Flow variability is a potential indicator of land use impacts on aquatic ecosystems and a dominating factor for lotic habitats. Vegetation management effects on the stream habitat conditions must be better understood to propose forest management activities that are compatible with general ecosystem management objectives (integrity, diversity, sustainability, etc.). In our study, we used long term flow data (1936-2004) from four gauged experimental watersheds (W1, W2, W17, W18) of Coweeta Hydrologic Laboratory in US to assess the impacts of pine conversion on flow characteristics by using paired watershed experimentation. In W1, all trees and shrubs were cut and burned in 1956-57 and white pine (
Sustainable management of forest ecosystems involves application of adequate forestry treatments towards maintaining their biodiversity, productivity, regeneration capacity, vitality and their potential to fulfill relevant functions (
Flow variability, a major aspect of hydrological behavior is a potential indicator of land use and a control on river ecology (
It is obvious that any disturbance that causes changes on sediment loads, water quality, and flow regime also has the potential to affect overall in-stream habitat quality. There are two main points that should be addressed: (1) the methodology required to identify flow variability; and (2) identification of forestry treatments that affect or control flow variability. Approaches that focus on benthic macroinvertebrates as monitors of stream health and thus incorporate hydraulic habitat conditions as part of bioassessment are widely accepted in recent decades (
A number of recent studies have proposed hydrological parameters that are indicative of stream ecological conditions. The IHA (Indicators of Hydrologic Alteration) method developed by Richter et al. (
Forestry treatments have rarely been subject to stream habitat condition assessments though many of them (cutting, conversion, prescribed burning, etc.) are expected to alter hydrological conditions in a watershed. Furthermore, impacts of forestry activities on stream habitat conditions, riparian ecosystems and extreme flows has been recently identified as a research gap (
Among the forestry treatments, species conversion has been an important forest practice for centuries. In Europe, conversion to pine started in the late 18th century for the restoration of degraded broad leaved forests (
In any case, a change in the species composition can potentially alter ecological conditions. A substantial amount of research has been conducted to understand the effects of changing species composition from many aspects. Most of the efforts to identify the hydrological consequences have focused on water yield (
The influence of species conversion on extreme flows was investigated with the emphasis on timing and magnitude (
In this study the influences of conversion for its potential to change the in-stream habitat conditions by using streamflow variability evaluation techniques.
The Coweeta Hydrologic Laboratory is located in the Nantahala Mountain Range of western North Carolina, latitude 35°03’ N, longitude 83°25’ W (
Flow series were analyzed as ratios of treatment watersheds (W1, W17) to control watersheds (W2, W18) to remove the influence of climatic variations. This is the advantage of paired watershed approach. The mean flow data has been given as an example to this (
Long term flow data belonging to 4 experimental watersheds (W1, W2, W17, W18 -
The effects of species conversion on interannual timing of 7Q flows was investigated using Mann Whitney-Pettitt test, a time series shift detection method (
We focused on the effects of conversion on low flows and assessed changes in flow dimensions of various recurrence intervals as a first step. Seven day consecutive flow rates at 2, 5, 10, 20, 50, and 100 years recurrences were calculated with traditional quantile estimation procedure for the period before treatment (1934-1956) and post treatment (1957-2004) periods.
7Q low flow duration curves (L-FDC) were prepared for all 4 watersheds to assess whether conversion affected low flows throughout the period in consideration. FDCs represent all flow values in a time interval in this case all 7Q low flows before the low flows changed due to conversion, and after. They can be prepared for various intervals and flow types. Percentage of years 7Q low flow values exceeded are plotted on the graph to see if there is a shift at 7Q low flows in general.
The flow variability in this section covers the time series analysis of ratios of high and low flow values to mean flows. Q90/50 and Q10/50 values for treatment watersheds were extracted from data series and standardized by dividing into control watershed values. Q50 represents the median flow, Q90 and Q10 are the flow values that 90 and 10 percent of the flows are above them. Therefore, Q90 is a low flow value, Q10 is a high flow value. The Q90/50 represents low flow variability, Q10/50 represents high flow variability.
The flow series of control and treatment watersheds indicated significant variations throughout the monitoring period. The reason of the variations could be caused by short term (cutting, fires etc.) and long term (species conversion, regrowth etc.) disturbances mentioned above. Long and short term changes in climatic variables are also visible on the time series given in
An important aspect of streamflow response to disturbance is the timing of response. Mann Whitney-Pettitt test revealed that the major long term variations on streamflow of treatment watersheds were caused by conversion to pine. The streamflows of these watersheds responded to the conversion with decreasing annual mean flows (AMR) starting in 1968. This means that an 11 years delay occurs for annual streamflows to respond to the conversion. Annual maximum (AXR), annual minimum (ANR), 7QMinimum and 7QMaximum flow ratio series were changed at different but close durations after conversion (
More specifically, annual mean flows started to decrease in 1968 in both watersheds (W1 and W17); however, the maximum annual flows were affected less and the change point for AXR was 1976 for W1 and 1972 for W17. The annual minimum flows were also affected strongly, but change points were similar for both watersheds (
Flow of a stream generally follows a regular pattern throughout the year with respect to rising and falling limbs of interannual hydrographs. The rising limb follows the rainy period and peaks at some point. The lowest point of the hydrograph generally corresponds to the end of a low or no precipitation period. The lowest and highest points may shift for or back according to the precipitation characteristics of the year affecting the life cycles of steam biota. The paired watershed approach provided us to evaluate the changes in timing of 7Qlow and high flows due to conversion to pine. The time series was divided into 3 periods to evaluate the interannual timing of 7 days minimum and maximum flows as explained in methods. The timing ratios of W1/W2 for 7Qmin flows were 1.09, 1.05, and 0.99, pointing out a date pulled forward due to conversion to pine (
The 7 day maximum flow of W1 was 110.72th day before the treatment, which corresponds to the second half of April. It moved forward to second half of March (82.76th day) in the first period (1970-86) after conversion and then delayed to second half of April again (113.35th day). When these values were divided to the control watershed values then the picture was opposite; a delayed timing for the first period compared to before treatment (from 0.98 to 1.09) and a very similar timing for the second period (1987-03) compared to again before treatment conditions. For the case of W17; 1.43 before pretreatment ratio dropped to 0.90 in the first period which is a very sharp change, but again raised to 1.00 at the second period. The reasons we discussed above for the low flows were also valid for 7 day maximum flows.
The conversion to pine resulted in changes in recurrence of 7 days minimum and maximum flows and also flow regimes. The effects the conversion was very clear on mean flows (
Streamflow affects the habitat quality of lotic populations. Furthermore, streamflow variability is a significant factor governing the distribution of stream flora and fauna. The importance of low and high flows in aquatic ecosystems is well established but the effects of forestry treatments on ecologic streamflow indices have not been well documented.
Seasonal and between-year variations in flow regime of streams are normal and the stream communities are generally able to withstand the variations. However, human impacts for management purposes may alter the ecosystem and its responses substantially.
In this study we dealt with timing, frequency, magnitude, and variability aspects of flow characteristics to evaluate the changes due to species conversion treatment at experimental watersheds. Paired watershed approach provided very precise results of changes in streamflow and helped to eliminate the influence of climatic variability. We could assess the change in flow responses due to conversion to pine with this well established method and long term flow data.
Many attributes of the watersheds may alter response of streamflow to a disturbance. In our study, W1 (south facing) and W17 (north facing) had similar responses to conversion to pine with respect to timing, magnitude and low/high flow variability. The only visible difference was about interannual changes in the timing of ecological low and high flows which suggests that species conversion is not the major factor and facing of the watersheds might be affecting the results. In other words, the changes in Julian date of ecologic maximum and minimum flows in time series could not be explained with the conversion to pine. The precipitation pattern should be evaluated with a more concentrated approach to discuss this issue.
The responses of treatment watersheds to conversion corresponded to similar years in case of 7Q maximum and minimum flows. The timing response of this ecologic flow indice was closer to annual mean flow values compared to annual maximum or minimum flow values especially in cases of maximum flows. The 7 day maximum and minimum flows were also affected from conversion for magnitude and frequency.
The low flow variability (
Based on these results we conclude that ecologic flow parameters including the timing and magnitude of 7Q high and low flows have been affected significantly but flow variability has not been changed due to conversion to pine.
Experimental watersheds of Coweeta Hydrologic Laboratory (
Annual mean flows and flow ratios of watersheds 1 and 2 for the whole evaluation period.
The change in the recurrence of 7 day minimum flows. The recurrence intervals are 2, 5, 10, 20, 50, and 100 years represented by shrinking circles respectively. The largest circle represents 2 years return period, the smallest one 100 years.
Low flow duration curves of the experimental watersheds.
Time series of Q90/50 flow series.
Time series of Q10/50 flow series.
Some properties of experimental watersheds (
Watershed Number | Name of Stream | Area(ha) | Elevation at Weir (m) | Maximum elevation (m) | Aspect |
---|---|---|---|---|---|
1 | Copper Branch | 16 | 705 | 988 | S |
2 | Shope Branch | 12 | 709 | 1004 | SSE |
17 | Hertzler Branch | 13 | 760 | 1021 | NW |
18 | Grady Branch | 13 | 726 | 993 | NW |
Primary change point (CP) years, direction of change (D: Decrease; I: Increase), and the significance of shift for all time series. Less significant CPs are also given as secondary.
Time series | W1/W2 | W17/W18 | ||
---|---|---|---|---|
Primary | Secondary | Primary | Secondary | |
AMR | 1968-D-0.99 | 1980-D-0.98 | 1968-D-0.99 | 1980-D-0.98 |
AXR | 1976-D-0.98 | n/a | 1972-D-0.99 | n/a |
ANR | 1965-D-0.99 | 1978-D-0.81 | 1966-D-0.99 | 1995-I-0.98 |
7QMin | 1965-D-0.99 | 1977-D-0.90 | 1968-D-0.99 | n/a |
7QMax | 1968-D-0.99 | n/a | 1969-D-0.99 | n/a |
Timing of 7Q minimum and maximum flows. (CV): coefficient of variation.
Dates | Series | Mean | CV | ||||
---|---|---|---|---|---|---|---|
1937-54 | 1970-86 | 1987-03 | 1937-54 | 1970-86 | 1987-03 | ||
Julian date of 7QMin flow | W1 | 266.17 | 282.06 | 232.18 | 0.36 | 0.26 | 0.45 |
W2 | 244.89 | 269.82 | 234.18 | 0.37 | 0.08 | 0.36 | |
W17 | 269.28 | 290.65 | 235.41 | 0.3 | 0.11 | 0.47 | |
W18 | 280.17 | 282.65 | 228.24 | 0.26 | 0.1 | 0.46 | |
W1/W2 | 1.09 | 1.05 | 0.99 | 0.97 | 3.07 | 1.23 | |
W17/W18 | 0.96 | 1.03 | 1.03 | 1.13 | 1.17 | 1.03 | |
Julian date of 7QMax flow | W1 | 110.72 | 82.76 | 113.35 | 0.96 | 0.92 | 0.94 |
W2 | 113.17 | 76.24 | 112.53 | 0.93 | 0.59 | 0.87 | |
W17 | 131.33 | 79.65 | 107.53 | 1.03 | 0.94 | 0.92 | |
W18 | 91.78 | 88.35 | 107.65 | 1.14 | 0.87 | 0.78 | |
W1/W2 | 0.98 | 1.09 | 1.01 | 1.03 | 1.56 | 1.08 | |
W17/W18 | 1.43 | 0.9 | 1 | 0.9 | 1.08 | 1.18 |