of Implications Climate Variability for Low Flows and Dilution of Discharges from Point Sources NOAA-CIRES Western Water Assessment
The quality of surface waters in many countries is The quality of surface waters in many countries is regulated largely by restrictions on the release of pollutants regulated largely by restrictions on the release of pollutants from point sources like wastewater treatment facilities from point sources like wastewater treatment facilities (Fig. 1). One of the most important problems related to the (Fig. 1). One of the most important problems related to the regulation of point-source discharges is estimating the regulation of point-source discharges is estimating the amount of dilution provided by the receiving water at the amount of dilution provided by the receiving water at the point of effluent discharge. Dilution flow is estimated point of effluent discharge. Dilution flow is estimated from the historical record of stream flow data by from the historical record of stream flow data by application of USEPA criteria that define low-flow application of USEPA criteria that define low-flow conditions (Fig. 2). The historical record of stream flow conditions (Fig. 2). The historical record of stream flow shows the effects of natural variation in climate at time shows the effects of natural variation in climate at time scales that tend to be long relative to the availability of scales that tend to be long relative to the availability of recorded flows. ENSO events, which recur with a recorded flows. ENSO events, which recur with a frequency of about 4-6 years, would be poorly represented frequency of about 4-6 years, would be poorly represented by hydrologic records of less than 50 years. Major by hydrologic records of less than 50 years. Major droughts are less frequent and less regular than ENSO droughts are less frequent and less regular than ENSO events, and thus can be accounted for only with even events, and thus can be accounted for only with even longer records. In Colorado, for example, there were five longer records. In Colorado, for example, there were five significant droughts during the 20th Century. significant droughts during the 20th Century.
Acute Minimum Flow,
James F. Saunders, III William M. Lewis, Jr. Center For Limnology
INTRODUCTION INTRODUCTION
250 200 150 100 50 0 1-y 3-y 5-y 10-y 20-y 74-y
The hydrograph of the Yampa River is typical of streams The hydrograph of the Yampa River is typical of streams draining basins in which much of the precipitation is in the draining basins in which much of the precipitation is in the form of snow (Fig.5). Hydrographs for two other streams form of snow (Fig.5). Hydrographs for two other streams show that the amount of seasonal variation is not constant show that the amount of seasonal variation is not constant across years, and the variability in annual flows reflects across years, and the variability in annual flows reflects variations in climate conditions (Figures 6-7). variations in climate conditions (Figures 6-7).
25000 Daily Average 20000 15000 10000 5000 0
Jan-80 Jan-82 Jan-84 Jan-86 Jan-88
TIME SCALES OF FLOW VARIATION TIME SCALES OF FLOW VARIATION
Figure 1. The water quality of Figure 1. The water quality of streams is affected by point source streams is affected by point source discharges. This large facility, discharges. This large facility, which serves 1.25 million people which serves 1.25 million people in the Denver Metropolitan area, in the Denver Metropolitan area, discharges 150 million gallons of discharges 150 million gallons of treated municipal wastewater to treated municipal wastewater to the South Platte River each day. the South Platte River each day.
Period of Record
Figure 3. The relationship between period of record (1- ,3-, Figure 3. The relationship between period of record (1- ,3-, 5-, 10-, 20-, and 74-y) and acute low flow conditions for the 5-, 10-, 20-, and 74-y) and acute low flow conditions for the Yampa River. Uncertainty diminishes as record length Yampa River. Uncertainty diminishes as record length increases. Bias is evident in records shorter than 10 years. increases. Bias is evident in records shorter than 10 years. The vertical bar encompasses 80% of the values; the The vertical bar encompasses 80% of the values; the horizontal bar indicates the median. horizontal bar indicates the median.
Mixed Concentratio
480 360 240
Figure 5. A 10-y record of flows in the Yampa River illustrates Figure 5. A 10-y record of flows in the Yampa River illustrates seasonal and annual variability. The seasonal pattern is dominated seasonal and annual variability. The seasonal pattern is dominated by a spring snow-melt peak and an extended period of base flow. by a spring snow-melt peak and an extended period of base flow.
70000 Total Annual Flo 60000 50000 40000 30000 20000 10000 0 1925 1945 1965 Water Year 1985
1.2
Figure 2. Low flow conditions Figure 2. Low flow conditions determine the amount of water determine the amount of water available for dilution of pollutants. available for dilution of pollutants. This photograph of the South This photograph of the South Platte River near Fort Lupton Platte River near Fort Lupton shows low flow conditions. shows low flow conditions.
The relationships between low flows and the length of the The relationships between low flows and the length of the flow record are quantified for two unregulated streams in flow record are quantified for two unregulated streams in Colorado: the Yampa River and Middle Boulder Creek. At Colorado: the Yampa River and Middle Boulder Creek. At issue is the extent to which the range of variability in issue is the extent to which the range of variability in stream flow can be captured by analysis of flows from a stream flow can be captured by analysis of flows from a given period of record (1-, 3-, 5-, 10-, or 20-y blocks of given period of record (1-, 3-, 5-, 10-, or 20-y blocks of contiguous daily flows taken from the 70+ y record). contiguous daily flows taken from the 70+ y record). Longer records are more likely to reflect a broad range of Longer records are more likely to reflect a broad range of natural climate variability, are but rare. Shorter records can natural climate variability, but are rare. Shorter records can introduce uncertainty or bias in the estimation of low-flow introduce uncertainty or bias in the estimation of low-flow conditions because events with periodicities of multiple conditions because events with periodicities of multiple years (e.g., ENSO events or droughts) are not represented years (e.g., ENSO events or droughts) are not represented adequately. adequately.
PURPOSE PURPOSE
120 0 Jan-30 Jan-32 Jan-34 Dry Jan-36 Jan-38 Wet Standard
1
0.8
0.6
0.4
Figure 4. Hypothetical water quality scenarios in which the Figure 4. Hypothetical water quality scenarios in which the concentration of a conservative substance is predicted in the concentration of a conservative substance is predicted in the Yampa River downstream of an effluent discharge. One Yampa River downstream of an effluent discharge. One scenario is based on a 3-y record during a relatively dry scenario is based on a 3-y record during a relatively dry period (10th percentile) and the other is based on a relatively period (10th percentile) and the other is based on a relatively wet period (90th percentile). Effluent concentrations based wet period (90th percentile). Effluent concentrations based on each flow scenario are applied to daily flows observed on each flow scenario are applied to daily flows observed during the 1930s, and the predicted in-stream concentrations during the 1930s, and the predicted in-stream concentrations are compared with the actual water quality standard. The are compared with the actual water quality standard. The standard would have been exceeded many more times if the standard would have been exceeded many more times if the permit limit had been set according to dilution flows permit limit had been set according to dilution flows observed during a wet period. observed during a wet period.
0.2
0
Figure 6. A 70-y record of annual flows in Middle Boulder Creek Figure 6. A 70-y record of annual flows in Middle Boulder Creek shows interannual variation over a broad range of climate shows interannual variation over a broad range of climate conditions in the 20th century. A 3-y moving average (red line) is conditions in the 20th century. A 3-y moving average (red line) is superimposed to highlight the effects of major droughts (denoted superimposed to highlight the effects of major droughts (denoted by solid bars in the upper range of the graph). by solid bars in the upper range of the graph).
Protection of aquatic life is one basis Protection of aquatic life is one basis for the determination of specific for the determination of specific numeric standards for a variety of numeric standards for a variety of substances reaching streams and substances reaching streams and rivers as a result of human activity. rivers as a result of human activity. Fishes and aquatic life in general Fishes and aquatic life in general have low tolerance for water have low tolerance for water pollution. pollution.
Record length has a very pronounced effect on estimates of Record length has a very pronounced effect on estimates of regulatory low flows. For annual minima and for regulatory low flows. For annual minima and for regulatory low flows based on 3-y and 5-y blocks of data, regulatory low flows based on 3-y and 5-y blocks of data, uncertainty is large (Fig. 3), and bias is evident. As record uncertainty is large (Fig. 3), and bias is evident. As record length increases, the range of values decreases and the length increases, the range of values decreases and the median converges on the value based on the entire record. median converges on the value based on the entire record. Acceptable conv...