[1] Changes in vegetation cover can significantly affect streamflow. Two common methods for estimating vegetation effects on streamflow are the paired catchment method and the time trend analysis technique. In this study, the performance of these methods is evaluated using data from paired catchments in Australia, New Zealand, and South Africa. Results show that these methods generally yield consistent estimates of the vegetation effect, and most of the observed streamflow changes are attributable to vegetation change. These estimates are realistic and are supported by the vegetation history. The accuracy of the estimates, however, largely depends on the length of calibration periods or pretreatment periods. For catchments with short or no pretreatment periods, we find that statistically identified prechange periods can be used as calibration periods. Because streamflow also responds to climate variability, in assessing streamflow changes it is necessary to consider the effect of climate in addition to the effect of vegetation. Here, the climate effect on streamflow was estimated using a sensitivity-based method that calculates changes in rainfall and potential evaporation. A unifying conceptual framework, based on the assumption that climate and vegetation are the only drivers for streamflow changes, enables comparison of all three methods. It is shown that these methods provide consistent estimates of vegetation and climate effects on streamflow for the catchments considered. An advantage of the time trend analysis and sensitivity-based methods is that they are applicable to nonpaired catchments, making them potentially useful in large catchments undergoing vegetation change.

.[1] Plantation forestry is an increasingly important land use in Australia. While plantations can present significant environmental benefits such as salinity and erosion controls, their impact on streamflow needs to be recognized. This study evaluated the impacts of plantation and climate variability on streamflow from catchments in Australia. In total 15 catchments were selected based on the availability of streamflow, meteorological, and plantation information. The catchment areas range from 0.6 to 1136 km 2 and represent different climatic conditions and management practices. The Mann-Kendall test was used to estimate trends in the annual streamflow, rainfall, potential evaporation, and runoff ratio. The plantation effect on streamflow was determined with the time-trend analysis method, while the effect of climate variability was calculated using the sensitivity-based approach. All the selected catchments showed reductions in annual streamflow and runoff ratio over the period of the records. The results indicate that plantation expansions accounted for 28% to 106% of the observed total streamflow reductions, while climate variability was responsible for 5% to 80% of the total streamflow reductions. It is evident that both plantation expansions and climate variability contributed to the observed streamflow reductions. In most of the catchments, the effect of plantation expansion on streamflow was more important than that of climate variability and is likely to have exacerbated the effect of climate variability on streamflow. Plantation expansions can reduce streamflow from both small and large catchments as this study demonstrated and the relative streamflow reduction can be linearly related to percentage plantation area in a catchment.

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Abstract:On the basis of the mean air temperature, precipitation, sunshine duration and pan evaporation at 23 meteorological stations in the headwater catchment of the Yellow River basin from 1960 to 2001, the long-term monotonic trend and abrupt changes for major climate variables have been investigated. The plausible monotonic trend of annual climatic time series are detected using a non-parametric method. The abrupt changes have been investigated in terms of a 5 year moving averaged annual series, using the moving t-test (MTT) method, Yamamoto method and Mann-Kendall method. The results showed that the annual air temperature has increased by 0Ð80°C in the headwater catchment of the Yellow River basin during the past 42 years. One obvious cold period and one warm period were detected. The warmest centre was located in the northern part of the basin. The long-term trend for annual precipitation was not significant during the same period, but a dry tendency was detected. According to the Kendall slope values, the declining centre for annual precipitation was located in the eastern part and the centre of the study area. The long-term monotonic trend for annual sunshine duration and pan evaporation were negative. The average Kendall slopes are 29Ð96 h/10 yr and 39Ð63 mm/10 yr, respectively. The tests for abrupt changes using MTT and Yamamoto methods show similar results. Abrupt changes occurred in the mid 1980s for temperature, in the late 1980s for precipitation and in the early 1980s for sunshine duration and pan evaporation. It can be seen that the abrupt changes really happened in the 1980s for the climate variables. Different results are shown using the Mann-Kendall method. Both the abrupt changes of temperature and precipitation took place in the early 1990s, and that of pan evaporation occurred in the 1960s. The only abrupt change in sunshine duration happened during the similar period (in the 1980s) with the results detected by the MTT and Yamamoto methods. The abrupt changes which occurred in the 1990s and 1960s are not detectable using the MTT and Yamamoto methods because of the data limitation. However, the results tested by the MTT and Yamamoto methods exhibited great consistency. Some of the reasons may be due to the similar principles for these two methods. Different methods testing the abrupt climatic changes have their own merits and limitations and should be compared based on their own assumption and applicable conditions when they are used.