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Abstract Background Forest disturbance induced changes in the coupling of forest carbon and water have important implications for ecosystem functioning and sustainable forest management. However, this is rarely investigated at the large watershed scale with cumulative forest disturbance. We used a combination of techniques including modeling, statistical analysis, and machine learning to investigate the effects of cumulative forest disturbance on water use efficiency (WUE, a proxy for carbon and water coupling) in the 19,200 km2 Chilcotin watershed situated in the central interior of British Columbia, Canada. Harvesting, wildfire, and a severe Mountain Pine Beetle (MPB) infestation have gradually cumulated over the 45-year study period, and the watershed reached a cumulative equivalent clear-cut area of 10% in 1999 and then 40% in 2016. Results Surprisingly, with the dramatic forest disturbance increase from 2000 to 2016 which was mainly due to MPB, watershed-level carbon stocks and sequestration showed an insignificant reduction. This resilience was mainly due to landscape-level carbon dynamics that saw a balance between a variety of disturbance rates and types, an accumulation of older stand types, and fast growing young regenerated forests. Watershed-level carbon sequestration capacity was sustained, measured by Net Primary Production (NPP). A concurrent significant decrease in annual evapotranspiration (ET), led to a 19% increase in WUE (defined as the ratio of NPP to ET), which is contrary to common findings after disturbance at the forest stand-level. During this period of high disturbance, ET was the dominant driver of the WUE increase. Conclusions We conclude that disturbance-driven forest dynamics and the appropriate scale must be considered when investigating carbon and water relationship. In contrast to the stand-level trade-off relationship between carbon and water, forested watersheds may be managed to maintain timber, carbon and water resources across large landscapes.

Background: Forest disturbance induced changes in the coupling of forest carbon and water have important implications for ecosystem functioning and sustainable forest management. However, this is rarely investigated at the large watershed scale with cumulative forest disturbance. We used a combination of techniques including modeling, statistical analysis, and machine learning to investigate the effects of cumulative forest disturbance on water use efficiency (WUE, a proxy for carbon and water coupling) in the 19,200 km2 Chilcotin watershed situated in the central interior of British Columbia, Canada. Harvesting, wildfire, and a severe Mountain Pine Beetle (MPB) infestation have gradually cumulated over the 45-year study period, and the watershed reached a cumulative equivalent clear-cut area of 10% in 1999 and then 40% in 2016. Results: Surprisingly, with the dramatic forest disturbance increase from 2000 to 2016 which was mainly due to MPB, watershed-level carbon stocks and sequestration showed an insignificant reduction. This resilience was mainly due to landscape-level carbon dynamics that saw a balance between a variety of disturbance rates and types, an accumulation of older stand types, and fast growing young regenerated forests. Watershed-level carbon sequestration capacity was sustained, measured by Net Primary Production (NPP). A concurrent significant decrease in annual evapotranspiration (ET), led to a 19% increase in WUE (defined as the ratio of NPP to ET), which is contrary to common findings after disturbance at the forest stand-level. During this period of high disturbance, ET was the dominant driver of the WUE increase. Conclusions: We conclude that disturbance-driven forest dynamics and the appropriate scale must be considered when investigating carbon and water relationship. In contrast to the stand-level trade-off relationship between carbon and water, forested watersheds may be managed to maintain timber, carbon and water resources across large landscapes. [ABSTRACT FROM AUTHOR]

The relationship between peak specific discharge and watershed area is examined for large, rare and extreme floods in Malaysia using a distributed hydrological model. Envelope curves for specific discharge for these events and the uncertainty is quantified. The relationships between rainfall duration and intensity as a function of watershed size were also examined. As a result, three main regions were defined to estimate the peak discharge as a function of watershed size for large, extreme and rare floods. The average magnitudes for the PMP and the world's extreme rainfall events were 5 and 12 times larger than the 100-year event, respectively. The envelope curves may assist engineers and other interested parties to estimate the peak discharge for watershed up to 100,000 km2, especially for ungauged watersheds. [ABSTRACT FROM AUTHOR]

Understanding hydrological responses to reforestation is an important subject in watershed management, particularly in large forested watersheds (>1000 km²). In this study, we selected two large forested watersheds (Pingjiang and Xiangshui) located in the upper reach of the Poyang Lake watershed, Southeastern China (with an area of 3261.4 and 1458 km², respectively) to assess the effects of large-scale reforestation on streamflow. Both watersheds have similar climate and experienced comparable and dramatic forest changes during the past decades, but with contrasted watershed properties (e.g., the topography is much steeper in Xiangshui than in Pingjiang), which provides us with a unique opportunity to compare the differences in hydrological recovery in two contrasted watersheds. Streamflow at different percentiles (e.g., 5%, 10%, 50% and 95%) were compared using a combination of statistical analysis with year-wise method for each watershed. The results showed that forest recovery had no significant effects on median flows (Q50%) in both watersheds. However, reforestation significantly reduced high flows in Pingjiang, but had limited influence in Xiangshui. Similarly, reforestation had significant and positive effects on low flows (Q95%) in Pingjiang, while it did not significantly change low flows in Xiangshui. Thus, hydrological recovery is limited and slower in the steeper Xiangshui watershed, highlighting that watershed property is also important for determining hydrological responses to reforestation. This finding has important implications for designing reforestation and watershed management strategies in the context of hydrological recovery. [ABSTRACT FROM AUTHOR]

The distributed parameter and continuous simulation Annualized Agricultural Non-Point Source (AnnAGNPS) model was implemented in the watershed Anzur (Spain) covered by olive groves, to assess its prediction capability of surface runoff at the large watershed scale in semi-arid conditions. A 5-year database reporting hydrological, geomorphological and land use characteristics of the watershed allowed model implementation. Almost 180 surface runoff events were modelled by AnnAGNPS and compared with the corresponding observations through statistical indexes and grouping the runoff events in order to evaluate the model at different temporal scales (event, monthly and seasonal). AnnAGNPS evaluation showed that, in general, runoff was estimated by the default model with low accuracy at all the investigated time scales, likely, as a result of a simple representation of spatial variability. Calibration (by reducing initial curve numbers (CN) of the olive groves) provided more accurate and satisfactory predictions of event, monthly and seasonal runoff volumes with a low effort in the parameterisation approach. The best model performance was achieved at the event scale. The runoff prediction reliability may be attributable to the AnnAGNPS inaccuracy in adjusting CN values during the continuous simulation of the soil moisture conditions, because estimations of daily evapotranspiration values are quite realistic. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The Wenchuan earthquake has caused abundance of loose materials supplies for debris flows. Many debris flows have occurred in watersheds in area beyond 20 km, presenting characteristics differing from those in small watersheds. The debris flows yearly frequency decreases exponentially, and the average debris flow magnitude increases linearly with watershed size. The rainfall thresholds for debris flows in large watersheds were expressed as I = 14.7 D (2 h < D < 56 h), which is considerably higher than those in small watersheds as I = 4.4 D (2 h < D < 37 h). A case study is conducted in Ergou, 39.4 km in area, to illustrate the formation and development processes of debris flows in large watersheds. A debris flow develops in a large watershed only when the rainfall was high enough to trigger the wide-spread failures and erosions on slope and realize the confluence in the watershed. The debris flow was supplied by the widely distributed failures dominated by rill erosions (14 in 22 sources in this case). The intermittent supplying increased the size and duration of debris flow. While the landslide dam failures provided most amounts for debris flows (57 % of the total amount), and amplified the discharge suddenly. During these processes, the debris flow velocity and density increased as well. The similar processes were observed in other large watersheds, indicating this case is representative. [ABSTRACT FROM AUTHOR]

The combined use of water erosion models and geographic information systems has facilitated soil loss estimation at the watershed scale. Tools such as the Geo-spatial interface for the Water Erosion Prediction Project (GeoWEPP) model provide a convenient spatially distributed soil loss estimate but require discretization to identify hillslopes and channels. In GeoWEPP, the TOpographic PArameteriZation (TOPAZ) model is used as an automated procedure to extract a watershed boundary, hillslopes and channels from a digital elevation model (DEM). Previous studies in small watersheds have shown that the size of the hillslopes and the channel distribution affect the model estimates, but in large watersheds, the effects on the soil loss estimates have yet to be tested. Therefore, the objective of this study was to evaluate the effect of discretization on the hillslope sediment yield estimates using GeoWEPP in two large watersheds (>10 km2). The watersheds were selected and discretized varying the TOPAZ parameters [critical source area (CSA) and minimum source channel length (MSCL)] in a 30-m resolution digital elevation model. The drainage networks built with TOPAZ were compared with each other using the drainage density index. The results showed that the discretization affected hillslope sediment yield estimates and their spatial distribution more than the total runoff. The drainage density index and the hillslope sediment yield were proportional but inversely related; thus, soil loss estimates were highly affected by the spatial discretization. As a result of this analysis, a method to choose the CSA and MSCL values that generates the greatest fraction of hillslopes having profile lengths less than 200m was developed. This slope length condition is particularly crucial when using the WEPP and GeoWEPP models, in order for them to produce realistic estimates of sheet and rill erosion. Finally, and as a result of this analysis, a more reliable method was developed for selecting the TOPAZ channel network parameters (CSA and MSCL). [ABSTRACT FROM AUTHOR]

Flow regime refers to five elements of streamflow including flow magnitude, frequency, timing, duration and change rate. In spite of wide recognition of its critical significance in aquatic functions and ecosystem integrity, its responses to forest or land use change are rarely and quantitatively assessed. This paper used the Meijiang watershed (6983.2 km2), situated in the upper reach of the Poyang Lake basin, as an example to first demonstrate how flow regimes were altered by deforestation and then show if the altered flow regimes were possibly recovered by consequent reforestation. Two breakpoints (year 1968 and year 1985) with significant annual streamflow changes were detected, and they were then used to define three distinct periods including the reference or control period (1957 to 1967), deforestation (1968 to 1984) and reforestation (1985-2006). The paired year approach was then applied to quantitatively analyse the responses of flow regimes to forest cover changes. Both high flows (daily flows≧Q5%) and low flows (daily flows≦Q95%) were assessed. For high flows, the deforestation significantly increased the averaged magnitudes by 10.4%, increased the return periods (5-10 year) by 23.4%, advanced the averaged timings by 10.7 h and extended the averaged durations over the thresholds by 4 days. In contrast, reforestation delayed the averaged timings by 10.5 h, reduced the averaged duration by 5 days and decreased the averaged magnitude by 17.9%. Regarding low flows, the deforestation decreased the flow magnitudes by 30.1% with 30.5 day advancing in the average timings. To our surprise, however, low flows were not significantly changed by reforestation. All above results clearly demonstrate that flow regimes of both high and low flows were significantly altered by deforestation, and recovery of those alterations through reforestation may take much longer time than expected probably because of severe soil erosion and resultant loss of soil infiltration capacity after deforestation. [ABSTRACT FROM AUTHOR]

In forested watersheds, forest changes and climatic variability have been commonly recognized as two major drivers for streamflow variations. Previous research has separated their relative contributions but mainly focused on either deforestation and climate or reforestation and climate, but rarely with single studies on both. This study used the Meijiang watershed (6983·2 km2), situated in the upper reach of the Poyang Lake basin, as an example to quantify how climate and forest changes (both deforestation and reforestation) consecutively affect streamflow dynamics. Two methods, namely modified double-mass curves and sensitivity-based approach, were used in this study. Two breakpoints (years 1968 and 1985) with significant annual streamflow changes were detected, and together with the control period, they were then used to define three distinct periods: the control (1957-1967), deforestation (1968-1984) and reforestation (1985-2006) periods. Our results show that in the deforestation period, the average annual streamflow increment attributed to deforestation was 112·78 mm year−1, while the annual streamflow variation attributed to climate variability was −111·39 mm year−1. In the reforestation period, the average annual streamflow decrease caused by reforestation was −51·04 mm year−1, while the annual streamflow variation attributed to climate variability was 52·52 mm year−1. The sensitivity-based approach also provided similar results. The positive and negative values in the streamflow changes suggest offsetting effects between forest changes and climate variability in both deforestation and reforestation periods. The similar magnitudes of streamflow changes demonstrate that the hydrological effects of forest changes can be as great as those caused by climate change. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The long-term hydrological response to reforestation is critical to regional water management, especially in areas where large-scale reforestation has been practiced. In this study, we investigated the long-term hydrological response to reforestation in the 579 km2 basin in southeastern China through ground-based monitoring of water yield, sedimentation, vegetation cover and climate in the basin. The vegetation dynamics were also examined by remote sensing data (MSS, Landsat and AVHRR NDVI). We found that forest cover increased 23% or 13 593 ha from 1975 to 2002. Meanwhile, annual water yield decreased 86-88 mm from 1971-1983 (i.e. pre-reforestation) to 1984-2009 (i.e. post-reforestation). These decreases were significant statistically according to both regression and double mass analyses. Time series analysis demonstrated that there was a clear decline in annual sediment yield and an increase in annual evapotranspiration (ET) over the period from 1971 to 2009. We concluded that reforestation could significantly reduce annual water yield and sediment yield in the basin because of the forest cover change and forest growth. This conclusion is consistent with findings from widely paired-watershed studies and literatures published on the impact of reforestation in large watersheds. Our results also have important strategic implications and provide insight into more sustainable forest management practices for the future. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Most storm water numerical models assume that the entire watershed area is covered by the design storm and shall be considered as the tributary area to the design point. Care must be taken when simulating storm runoff generation from a large watershed because the rain storm may only cover a portion of the watershed. Because the area-averaged rainfall depth decays with respect to the storm-cover area, the experience of the larger the watershed, the higher the flood flow is no longer true. In this study, a storm centering technique is developed to identify the conservative size of a storm cell so that the design runoff rate and volume can be maximized among various locations of a storm center. Without any storm water detention in the watershed, the product of a tributary area and area-weighted rainfall depth serves as the indicator of runoff accumulation through the waterway. When taking detention basins into consideration, the effect of flow attenuation is converted into an equivalent tributary area that is used to maximize the runoff volume at the design point. This maximization procedure has been tested by the Lower Detention Basin designed and built in Las Vegas. The method is simple, but sensitive enough to identify the critical storm size for conservative designs. [ABSTRACT FROM AUTHOR]