@article{BartholdTurnerElsenbeeretal.2017, author = {Barthold, Frauke Katrin and Turner, Benjamin L. and Elsenbeer, Helmut and Zimmermann, Alexander}, title = {A hydrochemical approach to quantify the role of return flow in a surface flow-dominated catchment}, series = {Hydrological processes}, volume = {31}, journal = {Hydrological processes}, number = {5}, publisher = {Wiley}, address = {Hoboken}, issn = {0885-6087}, doi = {10.1002/hyp.11083}, pages = {1018 -- 1033}, year = {2017}, abstract = {Stormflow generation in headwater catchments dominated by subsurface flow has been studied extensively, yet catchments dominated by surface flow have received less attention. We addressed this by testing whether stormflow chemistry is controlled by either (a) the event-water signature of overland flow, or (b) the pre-event water signature of return flow. We used a high-resolution hydrochemical data set of stormflow and end-members of multiple storms in an end-member mixing analysis to determine the number of end-members needed to explain stormflow, characterize and identify potential end-members, calculate their contributions to stormflow, and develop a conceptual model of stormflow. The arrangement and relative positioning of end-members in stormflow mixing space suggest that saturation excess overland flow (26-48\%) and return flow from two different subsurface storage pools (17-53\%) are both similarly important for stormflow. These results suggest that pipes and fractures are important flow paths to rapidly release stored water and highlight the value of within-event resolution hydrochemical data to assess the full range and dynamics of flow paths.}, language = {en} } @article{FathizadehHosseiniZimmermannetal.2017, author = {Fathizadeh, O. and Hosseini, Seyed Mehrdad and Zimmermann, Alexander and Keim, R. F. and Boloorani, A. Darvishi}, title = {Estimating linkages between forest structural variables and rainfall interception parameters in semi-arid deciduous oak forest stands}, series = {The science of the total environment : an international journal for scientific research into the environment and its relationship with man}, volume = {601}, journal = {The science of the total environment : an international journal for scientific research into the environment and its relationship with man}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0048-9697}, doi = {10.1016/j.scitotenv.2017.05.233}, pages = {1824 -- 1837}, year = {2017}, abstract = {An understanding of the relationship between canopy structure and the water balance is needed for predicting how forest structure changes affect rainfall partitioning and, consequently, water resources. The objective of this study was to predict rainfall interception (I) and canopy storage capacity (S) using canopy structure variables and to investigate how seasonal changes influence their relationship. The study was conducted in twelve 50 m x 50 m plots in the Zagros forest in the western Iranian state of Ilam, protected forests of Dalab region. Average cumulative I was 84.2mm, accounting for 10.2\% of cumulative gross precipitation (GP) over a 1-year period. Using a regression based method, S averaged similar to 1 mm and 0.1 mm in the leafed and leafless periods, respectively. There were no relationships between tree density and I: GP or S, but I: GP and S increased with leaf area index, canopy cover fraction, basal area, tree height, and diameter at breast height in the leafed period. In addition, wood area index and canopy cover fraction were related to I: GP or S in the leafless period. (C) 2017 Elsevier B.V. All rights reserved.}, language = {en} } @article{VossZimmermannZimmermann2016, author = {Voss, Sebastian and Zimmermann, Beate and Zimmermann, Alexander}, title = {Detecting spatial structures in throughfall data: The effect of extent, sample size, sampling design, and variogram estimation method}, series = {Journal of hydrology}, volume = {540}, journal = {Journal of hydrology}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0022-1694}, doi = {10.1016/j.jhydrol.2016.06.042}, pages = {527 -- 537}, year = {2016}, abstract = {In the last decades, an increasing number of studies analyzed spatial patterns in throughfall by means of variograms. The estimation of the variogram from sample data requires an appropriate sampling scheme: most importantly, a large sample and a layout of sampling locations that often has to serve both variogram estimation and geostatistical prediction. While some recommendations on these aspects exist, they focus on Gaussian data and high ratios of the variogram range to the extent of the study area. However, many hydrological data, and throughfall data in particular, do not follow a Gaussian distribution. In this study, we examined the effect of extent, sample size, sampling design, and calculation method on variogram estimation of throughfall data. For our investigation, we first generated non Gaussian random fields based on throughfall data with large outliers. Subsequently, we sampled the fields with three extents (plots with edge lengths of 25 m, 50 m, and 100 m), four common sampling designs (two grid-based layouts, transect and random sampling) and five sample sizes (50, 100, 150, 200, 400). We then estimated the variogram parameters by method-of-moments (non-robust and robust estimators) and residual maximum likelihood. Our key findings are threefold. First, the choice of the extent has a substantial influence on the estimation of the variogram. A comparatively small ratio of the extent to the correlation length is beneficial for variogram estimation. Second, a combination of a minimum sample size of 150, a design that ensures the sampling of small distances and variogram estimation by residual maximum likelihood offers a good compromise between accuracy and efficiency. Third, studies relying on method-of-moments based variogram estimation may have to employ at least 200 sampling points for reliable variogram estimates. These suggested sample sizes exceed the number recommended by studies dealing with Gaussian data by up to 100 \%. Given that most previous through fall studies relied on method-of-moments variogram estimation and sample sizes <<200, currently available data are prone to large uncertainties. (C) 2016 Elsevier B.V. All rights reserved.}, language = {en} } @article{ZimmermannVossMetzgeretal.2016, author = {Zimmermann, Alexander and Voss, Sebastian and Metzger, Johanna Clara and Hildebrandt, Anke and Zimmermann, Beate}, title = {estimating mean throughfall}, series = {Journal of hydrology}, volume = {542}, journal = {Journal of hydrology}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0022-1694}, doi = {10.1016/j.jhydrol.2016.09.047}, pages = {781 -- 789}, year = {2016}, abstract = {The selection of an appropriate spatial extent of a sampling plot is one among several important decisions involved in planning a throughfall sampling scheme. In fact, the choice of the extent may determine whether or not a study can adequately characterize the hydrological fluxes of the studied ecosystem. Previous attempts to optimize throughfall sampling schemes focused on the selection of an appropriate sample size, support, and sampling design, while comparatively little attention has been given to the role of the extent. In this contribution, we investigated the influence of the extent on the representativeness of mean throughfall estimates for three forest ecosystems of varying stand structure. Our study is based on virtual sampling of simulated throughfall fields. We derived these fields from throughfall data sampled in a simply structured forest (young tropical forest) and two heterogeneous forests (old tropical forest, unmanaged mixed European beech forest). We then sampled the simulated throughfall fields with three common extents and various sample sizes for a range of events and for accumulated data. Our findings suggest that the size of the study area should be carefully adapted to the complexity of the system under study and to the required temporal resolution of the throughfall data (i.e. event-based versus accumulated). Generally, event-based sampling in complex structured forests (conditions that favor comparatively long autocorrelations in throughfall) requires the largest extents. For event-based sampling, the choice of an appropriate extent can be as important as using an adequate sample size. (C) 2016 Elsevier B.V. All rights reserved.}, language = {en} } @article{ZimmermannUberZimmermannetal.2015, author = {Zimmermann, Alexander and Uber, Magdalena and Zimmermann, Beate and Levia, Delphis F.}, title = {Predictability of stemflow in a species-rich tropical forest}, series = {Hydrological processes}, volume = {29}, journal = {Hydrological processes}, number = {23}, publisher = {Wiley-Blackwell}, address = {Hoboken}, issn = {0885-6087}, doi = {10.1002/hyp.10554}, pages = {4947 -- 4956}, year = {2015}, abstract = {Numerous studies investigated the influence of abiotic (meteorological conditions) and biotic factors (tree characteristics) on stemflow generation. Although these studies identified the variables that influence stemflow volumes in simply structured forests, the combination of tree characteristics that allows a robust prediction of stemflow volumes in species-rich forests is not well known. Many hydrological applications, however, require at least a rough estimate of stemflow volumes based on the characteristics of a forest stand. The need for robust predictions of stemflow motivated us to investigate the relationships between tree characteristics and stemflow volumes in a species-rich tropical forest located in central Panama. Based on a sampling setup consisting of ten rainfall collectors, 300 throughfall samplers and 60 stemflow collectors and cumulated data comprising 26 rain events, we derive three main findings. Firstly, stemflow represents a minor hydrological component in the studied 1-ha forest patch (1.0\% of cumulated rainfall). Secondly, in the studied species-rich forest, single tree characteristics are only weakly related to stemflow volumes. The influence of multiple tree parameters (e.g. crown diameter, presence of large epiphytes and inclination of branches) and the dependencies among these parameters require a multivariate approach to understand the generation of stemflow. Thirdly, predicting stemflow in species-rich forests based on tree parameters is a difficult task. Although our best model can capture the variation in stemflow to some degree, a critical validation reveals that the model cannot provide robust predictions of stemflow. A reanalysis of data from previous studies in species-rich forests corroborates this finding. Based on these results and considering that for most hydrological applications, stemflow is only one parameter among others to estimate, we advocate using the base model, i.e. the mean of the stemflow data, to quantify stemflow volumes for a given study area. Studies in species-rich forests that wish to obtain predictions of stemflow based on tree parameters probably need to conduct a much more extensive sampling than currently implemented by most studies. Copyright (c) 2015 John Wiley \& Sons, Ltd.}, language = {en} } @article{AichZimmermannElsenbeer2014, author = {Aich, Valentin and Zimmermann, Alexander and Elsenbeer, Helmut}, title = {Quantification and interpretation of suspended-sediment discharge hysteresis patterns: How much data do we need?}, series = {Catena : an interdisciplinary journal of soil science, hydrology, geomorphology focusing on geoecology and landscape evolution}, volume = {122}, journal = {Catena : an interdisciplinary journal of soil science, hydrology, geomorphology focusing on geoecology and landscape evolution}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0341-8162}, doi = {10.1016/j.catena.2014.06.020}, pages = {120 -- 129}, year = {2014}, abstract = {Sediment-discharge hysteresis loops are frequently analyzed to facilitate the understanding of sediment transport processes. Hysteresis patterns, however, are often complex and their interpretation can be complicated. Particularly, quantifying hysteresis patterns remains a problematic issue. Moreover, it is currently unknown how much data is required for analyzing sediment-discharge hysteresis loops in a given area. These open questions and challenges motivated us to develop a new method for quantifying suspended-sediment hysteresis. Subsequently, we applied the new hysteresis index to three suspended-sediment and discharge datasets from a small tropical rainforest catchment. The datasets comprised a different number of events and sampling sites. Our analyses show three main findings: (1) datasets restricted to only few events, which is typical for rapid assessment surveys, were always sufficient to identify the dominating hysteresis pattern in our research area. Furthermore, some of these small datasets contained multiple-peak events that allowed identifying intra-event exhaustion effects and hence, limitations in sediment supply. (2) Datasets comprising complete hydrological years were particularly useful for analyzing seasonal dynamics of hysteresis. These analyses revealed an exhaustion of hysteresis on the inter-event scale which also points to a limited sediment supply. (3) Datasets comprising measurements from two consecutive gauges installed at the catchment outlet and on a slope within that catchment allowed analyzing the change of hysteresis patterns along the flowpath. On the slope, multiple-peak events showed a stronger intra-event exhaustion of hysteresis than at the catchment outlet. Furthermore, exhaustion of hysteresis on the inter-event scale was not evident on the slope but occurred at the catchment outlet. Our results indicate that even small sediment datasets can provide valuable insights into sediment transport processes of small catchments. Furthermore, our results may serve as a first guideline on what to expect from an analysis of hysteresis patterns for datasets of varying quality and quantity. (c) 2014 Elsevier B.V. All rights reserved.}, language = {en} } @article{ZimmermannZimmermann2014, author = {Zimmermann, Alexander and Zimmermann, Beate}, title = {Requirements for throughfall monitoring: The roles of temporal scale and canopy complexity}, series = {Agricultural and forest meteorology}, volume = {189}, journal = {Agricultural and forest meteorology}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0168-1923}, doi = {10.1016/j.agrformet.2014.01.014}, pages = {125 -- 139}, year = {2014}, abstract = {A wide range of basic and applied problems in water resources research requires high-quality estimates of the spatial mean of throughfall. Many throughfall sampling schemes, however, are not optimally adapted to the system under study. The application of inappropriate sampling schemes may partly reflect the lack of generally applicable guidelines on throughfall sampling strategies. In this study we conducted virtual sampling experiments using simulated fields which are based on empirical throughfall data from three structurally distinct forests (a 12-year old teak plantation, a 5-year old young secondary forest, and a 130-year old secondary forest). In the virtual sampling experiments we assessed the relative error of mean throughfall estimates for 38 different throughfall sampling schemes comprising a variety of funnel- and trough-type collectors and a large range of sample sizes. Moreover, we tested the performance of each scheme for both event-based and accumulated throughfall data. The key findings of our study are threefold. First, as errors of mean throughfall estimates vary as a function of throughfall depth, the decision on which temporal scale (i.e. event-based versus accumulated data) to sample strongly influences the required sampling effort. Second, given a chosen temporal scale throughfall estimates can vary considerably as a function of canopy complexity. Accordingly, throughfall sampling in simply structured forests requires a comparatively modest effort, whereas heterogeneous forests can be extreme in terms of sampling requirements, particularly if the focus is on reliable data of small events. Third, the efficiency of trough-type collectors depends on the spatial structure of throughfall. Strong, long-ranging throughfall patterns decrease the efficiency of troughs substantially. Based on the results of our virtual sampling experiments, which we evaluated by applying two contrasting sampling approaches simultaneously, we derive readily applicable guidelines for throughfall monitoring. (C) 2014 Elsevier B.V. All rights reserved.}, language = {en} } @article{ZimmermannZimmermannTurneretal.2014, author = {Zimmermann, Beate and Zimmermann, Alexander and Turner, Benjamin L. and Francke, Till and Elsenbeer, Helmut}, title = {Connectivity of overland flow by drainage network expansion in a rain forest catchment}, series = {Water resources research}, volume = {50}, journal = {Water resources research}, number = {2}, publisher = {American Geophysical Union}, address = {Washington}, issn = {0043-1397}, doi = {10.1002/2012WR012660}, pages = {1457 -- 1473}, year = {2014}, abstract = {Soils in various places of the Panama Canal Watershed feature a low saturated hydraulic conductivity (K-s) at shallow depth, which promotes overland-flow generation and associated flashy catchment responses. In undisturbed forests of these areas, overland flow is concentrated in flow lines that extend the channel network and provide hydrological connectivity between hillslopes and streams. To understand the dynamics of overland-flow connectivity, as well as the impact of connectivity on catchment response, we studied an undisturbed headwater catchment by monitoring overland-flow occurrence in all flow lines and discharge, suspended sediment, and total phosphorus at the catchment outlet. We find that connectivity is strongly influenced by seasonal variation in antecedent wetness and can develop even under light rainfall conditions. Connectivity increased rapidly as rainfall frequency increased, eventually leading to full connectivity and surficial drainage of entire hillslopes. Connectivity was nonlinearly related to catchment response. However, additional information on factors such as overland-flow volume would be required to constrain relationships between connectivity, stormflow, and the export of suspended sediment and phosphorus. The effort to monitor those factors would be substantial, so we advocate applying the established links between rain event characteristics, drainage network expansion by flow lines, and catchment response for predictive modeling and catchment classification in forests of the Panama Canal Watershed and in similar regions elsewhere.}, language = {en} } @article{SchmidtZimmermannElsenbeer2014, author = {Schmidt, Lena Katharina and Zimmermann, Alexander and Elsenbeer, Helmut}, title = {Ant mounds as a source of sediment in a tropical rainforest?}, series = {Hydrological processes}, volume = {28}, journal = {Hydrological processes}, number = {13}, publisher = {Wiley-Blackwell}, address = {Hoboken}, issn = {0885-6087}, doi = {10.1002/hyp.10222}, pages = {4156 -- 4160}, year = {2014}, abstract = {In Lutzito catchment on Barro Colorado Island, Panama, extraordinarily high suspended-sediment yields of 1-2Mgha-1year-1 were generated despite the dense forest cover coinciding with erosion-resistant soils. We hypothesized that ant mounding activity is an important zoogeomorphological mechanism in this area, providing relevant quantities of easily transportable material at the soil surface. To test this hypothesis, all ant mound material was collected collected for dry mass determination from thirty 4m2 plots installed in the study area every 1-3days during the 39-day sampling period. Additionally, three ground-nesting ant species responsible for mounds in the study area, Ectatomma ruidum, Trachymyrmex cornetzi and Strumigenys marginiventris, were identified. On the basis of the total of 1.38kg of material collected in the wet season of 2011, the estimate for the whole 8months wet season amounts to 725kgha-1. As this value is in the same order of magnitude as sediment output, it shows that ants may act as important ecosystem engineers and contribute to sediment production here by providing large quantities of fine-grained, readily erodible material at the soil surface for subsequent transport to the streambed. Copyright (c) 2014 John Wiley \& Sons, Ltd.}, language = {en} } @article{ZimmermannZimmermannScheckenbachetal.2013, author = {Zimmermann, B. and Zimmermann, Alexander and Scheckenbach, H. L. and Schmid, T. and Hall, Jefferson. S. and van Breugel, Michiel}, title = {Changes in rainfall interception along a secondary forest succession gradient in lowland Panama}, series = {Hydrology and earth system sciences : HESS}, volume = {17}, journal = {Hydrology and earth system sciences : HESS}, number = {11}, publisher = {Copernicus}, address = {G{\"o}ttingen}, issn = {1027-5606}, doi = {10.5194/hess-17-4659-2013}, pages = {4659 -- 4670}, year = {2013}, abstract = {Secondary forests are rapidly expanding in tropical regions. Yet, despite the importance of understanding the hydrological consequences of land-cover dynamics, the relationship between forest succession and canopy interception is poorly understood. This lack of knowledge is unfortunate because rainfall interception plays an important role in regional water cycles and needs to be quantified for many modeling purposes. To help close this knowledge gap, we designed a throughfall monitoring study along a secondary succession gradient in a tropical forest region of Panama. The investigated gradient comprised 20 forest patches 3 to 130 yr old. We sampled each patch with a minimum of 20 funnel-type throughfall collectors over a continuous 2month period that had nearly 900 mm of rain. During the same period, we acquired forest inventory data and derived several forest structural attributes. We then applied simple and multiple regression models (Bayesian model averaging, BMA) and identified those vegetation parameters that had the strongest influence on the variation of canopy interception. Our analyses yielded three main findings. First, canopy interception changed rapidly during forest succession. After only a decade, throughfall volumes approached levels that are typical for mature forests. Second, a parsimonious (simple linear regression) model based on the ratio of the basal area of small stems to the total basal area outperformed more complex multivariate models (BMA approach). Third, based on complementary forest inventory data, we show that the influence of young secondary forests on interception in realworld fragmented landscapes might be detectable only in regions with a substantial fraction of young forests. Our re-sults suggest that where entire catchments undergo forest regrowth, initial stages of succession may be associated with a substantial decrease of streamflow generation. Our results further highlight the need to study hydrological processes in all forest succession stages, including early ones.}, language = {en} }