represent the interquartile spread between the 25 th and 75 th percentiles, and T-bars represent the 10 th and 90 th percentiles of median monthly flows. Figure 2-6. Median monthly flows for the Cache River at the USGS gaging station at Cotton Plant AR. Box tops and bottoms represent the interquartile spread between the 25 th and 75 th percentiles, and T-bars represent the 10 th and 90 th percentiles of median monthly flows. Cache River 319 Study 2-11 Section 2

12 Cache River at Egypt, AR 10,000 Flow Duration 1,000 Daily Flow (cfs) Daily Mean Flows ( ) Percentage of Time Flow Equaled or Exceeded Figure 2-7. Flow duration curve for the Cache River at the USGS gaging station at Egypt AR. Cache River at Patterson, AR 100,000 Flow Duration 10,000 Daily Flow (cfs) 1, Daily Mean Flows ( ) Percentage of Time Flow Equaled or Exceeded Figure 2-8. Flow duration curve for the Cache River at the USGS gaging station at Patterson AR. Cache River 319 Study 2-12 Section 2

13 Cache River near Cotton Plant, AR 10,000 Flow Duration 1,000 Daily Flow (cfs) Daily Mean Flows ( ) Percentage of Time Flow Equaled or Exceeded Figure 2-9. Flow duration curve for the Cache River at the USGS gaging station at Cotton Plant AR. Cache River at Egypt, AR 10,000 Flood Frequency 9,000 8,000 Annual Peak Flow (cfs) 7,000 6,000 5,000 4,000 3,000 2,000 1, Recurrence Interval (yrs) Figure Annual peak flow flood frequency curve for the Cache River at the USGS gaging station at Egypt AR. Cache River 319 Study 2-13 Section 2

14 Cache River at Patterson, AR 14,000 Flood Frequency 12,000 Annual Peak Flow (cfs) 10,000 8,000 6,000 4,000 2, Recurrence Interval (yrs) Figure Annual peak flow flood frequency curve for the Cache River at the USGS gaging station at Patterson AR. Cache River near Cotton Plant, AR 12,000 Flood Frequency 10,000 Annual Peak Flow (cfs) 8,000 6,000 4,000 2, Recurrence Interval (yrs) Figure Annual peak flow flood frequency curve for the Cache River at the USGS gaging station at Cotton Plant AR. Cache River 319 Study 2-14 Section 2

15 Cache River at Patterson, AR 250 Low-Flow Frequency d Low Flow ( ) Flow (cfs) Recurrence Interval (yrs) Figure Seven-day low flow frequency curve for the Cache River at the USGS gaging station at Patterson AR. Cache River near Cotton Plant, AR 140 Low-Flow Frequency d Low Flow ( ) 100 Flow (cfs) Recurrence Interval (yrs) Figure Seven-day low flow frequency curve for the Cache River at the USGS gaging station at Cotton Plant AR. Cache River 319 Study 2-15 Section 2

16 Figure Hydrograph for the Bayou DeView at the USGS gaging station at Morton AR covering the time period of the Cache River 319 study. Figure Median monthly flows for the Bayou DeView at the USGS gaging station at Morton AR. Box tops and bottoms represent the interquartile spread between the 25 th and 75 th percentiles, and T-bars represent the 10 th and 90 th percentiles of median monthly flows. Cache River 319 Study 2-16 Section 2

17 Bayou DeView near Morton, AR 10,000 Flow Duration 1,000 Daily Flow (cfs) Daily Mean Flows ( ) Percentage of Time Flow Equaled or Exceeded Figure Flow duration curve for the Bayou DeView at the USGS gaging station at Morton Plant AR. Bayou DeView near Morton, AR 8,000 Flood Frequency 7,000 6,000 Annual Peak Flow (cfs) 5,000 4,000 3,000 2,000 1, Recurrence Interval (yrs) Figure Annual peak-flow flood frequency curve for the Bayou DeView at the USGS gaging station at Morton AR. Cache River 319 Study 2-17 Section 2

18 Figure Seven-day low flow frequency curve for the Bayou DeView at the USGS gaging station at Morton AR. Figure April median streamflows for Cache River at the USGS gaging station at Patterson AR. Cache River 319 Study 2-18 Section 2

19 Figure August median streamflows for Cache River at the USGS gaging station at Patterson AR. Figure Streamflows for the 1-, 3-, 7-, 30-, and 90-day maximum annual high-flow events for the Cache River at the USGS gaging station at Patterson AR. Cache River 319 Study 2-19 Section 2

20 Figure Streamflows for the 1-, 3-, 7-, 30-, and 90-day minimum annual low-flow events for the Cache River at the USGS gaging station at Patterson AR. Figure Annual number and mean duration of high-flow events with a streamflow equal to or greater than the 25 th percentile value of the flow duration curve for the Cache River at the USGS gaging station at Egypt AR (see Figure 2-7). Cache River 319 Study 2-20 Section 2

21 Figure Annual number and mean duration of low-flow events with a streamflow equal to or less than the 75 th percentile value of the flow duration curve for the Cache River at the USGS gaging stain at Patterson AR (see Figure 2-7). Figure Annual number of days with streamflow equal to 0 cfs for the Cache River at the USGS gaging station at Egypt AR. Cache River 319 Study 2-21 Section 2

22 Figure Mean annual rate of increase (for rising hydrograph trends) and decrease (for falling hydrograph trends) for the Cache River at Egypt AR. Figure Date of annual 1-day minimum and maximum flows for the Cache River at the USGS gaging station at Egypt AR. Note that dates are on the Julian calendar with January 1 corresponding to a value of 0 and December 26 corresponding to a value of 360. Cache River 319 Study 2-22 Section 2