Streamflow Response Research Articles

Streamflow response to headwater reforestation in the Ganaraska River basin, southern Ontario, Canada

AbstractIncreased attention is being paid to the role of reforestation in strategies for sustainable forest management, landscape restoration and carbon sequestration. Reforestation of drainage basins is generally assumed to reduce annual streamflow as well as peak and low flows. However, most studies of reforestation effects on streamflow have been conducted on small experimental basins, and the applicability of their results to larger basins is unclear. This study revisits an earlier examination of streamflow response to headwater reforestation in a 267 km2 basin (Ganaraska River) in southern Ontario, Canada for 1945–2007. Forest cover in the basin headwaters increased from 13 km2 in 1945 to 31 km2 by 1990, with most of this change between 1950 and 1965. Streamflow metrics from the reforested basin and two headwater sub‐basins were compared to those from a nearby basin and sub‐basin of similar size and physiography to the Ganaraska basins but without extensive headwater reforestation. No temporal trends were found for inter‐basin differences in annual runoff or runoff ratios for the entire Ganaraska basin or its largest sub‐basin; however, reforestation appears to have reduced several metrics of peak streamflow at the basin and sub‐basin scale. Relationships between high flows classified according to generating event type and the associated precipitation depth suggest that expansion of forest cover in the Ganaraska River basin (GRB) and associated changes in microclimatic conditions have reduced the potential for frozen soil to generate surface runoff and high flows in response to rain‐on‐snow during spring snowmelt. This was accompanied by increased low flows from the GRB and its sub‐basins from 1960 to 2007 relative to the control basin, suggesting that the expanded forest cover has enhanced groundwater recharge by prolonging spring snowmelt and promoting infiltration of rainfall and snowmelt inputs relative to non‐forested areas. Some discrepancies between these results and those of other studies of streamflow response to reforestation may be attributed to the relatively large size of the GRB and the limited extent of reforestation. Nevertheless, the results highlight the importance of considering how reforestation may affect key hydrologic processes in a given landscape when predicting the resulting streamflow response. Copyright © 2011 John Wiley & Sons, Ltd.

Hydrologic responses to climate change: considering geographic context and alternative hypotheses

Received 6 January 2011 Accepted 14 January 2011 One of the most significant consequences of climate warming is the likely change in streamflow as a result of warming air temperatures. Hydrologists have responded to the challenge of understanding these effects. Many recent studies quantify historical trends in streamflow and usually attribute these trends to climate warming, via altered evapotranspiration and snowpack (Figure 1.a). However, without questioning the fundamental reality of a warming climate, hydrologists should also consider biotic and social processes whose omission may produce misleading interpretations about climate change effects on hydrology. The aim of this commentary is to raise awareness of ecological and social processes that may confound the interpretation of climate effects on hydrology, to review how the geographic context of streamflow records affects interpretation of the climate signal, and to suggest a ‘checklist’ of working hypotheses that can be used to structure studies of streamflow responses to climate change. A wide variety of trends in streamflow have been detected and attributed to climate change and variability, but a few themes dominate the literature. The most common studies report earlier snowmelt, a shift to earlier streamflow timing, altered spring maximum flows, and/or intensified summer drought (Adam et al., 2009; Barnett et al., 2008; Brabets and Walvoord, 2009; Burn et al., 2010; Cuo et al., 2009; Hamlet et al., 2007; Hodgekins et al., 2003; Hodgekins and Dudley, 2006; Huntington et al., 2004; Jefferson et al., 2008; Knowles et al., 2006; Lee et al., 2004; Mote et al., 2003; Shepherd et al., 2010; Stewart et al., 2005; Stewart, 2009; Wilson et al., 2010; Xu et al., 2009). These studies focus on mountainous regions or near-polar latitudes of the Northern Hemisphere, and the relationships among warming, snowmelt, and streamflow vary with geographic location, elevation, and latitude. Another frequent finding is a trend of increased streamflow (annual, winter, and/or spring) associated with increased precipitation or temperature, or both (Andreadis and Lettenmaier, 2006; Birsan et al., 2005; Chen et al., 2006; Gautam et al., 2010; Johnston and Schmagin, 2008; Lins and Slack, 1999; Liu et al., 2010; Milliman et al., 2008; Peterson et al., 2002; St. George, 2007; Wilson et al., 2010; Xu et al., 2009; Zhang et al., 2001; Zhang and Schilling, 2006). The flow quantiles affected vary, with some studies reporting increased low flows (Liu et al., 2010) while others project increased flood risk (Allamano et al., 2009, but see Wilby et al., 2008). The climate-streamflow trend literature also contains considerable discussion of methods. Most studies use the Mann-Kendall non-parametric test (Hirsch and Slack, 1984; Helsel and Hirsch, 2002). Moreover, there is broad recognition that trends can be confounded with long-term climate cycles (Burn, 2008; Huntington et al., 2004; Lee et al., 2004; Marengo, 2009; St. George, 2007; Weider and Boutt, 2010; Woo et al., 2006) and that trends are sensitive to the start date of the record (e.g. Wilby et al., 2008). These latter issues are not addressed in this commentary.

Possible link between irrigation in the U.S. High Plains and increased summer streamflow in the Midwest

We have previously presented evidence that higher rates of evapotranspiration (ET) associated with irrigation in the U.S. High Plains has likely caused an increased downwind precipitation (P). July P over the Midwest increased by 20%–30% from the preirrigation period (1900–1950) to the postirrigation (1950–2000) period. In this study, we test the hypothesis that the increased July P has had hydrologic consequences, possibly increasing groundwater storage and streamflow. Seasonal analyses of hydrologic variables over Illinois suggest that the water table and streamflow response lags P − ET by 1–2 months, indicating August and September as the months when the increased July P may be detected. We analyzed long‐term observations of water table depth at 10 wells in Illinois and streamflow at 46 gauges in Illinois‐Ohio basins. The Mann‐Kendal test for trends suggests field significant increases in groundwater storage and streamflow in August–September over the period of irrigation expansion. Examination of soil moisture response to present‐day above‐normal July P suggests that the increased July P can reach the water table in normal to wet years. Mann‐Kendall tests suggest that there has been no change in pan evaporation and atmospheric vapor pressure deficit. This implies that soil water availability is the driver of changes in ET, and the increased P may have possibly increased ET. Other studies in the literature give further evidence of increased ET due to increased P. By ruling out a reduction in ET, we suggest that the observed increase in groundwater storage and streamflow in the Midwest is linked to the increased July precipitation attributed to High Plains irrigation. We note that the increases in late summer streamflow are rather small when placed in the context of seasonal dynamics, but they are conceptually important in that they point to a different cause of change.

Streamflow responses to past and projected future changes in climate at the Hubbard Brook Experimental Forest, New Hampshire, United States

Climate change has the potential to alter streamflow regimes, having ecological, economic, and societal implications. In the northeastern United States, it is unclear how climate change may affect surface water supply, which is of critical importance in this densely populated region. The objective of this study was to evaluate the impact of climate change on the timing and quantity of streamflow at small watersheds at the Hubbard Brook Experimental Forest in New Hampshire. The site is ideal for this analysis because of the availability of long‐term hydroclimatological records for analyzing past trends and ample data to parameterize and test hydrological models used to predict future trends. In this study, future streamflow projections were developed with the forest watershed model PnET‐BGC, driven by climate change scenarios from statistically downscaled outputs of atmospheric‐ocean general circulation models. Results indicated that earlier snowmelt and the diminishing snowpack is advancing the timing and reducing the magnitude of peak discharge associated with snowmelt. Past increases in precipitation have caused annual water yield to increase significantly, a trend that is expected to continue under future climate change. Significant declines in evapotranspiration have been observed over the long‐term record, although the cause has not been identified. In the future, evapotranspiration is expected to increase in response to a warmer and wetter environment. These increases in evapotranspiration largely offset increases in precipitation, resulting in relatively little change in streamflow. Future work should aim to decrease uncertainty in the climate projections, particularly for precipitation, obtain a better understanding of the effect of CO2 on vegetation, determine if climate‐induced changes in tree species composition will influence discharge, and assess the impacts of changing hydrology on downstream water supplies.

Vegetation impact on mean annual evapotranspiration at a global catchment scale

Research into the role of catchment vegetation within the hydrologic cycle has a long history in the hydrologic literature. Relationships between vegetation type and catchment evapotranspiration and runoff were primarily assessed through paired catchment studies during the 20th century. Results from over 200 paired catchment studies from around the world have been reported in the literature. Two constraints on utilizing the results from paired catchment studies in the wider domain have been that the catchment areas studied are generally (1) small (<10 km2) and (2) from a narrow range of climate types. The majority of reported paired catchment studies are located in the USA (∼47%) and Australia (∼27%) and experience mainly temperate (Köppen C) and cold (Köppen D) climate types. In this paper we assess the impact of vegetation type on mean annual evapotranspiration through a large, spatially, and climatically diverse data set of 699 catchments from around the world. These catchments are a subset of 861 unregulated catchments considered for the analysis. Spatially averaged precipitation and temperature data, in conjunction with runoff and land cover information, are analyzed to draw broad conclusions about the vegetation impact on mean annual evapotranspiration. In this analysis any vegetation impact signal is assessed through differences in long‐term catchment average actual evapotranspiration, defined as precipitation minus runoff, between catchments grouped by vegetation type. This methodology differs from paired catchment studies where vegetation impact is assessed through streamflow responses to a controlled, within catchment, land cover change. The importance of taking the climate type experienced by the catchments into account when assessing the vegetation impact on evapotranspiration is demonstrated. Tropical and temperate forested catchments are found to have statistically significant higher median evapotranspiration, by about 170 mm and 130 mm, respectively, than non‐forested catchments. Unexpectedly, cold forested catchments exhibit significantly lower median evapotranspiration, by about 90 mm, than non‐forested catchments. No significant difference was found between median evapotranspiration of temperate evergreen and deciduous forested catchments though sample sizes were small. Temperate evergreen needleleaf forested catchments were found to have significantly higher median evapotranspiration than evergreen broadleaf forested catchments though sample sizes were small. The significant temperate forest versus non‐forest difference in median evapotranspiration was found to persist for catchments with areas <1,000 km2, but not for catchments with areas ≥1,000 km2, which is consistent with the suggestion that the vegetation impact on evapotranspiration diminishes as catchment area increases. In summary, the results presented here are consistent with those drawn from reviews of paired catchment results. However, this paper demonstrates the value of a diverse hydroclimatic data set when assessing the vegetation impact on evapotranspiration as the magnitude of impact is observed to vary across climate types.

Streamflow dynamics in a Mediterranean temporary river

Mediterranean rivers are characterized by the irregularity of flow, harsh hydrological fluctuations and a profound transformation as the result of human activity. In this study, we investigate the streamflow response of a Mediterranean temporary river in which different groundwater, agriculture and urban contributions play an important role. Streamflow was measured at three nested gauging stations installed along Na Borges River, a lowland agricultural catchment (319 km2) on the island of Mallorca. Based on two hydrological years (2004/05 and 2005/06), potential evapotranspiration and surface water–groundwater interaction generated a succession of four different hydrological periods playing an important role in baseflow dynamics. The runoff coefficients were very low (<5%). At the event scale, groundwater also controlled runoff response, being very different according to hydrogeology, antecedent conditions and human impacts. During dry seasons, wastewater and karstic spring discharges maintain an influent regime into some streams. As a result, intense rainstorms in late summer generated water volumes over the impervious urban surfaces involved, with the result that quickflow was significant because the hydrological pathways were active. Citation Estrany, J., Garcia, C. & Alberich, R. (2010) Streamflow dynamics in a Mediterranean temporary river. Hydrol. Sci. J. 55(5), 717–736.

The long-term response of stream flow to climatic warming in headwater streams of interior AlaskaThis article is one of a selection of papers from The Dynamics of Change in Alaska’s Boreal Forests: Resilience and Vulnerability in Response to Climate Warming.

Warming in the boreal forest of interior Alaska will have fundamental impacts on stream ecosystems through changes in stream hydrology resulting from upslope loss of permafrost, alteration of availability of soil moisture, and the distribution of vegetation. We examined stream flow in three headwater streams of the Caribou–Poker Creeks Research Watershed (CPCRW) in interior Alaska over a 30-year period to determine (i) how stream flow varied among streams draining watersheds with varying extents of permafrost and (ii) evaluate if stream hydrology is changing with loss of permafrost. The three streams drained subcatchments with permafrost extents ranging from 4% to 53%. For each stream, runoff data were analyzed by separating base and storm flow contributions using a local-minimum method and with analysis of flood recession curves. Mean daily runoff during the ice-free season did not significantly vary among streams (mean = 0.57 mm·d–1), although the watersheds with lower permafrost had a greater contribution of base flow. Across years, flow was variable and was related with summer temperature in the watershed with low permafrost and with precipitation in the watershed with high permafrost. With climate warming and loss of permafrost, stream flows will become less responsive to precipitation and headwater streams may become ephemeral.

Streamflow response to fire in large catchments of a Mediterranean-climate region using paired-catchment experiments

Understanding post-fire streamflow dynamics in large California catchments is limited by a lack of direct empirical evidence. Scaling results from small experimental catchments to large catchments for practical applications is challenging. We investigated the possibility of using streamflow data from an existing gauge network in central and southern California to examine the effects of fire on streamflow using a paired-catchment approach. Post-fire streamflow change was examined in six paired catchments at annual, seasonal and monthly time-periods. Prediction intervals associated with the pre-fire calibration regression models were used to identify statistically significant changes in post-fire streamflow. The identification of suitable paired test and control catchments presented a major challenge, despite the large number of potential catchments in the network. The best calibration results were associated with catchment pairs that had similar orographic controls over rainfall, with proximity to one another being a secondary control. The effect of fires on streamflow, regardless of time-period examined, was found to be variable, depending mostly on post-fire wetness conditions. No relation was evident between post-fire streamflow change and catchment size or area burnt.

Streamflow response and sediment yield after farmland abandonment: results from a small experimental catchment in the Central Spanish Pyrenees

A small catchment affected by agricultural practices in the past, then progressively abandoned and naturally re-vegetated was monitored in the central Spanish Pyrenees. The results obtained over a 9-year period revealed the complexity of its hydrological and geomorphological behaviour. Several runoff generation processes can occur at the same time and in different parts of the catchment, depending on the water reserves conditions and rainfall characteristics. Sediment response is mainly controlled by the spatial and temporal dynamics of water and sediment contributing areas within the catchment. The sediment output illustrates the complexity of the geomorphic response of mountain environments modified by past farmland activities.

Assessing the impact of mixing assumptions on the estimation of streamwater mean residence time

AbstractCatchment streamwater mean residence time (Tmr) is an important descriptor of hydrological systems, reflecting their storage and flow pathway properties. Tmr is typically inferred from the composition of stable water isotopes (oxygen‐18 and deuterium) in observed rainfall and discharge. Currently, lumped parameter models based on convolution and sinewave functions are usually used for tracer simulation. These traditional models are based on simplistic assumptions that are often known to be unrealistic, in particular, steady flow conditions, linearity, complete mixing and others. However, the effect of these assumptions on Tmr estimation is seldom evaluated. In this article, we build a conceptual model that overcomes several assumptions made in traditional mixing models. Using data from the experimental Maimai catchment (New Zealand), we compare a complete‐mixing (CM) model, where rainfall water is assumed to mix completely and instantaneously with the total catchment storage, with a partial‐mixing (PM) model, where the tracer input is divided between an ‘active’ and a ‘dead’ storage compartment. We show that the inferred distribution of Tmr is strongly dependent on the treatment of mixing processes and flow pathways. The CM model returns estimates of Tmr that are well identifiable and are in general agreement with previous studies of the Maimai catchment. On the other hand, the PM model—motivated by a priori catchment insights—provides Tmr estimates that appear exceedingly large and highly uncertain. This suggests that water isotope composition measurements in rainfall and discharge alone may be insufficient for inferring Tmr. Given our model hypothesis, we also analysed the effect of different controls on Tmr. It was found that Tmr is controlled primarily by the storage properties of the catchment, rather than by the speed of streamflow response. This provides guidance on the type of information necessary to improve Tmr estimation. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Implications of Ensemble Quantitative Precipitation Forecast Errors on Distributed Streamflow Forecasting

Abstract Evaluating the propagation of errors associated with ensemble quantitative precipitation forecasts (QPFs) into the ensemble streamflow response is important to reduce uncertainty in operational flow forecasting. In this paper, a multifractal rainfall downscaling model is coupled with a fully distributed hydrological model to create, under controlled conditions, an extensive set of synthetic hydrometeorological events, assumed as observations. Subsequently, for each event, flood hindcasts are simulated by the hydrological model using three ensembles of QPFs—one reliable and the other two affected by different kinds of precipitation forecast errors—generated by the downscaling model. Two verification tools based on the verification rank histogram and the continuous ranked probability score are then used to evaluate the characteristics of the correspondent three sets of ensemble streamflow forecasts. Analyses indicate that the best forecast accuracy of the ensemble streamflows is obtained when the reliable ensemble QPFs are used. In addition, results underline (i) the importance of hindcasting to create an adequate set of data that span a wide range of hydrometeorological conditions and (ii) the sensitivity of the ensemble streamflow verification to the effects of basin initial conditions and the properties of the ensemble precipitation distributions. This study provides a contribution to the field of operational flow forecasting by highlighting a series of requirements and challenges that should be considered when hydrologic ensemble forecasts are evaluated.

Climate and terrain factors explaining streamflow response and recession in Australian catchments

Abstract. Daily streamflow data were analysed to assess which climate and terrain factors best explain streamflow response in 183 Australian catchments. Assessed descriptors of catchment response included the parameters of fitted baseflow models, and baseflow index (BFI), average quick flow and average baseflow derived by baseflow separation. The variation in response between catchments was compared with indicators of catchment climate, morphology, geology, soils and land use. Spatial coherence in the residual unexplained variation was investigated using semi-variogram techniques. A linear reservoir model (one parameter; recession coefficient) produced baseflow estimates as good as those obtained using a non-linear reservoir (two parameters) and for practical purposes was therefore considered an appropriate balance between simplicity and explanatory performance. About a third (27–34%) of the spatial variation in recession coefficients and BFI was explained by catchment climate indicators, with another 53% of variation being spatially correlated over distances of 100–150 km, probably indicative of substrate characteristics not captured by the available soil and geology data. The shortest recession half-times occurred in the driest catchments and were attributed to intermittent occurrence of fast-draining (possibly perched) groundwater. Most (70–84%) of the variation in average baseflow and quick flow was explained by rainfall and climate characteristics; another 20% of variation was spatially correlated over distances of 300–700 km, possibly reflecting a combination of terrain and climate factors. It is concluded that catchment streamflow response can be predicted quite well on the basis of catchment climate alone. The prediction of baseflow recession response should be improved further if relevant substrate properties were identified and measured.

Impacts of climate change/variability on the streamflow in the Yellow River Basin, China

Changes of streamflow reflect combined effects of climate, soil and vegetation in the basin scale. This study was conducted to investigate the response of streamflow to the climate changes/variability in different scales of the Yellow River Basin (YRB). The spatial distribution and temporal trends were explored for precipitation and potential evapotranspiration (PE) during 1961–2000 to illustrate climate change/variability and impacts of climate change/variability on streamflow were explained by investigating the relationship of precipitation, PE and streamflow in the YRB. The results presented that: (i) precipitation and PE exhibited different spatial distribution patterns and temporal trends in different regions, and most stations showed negative trends for precipitation in the basin; (ii) the relationship of streamflow with precipitation and PE showed high nonlinearity, and the magnitudes and patterns of streamflow response to precipitation and PE displayed different patterns varied with the dry conditions in different region or years; and (iii) the precipitation elasticity of streamflow ( ɛ P ) was 1.80, 1.08, 1.78 and 1.95 in Lanzhou, Toudaoguai, Huayuankou and Lijin respectively, while the PE elasticity of streamflow ( ɛ ET ) was −3.41, −4.40, −4.52 and −4.20 in above four scales, respectively, from which can be seen that streamflow was more sensitive to precipitation in wet region than in arid region and inversely it was more sensitive to PE in arid regions than in wet regions. Furthermore, precipitation elasticity of streamflow calculated from the partial correlation presented a reasonable result to show the combined effect of precipitation and PE on streamflow.

Streamflow response to climate variability and human activities in the upper catchment of the Yellow River Basin

Both sensitivity-based method and simulation method are used to analyze the streamflow response to climate variability and human activities in the upper catchment of the Yellow River Basin (UYRB) in this study. The separation regime of effects from climate variability and human activities is investigated. Results show that the changes of streamflow are more sensitive to precipitation than potential evapotranspiration (PET). Effect of climate variability on streamflow estimated using the sensitivity-based method is weak in the upper catchment of Jimai station, and strong in the upper catchment of Lanzhou station, where the climate effects accounted for about 50% of total streamflow changes. Effects of human activities on streamflow accounted for about 40% in the UYRB, with weaker effects in the upper catchment of Tangnaihai station than those in the upper catchment of Lanzhou station. Both climate variability and human activities are main factors to affect the changes of streamflow in the UYRB.

Simulating streamflow response to climate scenarios in central Canada using a simple statistical downscaling method

Despite the importance of surface water in Manitoba, Canada, there has been relatively little investigation of the impact of climate change on the hydrology of the province. This study exam- ines streamflow characteristics in northern Manitoba basins (Taylor and Burntwood River basins) under climate scenarios generated by global climate models (GCM) from 2 agencies (the UK Hadley Centre and the Canadian Centre for Climate Modelling and Analysis). The hydrological model SLURP (Semi-distributed Land Use-based Runoff Processes) was run with perturbed meteorological data based on the GCM simulations under 2 greenhouse gas emission scenarios (A2 and B2) for 2 future periods (2041-2070 and 2071-2099). A method to incorporate the changed variability in future temperature and precipitation to the climate scenarios was also tested. All scenarios project wetter and warmer climates, and simulation results show that the mean annual runoff is likely to increase in every scenario. The largest seasonal increase occurs in late spring and the least in winter and sum- mer. The daily flow exceeded by 80% of the total records is also projected to substantially increase, resulting in much fewer days with extreme low flow compared to the control period. Results indicate that the combination of a hydrological model with simple GCM-based scenarios can produce results comparable to those obtained from regional climate modelling. Explicit incorporation of changed variability in climate scenarios results in higher runoff than consideration of only changes in means.

Streamflow responses to vegetation manipulations along a gradient of precipitation in the Colorado River Basin

The Colorado River Basin has been, and continues to be, the focus of a wide diversity of research efforts to learn more about the effects of natural and human-induced disturbances on the processes and functioning of the basin's upland watersheds. These watersheds are situated at the headwaters of streams and rivers that supply much of the water to downstream users in the western United States. Responses of streamflow to vegetation manipulations have been, and are, one of the research foci in this water-deficient part of the country. The watershed-scale research, led by the U.S. Forest Service and its cooperators, has spanned nearly a century and included an array of vegetation types along a wide range of precipitation gradients. Results from this research have shown that vegetation can be managed to enhance annual water yields while still providing the other natural resource benefits. Analyses of the research results suggest that the effect of vegetation manipulation on streamflow is associated with precipitation–elevational gradient and, therefore, vegetation type. An annual water yield increase between 25 and 100 mm could be achieved by implementing vegetation manipulations in the high elevation subalpine and mixed conifer forests, the ponderosa pine forests (in the Lower Basin), and portions of the low elevation chaparral shrublands. Negligible effects or small increases in water yield were observed for treating sagebrush, pinyon-juniper woodland and desert scrubs. Results from this research have improved our understanding of the basin's hydrology and provided much needed insights to manage forest to mitigate global climate change induced hydrologic impact and meet the increased needs of people living in the basin.

Seasonal patterns of suspended sediment transport in an abandoned farmland catchment in the Central Spanish Pyrenees

AbstractThe suspended sediment response of a small catchment subjected to farmland abandonment and subsequent plant recolonization was studied in relation to its hydrological functioning. The analysis of data over a seven‐year period demonstrated that suspended sediment yield was greatly influenced by the occurrence of intense, low‐frequency events. Greater amounts of suspended sediment were exported during spring, when the catchment was hydrologically more active. Rainfall intensity and baseflow at the start of a flood event had a strong influence on the sediment response, suggesting that several hydrological processes were active within the catchment. SSC (suspended sediment concentration)‐Q hysteretic loop analysis at the event scale aided understanding of the sedimentological and hydrological behaviour of the catchment. During the study period the SSC‐Q loops showed a high degree of seasonality and two main patterns strongly related to catchment wetness were distinguished. When the catchment was dry (mainly during summer and the beginning of autumn) the predominant process was infiltration excess runoff over sparsely vegetated areas close to the main channel. Under these conditions, floods exhibited a counter‐clockwise hysteretic loop and were characterized by a small streamflow response, short duration and high SSC. Under wet conditions (mainly during winter and spring), saturation excess runoff was increasingly dominant over vegetated areas. Under these conditions, floods exhibited a clockwise hysteretic loop, and were characterized by a larger streamflow response, longer duration and higher suspended sediment yield. The lower SSC during the falling stage of the hydrograph is likely to be due to dilution effects related to the contribution of clean water resulting from enlargement of the saturated areas, together with an increase in the baseflow discharge. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Environmental Consequences of the Demise in Swidden Cultivation in Montane Mainland Southeast Asia: Hydrology and Geomorphology

The hydrological and geomorphological impacts of traditional swidden cultivation in Montane Mainland Southeast Asia are virtually inconsequential, whereas the impacts associated with intensified replacement agricultural systems are often much more substantial. Negative perceptions toward swiddening in general by governments in the region beginning half a decade ago have largely been based on cases of forest conversion and land degradation associated with (a) intensified swidden systems, characterized by shortened fallow and extended cropping periods and/or (b) the widespread cultivation of opium for cash after the Second World War. Neither of these practices should be viewed as traditional, subsistence-based swiddening. Other types of intensive agriculture systems are now replacing swiddening throughout the region, including semi-permanent and permanent cash cropping, monoculture plantations, and greenhouse complexes. The negative impacts associated with these systems include changes in streamflow response, increased surface erosion, a higher probability of landslides, and the declination in stream water quality. Unlike the case for traditional swiddening, these impacts result because of several factors: (1) large portions of upland catchments are cultivated simultaneously; (2) accelerated hydraulic and tillage erosion occurs on plots that are cultivated repetitively with limited or no fallowing to allow recovery of key soil properties, including infiltration; (3) concentrated overland flow and erosion sources are often directly connected with the stream network; (4) root strength is reduced on permanently converted hillslopes; (5) surface and ground water extraction is frequently used for irrigation; and (6) and pesticides and herbicides are used. Furthermore, the commercial success of these systems relies on the existence of dense networks of roads, which are linear landscape features renowned for disrupting hydrological and geomorphological systems. A new conservation focus is needed to reduce the impacts of these intensified upland agricultural practices.

Streamflow response to Pinus plantation harvesting: Canobolas State forest, southeastern Australia

AbstractStreamflows were measured in two Pinus radiata plantation catchments and one native eucalypt forest catchment in Canobolas State forest from 1999 to 2007. In 2002/2003, clearfall harvesting of 43·2 and 40·3% of two plantation catchments occurred, respectively. Water yields increased by 54 mm (52%), 71 mm (35%) and 50 mm (19%) in the first three years post‐harvest in treated catchment A and by 103 mm (118%), 157 mm (82%) and 119 mm (48%) in treated catchment B relative to the native forest control catchment. In the fourth post‐harvest water year annual rainfall was only 488 mm, which resulted in negligible run‐off in all catchments, regardless of forest cover. In both plantation catchments, monthly streamflows increased significantly (p = 0·01, p &lt; 0·001) due to a significant increase in baseflows (p &lt; 0·001) after harvesting. Monthly stormflows were not significantly affected by harvesting. Flow duration curve analyses indicated a variable response between the two plantation catchments. Treated catchment A was converted from an ephemeral stream flowing 42% of the time pre‐harvest to a temporary stream flowing 82% of the time post‐harvest. These changes occurred throughout all seasons of the year but were most pronounced during summer and autumn when baseflows were maintained post‐harvest but were not observed under native forest or mature pine plantations. By contrast, flow duration increased in treated catchment B from 12% of the time pre‐harvest to 38% of the time post‐harvest with the greatest changes measured during the winter and spring months when streamflow would normally occur under native forest conditions. These observations have important implications for the development of models of plantation water use to be utilized in water resource planning in Australia. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Streamflow Response to Climate and Landuse Changes in a Coastal Watershed in North Carolina

It is essential to examine the sensitivity of hydrologic responses to climate and landuse change across different physiographic regions in order to formulate sound water management policies for local response to projected global change. This study used the U.S. Geological Survey's Precipitation Runoff Modeling System (PRMS) model to examine the potential impacts of climate and landuse changes on the monthly streamflow of the Trent River basin on the lower coastal plain of eastern North Carolina. The model was first calibrated and then validated using measured, historic, long-term daily streamflow. The model performed satisfactorily for simulating monthly streamflow, as indicated by an overall Nash-Sutcliffe simulation efficiency greater than 0.85. We examined the sensitivity of streamflow to changes in air temperature and precipitation. The simulations suggested that streamflow of individual years could change from -93% to 238%, depending on the two global circulation model (GCM) scenarios used (i.e., HadCMSul2 and CGC1). Streamflow of the Trent River will decrease with an increase in air temperature, and increase (or decrease) with an increase (or decrease) in precipitation. Streamflow was more sensitive to prescribed changes in precipitation than to air temperature for the study area, given its high and stable evapotranspiration rates in the humid climatic environment. Seven hypothetical landuse change scenarios representing forest conversion to crop lands and urban areas indicated that water yield could increase by 14% to 20%. The likely impacts of landuse changes may not be as high as those caused by predicted changes in climate, but moderate urbanization and extreme hydrologic events caused by climate change could pose significant water quantity and quality problems in the Trent River basin.