The process of distinguishing the natural flow compo- nent from the measured artificially influenced flow record is called naturalization. A key application is to identify whether observed changes in river flow are due to natural variability or to artificial influences changing over time.
The adjustment of gauged daily or monthly flows is normally carried out for the largest and most easily quantified artificial influences, for example, “exported” abstractions above the gauging station or “imported” effluent discharges into a river. Long series of natura- lized flows can be exceptionally valuable in detecting low-flow trends. Normally, no attempt is made to account for the often subtle impacts of land-use change, and these may be important.
Essentially, naturalization involves the adjustment of the observed (artificially influenced) flow record according to the known artificial influences upstream. The procedure requires a systematic record of the in- fluences, including the times, rates and durations of abstractions, discharges and reservoir releases. The data should be in compatible units, and an appropriate method to temporally disaggregate the data should be applied, as necessary (for example, from monthly to daily). On any day, the natural flow, Qn, at a gauging station can be approximated by simply adjusting the observed flow, Q, according to the net upstream artificial
0 0.1 0.2 0.3 0.4 0.5 0 5 10 15 20 Downstream distance (km) Q ( m 3/s ) Q ( m 3/s ) 0 0.1 0.2 0.3 0.4 0.5 0 5 10 15 20 Downstream distance (km) 0.3 Figure 10.4
Flow accretion diagram (left); residual flow diagram (right)
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influences. For example, if upstream of a gauging sta- tion there is a constant abstraction for water supply of 15 m³/s and an “imported” sewage effluent discharge of 5 m³/s, then the net impact is a reduction in river flow of 10 m³/s. In order to derive a naturalized flow record from a time series of gauged flows, it would be necessary to add 10 m³/s to each measured mean daily discharge. If abstractions or discharges vary on a daily basis, the net impact must be calculated daily and the time series of gauged flows adjusted accor- dingly. Given that there are often several artificial influences in a catchment, it is important to identify all upstream impacts in the catchment and to use ab- straction and discharge data to calculate the total net impact in order to adjust a gauged time series. These may vary seasonally and over decadal time scales. If abstraction and discharge measurements are not taken, then they will have to be estimated using the best data available (section 10.3).
It should be noted that there could be a high degree of uncertainty in estimating artificial influences. While, typically, there may be an error of ±5 per cent in the observed flow, the error associated with the artificial influence can be around ±40 per cent, or higher. An example of a naturalized hydrograph for the River Thames, a heavily influenced catchment in the United Kingdom, is shown in Figure 10.5. In January 1997, the naturalized discharge was approximately 20 m³/s higher than the gauged discharge measured at Kingston. This is the result of a net reduction in river flows caused by high abstraction rates for public water supply in London. Figure 10.6 illustrates the time series of 30-day annual minima flows derived from the 120-year gauged and
naturalized time series (Figure 10.5) for the River Thames. It shows that the gauged flow series indicates a decrease in 30-day minimum flows. However, once the record is adjusted for the artificial component (prima- rily, increasing abstractions for public water supply in London), recent low flows are seen to be substantially higher than those which characterized the early record. 10.6 Climate and land-use change
The impacts of both climate and land-use change on low flows are complex. Chapter 9 reviews the model- ling procedures that can be used to estimate the impact of changing land use and the sensitivity of low flows to different climate change scenarios. It is often difficult to differentiate between the effects of land-use change and other human influences and climate variability. Approaches include paired catchment studies with
contrasting land use and using hydrological models to simulate the impact of changing vegetation parameters on low-flow response.
In a review of land-use change impacts on low flows, Tallaksen and van Lanen (2004) identified a wide range
of different responses, depending on land-use change and climate regime. With respect to climate variability, although individual catchment studies have identified positive and negative trends in long time series of low flows, studies over large regions do not indicate con- sistent patterns (Hisdal and others, 2001). Analysing the sensitivity of low flows to changing climate inputs provides a methodology for estimating the range of low-flow responses to climate change. However, these produce only scenarios of possible outcomes and not forecasts of future events.
0 50 100 150 200 J F M A M J J A S O N D Gauged Naturalized Q ( m 3 /s) Month Figure 10.5 Disaggregation of river flows into gauged and natural components (River Thames at Kingston, United Kingdom, 1997) estImatIng loW FloWs In aRtIFICIally InFluenCeD RIVeRs
96 0 10 20 30 40 50 60 1883 1893 1903 1913 1923 1933 1943 1953 1963 1973 1983 1993 2003 30 -d ay m im im um fl ow (c um ec s) 0 10 20 30 40 50 60 1883 1893 1903 1913 1923 1933 1943 1953 1963 1973 1983 1993 2003 30 -d ay m in im um fl ow (c um ec s) – n at ur al iz ed xx
Figure 10.6 Trend line for 30-day annual minima for gauged and natural flow series (River Thames in the United Kingdom) (Source: Terry Marsh, Centre for Ecology and Hydrology, Wallingford, United Kingdom)
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References
Bullock A., A. Gustard, K. Irving, A. Sekulin and A. Young, 1994: Low Flow Estimation in Artificially
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European Environment Agency (EEA), 1998: Europe’s Environment: The Second Assessment. Copenhagen. Gustard, A., G.A. Cole, D.W. Marshall and A. Bayliss, 1987: A study of compensation flows in the UK. IH
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Tallaksen, L.M. and H.A.J. van Lanen (eds), 2004: Hydrological Drought: Processes and Estimation
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Theis, C.V., 1941: The effects of a well on the flow of a nearby stream. EOS Trans. AGU, Volume 22,
pp. 734–738. estImatIng loW FloWs In aRtIFICIally InFluenCeD RIVeRs
98 loW-FloW FoReCastIng