Assessment of harvesting treatment effects on the water balance of forested basinsprecipitation network design considerations

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1 Integrated Design of Hydrological Networks (Proceedings of the Budapest Symposium, July 1986). IAHS Publ. no. 158,1986. Assessment of harvesting treatment effects on the water balance of forested basinsprecipitation network design considerations Evaluation des effets du traitement de collecte des eaux sur le bilan d'eau des bassins forestiers-considérations sur la planification des réseaux de précipitation INTRODUCTION R.B.B. DICKISON, D.C. PALMER & D.A. DAUGHARTY Department of Forest Resources University of New Brunswick Fredericton, New Brunswick, Canada ABSTRACT Recent experience in an eastern Canadian water-balance study revealed serious problems in assessing the effect of a forest harvesting operation on stream discharge using the conventional paired-basin method. Because the research basins were not contiguous, networks of precipitation stations were installed in both basins to allow the difference in precipitation to be measured as a separate variable. However, there was an apparent change in precipitation relationships between the two basins following removal of the forest cover surrounding the measurement sites in the treatment basin. This change was attributed to a reduction in catch efficiency of the precipitation gages during snowfall. An alternative precipitation network design is proposed for use in areas where snowfall is a significant component of the annual precipitation. Removal of forest cover from small drainage basins has been shown to alter the annual water balance by increasing the discharge in some locations by as much as 660 mm/year (Bosch & Hewlett, 1982). Although some scientists (for example, Garczynski, 1980) continue to question whether the stream discharge from large river basins is altered upward or downward by removal of forest cover, the effect on small headwater basins is hardly debatable. The method of assessing treatment effects on small basins used almost universally is the paired-catchment (drainage basin) method pioneered in the United States over 50 years ago (Hewlett, 1981). Essentially, the method involves reserving one basin as a control while another is subjected to the designated treatment. Before treatment, contemporaneous measurements are conducted on both basins during a calibration period, usually of several years' duration. Recent experience in an eastern Canadian study (the Nashwaak Project) revealed serious problems in certain circumstances when this conventional method was used for the precipitation network (Dickison & Daugharty, 1982). This paper introduces an alternative 97

2 98 R.B.B.Dickison et al. method now being tested at the site of this study, and an assessment of the use of the method to reevaluate relationships based solely on the original design. REVIEW OF THE NASHWAAK PROJECT Study site The study site is located in central New Brunswick, Canada, about 150 km inland from the Atlantic coast, in a region of mixed coniferdeciduous forest cover where the topography varies from about 200 to 400 m elevation. When the study was initiated in 1970, access and financial limitations resulted in the selection of research basins which were not contiguous, but were considered to be sufficiently similar in a hydrological sense to allow the paired-catchment method to be applied. The location of the study site and the research basins are shown in Fig.l. The control area (Hayden Brook basin) is 660 hectares in area, and the treatment area (Narrows Mountain Brook basin) is 391 hectares. Precipitation networks The precipitation networks for the project are shown in Fig.l. At the beginning of the project in 1970, the networks consisted of six stations in the control area (stations Hl-6) and four stations in the treatment area (stations Ml-4); a headquarters station, located between the two basins, was established for the measurement of several meteorological variables. Additional stations added in 1983 (Cl-5) will be discussed later. All precipitation stations were placed in small clearings within the forest. These clearings were made about 2-4H in diameter (where H represents tree height), small enough to avoid undue exposure effects but large enough to prevent tree crowns from projecting into a 45 line-of-sight from the gage orifices. Each station was equipped with an unshielded standpipe-storage gage, supplemented during the summer season by the installation of Canadian Atmospheric Environment Service (AES) standard rain gages. Assessment of treatment effect on annual stream discharge Stream discharge data were collected by the Water Survey of Canada. For the 7-year period prior to removal of the forest cover from the treatment area in 1978 to 1979, the precipitation and discharge measurements from the two areas were compared, and an annual discharge relationship developed as follows: QNMB = Q HB DP (1) where Q^Mg = annual discharge (mm) from Narrows Mountain Brook basin, Qjjg = annual discharge (mm) from Hayden Brook basin, and

3 Water balance of forested basins 99 DP = difference in annual precipitation (mm) between the two basins (P^MB ~" P HB)' This equation explained 99.2% of the variance, with a standard error of 34.2 mm. Mean QNMJJ before treatment was 880 mm. FIG.l Map of the study area showing locations of precipitation stations. Following the clearcutting treatment, this relationship was used to evaluate the first-year effect of the treatment, on the following basis: Q = Q~NMB ~ QNMB (2) where the discharge values are in millimeters as previously given, and Q~ represents the predicted discharge based on equation (1). From this relationship, the estimate of the treatment effect was an

4 100 R.B.B.Dickison et al. increase of 120 mm, 11.9% of Q~, in the first year following treatment. Revisions to the discharge data, provided by the Water Survey of Canada after a data review, make this a slightly smaller increase than was previously reported (Dickison et al., 1981). The paired-catchment method was modified in this case to include DP because the experimental basins were separated by approximately 4.5 km from centroid to centroid, which is too far for acceptance of the basic assumption of no difference in precipitation between the two areas. In a later assessment of the effect on snowmelt runoff (Dickison & Daugharty, 1982), it appeared that the precipitation measured in the Narrows Mountain Brook basin was significantly reduced following removal of the protective forest cover surrounding the gaging sites, at least during the snowfall season. Compared to predicted values based on the relationship with the control area obtained during the calibration period of November to April, precipitation in the treatment basin averaged about 12% less during the 4 years immediately following treatment. If this difference is an artifact of the altered gage exposure, it introduces a bias in the determination of DP, and a consequent bias in the determination of the effect of the treatment on the water balance. Assessment of precipitation measurement bias In order to determine the causes and extent of the bias in our determination of precipitation, a separate study was initiated (Palmer, 1985). An external concentric "ring" of precipitation gaging sites (Cl-5) was established within the forested area surrounding the experimental basin (Fig.l). Within the basin, regenerating growth from the clearcut forest surrounding the original gaging sites was cut away to again provide a condition of exposure as similar as possible to that in the year following the harvesting treatment. The underlying hypothesis of this revised paired-catchment method is that the external network provides as good an estimate of a small basin's precipitation as a network within the basin itself. In this case, it was impossible to establish the validity of this hypothesis by a direct comparison because the catch efficiency of the internal network gages had been altered by the treatment. The test, therefore, was the comparative relationship between the external network and the control network in Hayden Brook basin. An important source of error in this determination is the natural storm-to-storm variability. Preferably, the relationship would be developed from measurements taken over a period of sufficient length for the mean difference to approach zero; alternatively, one may subjectively remove from the sample those cases (e.g. events within the years) where an examination of the overall storm precipitation distribution reveals a spatial pattern which is independent of local influences. This latter approach was used by Biais (1985) in an initial examination of the relationship for summer rainfall. Two winter seasons and two summer seasons have now been analyzed for the study site (Table 1) incorporating all precipitation events for which comparative measurements are available. The concentric networks (Ml-4 and Cl-5) have summer rainfall totals within 1 and 2 mm in the two seasons, even though the storm-to-storm variability

5 Water balance of forested basins 101 TABLE 1 networks Comparative precipitation catches (mm) in different Season Controlforested Treatment Internal/unforested External/fores ted (Hl-6) (Ml-4) (Cl-5) (mm) (mm) (% of control) (mm) (% of control) Winter (5 Dec. - 3 Apr.) Summer (22 May - 18 Sept.) Winter (30 Nov Feb.) Summer (3 July - 30 Sept.) has not been excluded. This analysis, therefore, confirms the conclusion of Biais (1985) that catch efficiencies at sites within the basin have not been altered for rainfall events (summer seasons). The influence of exposure on catch efficiency is therefore confined to snowfall events (winter seasons). The better catch efficiency for the summer precipitation measurements is largely due to the use of the small AES standard rain gages. They are mounted with their orifices only about 30 cm above ground, where the wind is sufficiently dampened that catch efficiencies are unaffected, even where the gaging sites are quite exposed. During winter, data were available only from the larger storage gages, mounted with their orifices about 2 m above ground; thus, catch efficiencies were severely reduced wherever the site was exposed to wind, especially for snow precipitation. This is apparent from the differences between the internal network (82.3% and 88.1% of the control basin) and the external network (95.1% and 96.0%). The difference from one season to another may be largely attributed to differences in the proportion of rainfall comprising the winter-season precipitation. The data in Table 1 show a slight difference in precipitation between the two basins, of -3.4% in the treatment basin for the summer and winter periods combined, using the external network as a true measure of the comparative precipitation. This is slightly outside the -3.3% to +2.6% range of annual differences during the 7-year calibration period, for which the mean difference was -1.0%, although neither the seasonal pretreatment nor the posttreatment

6 102 R.B.B.Dickison et al. differences were found to be statistically significant. In a determination of real water balance in a basin, the best estimate of the true precipitation must be obtained. However, if a reliable index to the difference in precipitation between two basins is needed, this determination will not be affected if the gages in both areas are similarly exposed. CONCLUSIONS The results of this analysis indicate that the conventional pairedcatchment method for calibration must be modified for use in areas where snowfall is a significant component of the precipitation and where the research basins are not contiguous. Contiguous or not, if precipitation is in the analysis of covariance by regression (as it should be in all stormflow studies) the possibility of an altered catch must be accounted for (J.D. Hewlett, pers. comm., November 26, 1985). It is recognized that an essential condition of the method, as originally conceived, was that the experimental basins are small and contiguous so that differences in precipitation between the basins could be minimized. This ideal situation, however, may not always be practical, due to physiographic conditions or access limitations. By establishing the treatment-area network external to the basin, DP can be incorporated into a regression equation, similar to equation (1). Following treatment, the measured precipitation would not be affected by a change in gage exposure. One concern with the proposed design is that site conditions outside the basin differ from those within; however, in randomly undulating areas, opposing slopes outside of the basin divide are generally similar to the slopes within the drainage basin, and the effects of opposing aspects tend to be compensating. This could be tested by operating both an internal and an external network during the calibration period. In this study, the external network was not established until after the harvesting treatment, but an assessment of the representativeness of the new network obtained by comparing the relationships in the control area with those obtained for the original treatment basin network prior to removal of the forest cover showed the difference to be insignificant. ACKNOWLEDGEMENTS Financial support for this study has been provided by the Canadian Forestry Service, the Atmospheric Environment Service, and the Inland Waters Directorate, Environment Canada, and the New Brunswick Department of Natural Resources. REFERENCES Biais, D.R.J. (1985) Analysis of rainfall catches for the Nashwaak Experimental Watershed Project in New Brunswick. BScF Thesis, Univ. New Brunswick, Fredericton, N.B., Canada. Bosch, J.M. & Hewlett, J.D. (1982) A review of catchment experiments to determine the effect of vegetation changes on water yield and évapotranspiration. J. Hydrol. (55), 3-23.

7 Water balance of forested basins 103 Dickison, R.B.B. & Daugharty, D.A. (1983) The effects on snowmelt runoff of the removal of forest cover. In: Proc. Fourth Northern Research Basin Symp. Workshop: Effect of Distribution of Snow and Ice on Streamflow, Ullensvang, Norway, March 22-25, Norweg. Nat. Coram, for Hydrol. Rep., No. 12, Oslo, Norway. Dickison, R.B.B., Daugharty, D.A. & Randall, D.K. (1981) Some preliminary results of the hydrological effects of clearcutting a small watershed in New Brunswick. In: Proc. 5th Can. Hydrotech. Conf., Can. Soc. Civ. Engr., May 26 and 27, 1981, Fredericton, N.B., Canada. Garczynski, F. (1980) Effect of percentage forest cover on the hydrological regime in three regions of the USA. In: The influence of man on the hydrological regime with special reference to representative and experimental basins (Proc. Helsinki Symp., June 1980), IAHS Publ. no Goodison, B.E. (1978) Accuracy of Canadian snow gage measurements. Appl. Met. (17), Hewlett, J.D. (1981) Principles of Forest Hydrology. Univ. Georgia Press, Athens, Georgia, U.S.A. Palmer, D.C. (1985) Correction of winter precipitation values following a change of gage exposure. In: Proc. 42nd East. Snow Conf., June 6 and 7, 1985, Montreal, Quebec, Canada. (In press) ADDENDUM The authors recently learned that Hewlett (1970) recommended a similar approach to that described in this paper: "(Precipitation stations) should be located around the experimental area but preferably not within the catchment on which a change in vegetal structure is expected to occur." We regret having overlooked this paper, but welcome the opportunity to reemphasize Dr. Hewlett's recommendation and provide supporting documentation. REFERENCE Hewlett, J.D. (1970) Review of the catchment experiment to determine water yield. In: Proc. Joint FAO/U.S.S.R. Internat. Sympos. on Forest Influences and Watershed Management, FAO, August 17 - September 6, 1970, Moscow, U.S.S.R.

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