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Institute
Previous hydrometric studies demonstrated the prevalence of overland flow as a hydrological pathway in the tropical rain forest catchment of South Creek, northeast Queensland. The purpose of this study was to consider this information in a mixing analysis with the aim of identifying sources of, and of estimating their contribution to, storm flow during two events in February 1993. K and acid-neutralizing capacity (ANC) were used as tracers because they provided the best separation of the potential sources, saturation overland flow, soil water from depths of 0.3, 0.6, and 1.2 m, and hillslope groundwater in a two-dimensional mixing plot. It was necessary to distinguish between saturation overland flow, generated at the soil surface and following unchanneled pathways, and overland flow in incised pathways. This latter type of overland flow was a mixture of saturation overland flow (event water) with high concentrations of K and a low ANC, soil water (preevent water) with low concentrations of K and a low ANC, and groundwater (preevent water) with low concentrations of K and a high ANC. The same sources explained the streamwater chemistry during the two events with strongly differing rainfall and antecedent moisture conditions. The contribution of saturation overland flow dominated the storm flow during the first, high-intensity, 178-mm event, while the contribution of soil water reached 50% during peak flow of the second, low-intensity, 44-mm event 5 days later. This latter result is remarkably similar to soil water contributions to storm flow in mountainous forested catchments of the southeastern United States. In terms of event and preevent water the storm flow hydrograph of the high-intensity event is dominated by event water and that of the low-intensity event by preevent water. This study highlights the problems of applying mixing analyses to overland flow-dominated catchments and soil environments with a poorly developed vertical chemical zonation and emphasizes the need for independent hydrometric information for a complete characterization of watershed hydrology and chemistry.
A hydrochemical approach to quantify the role of return flow in a surface flow-dominated catchment
(2017)
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.
Investigations of spatial patterns in forest tree species composition are essential in the understanding of landscape dynamics, especially in areas of land-use change. The specific environmental factors controlling the present patterns, however, vary with the scale of observation. In this study we estimated abundance of adult trees and tree regeneration in a Southern Alpine valley in Ticino, Switzerland. We hypothesized that, at the present scale, spatial pattern of post-cultural tree species does not primarily depend on topographic features but responds instead to small- scale variation in historical land use. We used multivariate regression trees to relate species abundances to environmental variables. Species matrices were comprised of single tree species abundance as well as species groups. Groups were formed according to common ecological species requirements with respect to shade tolerance, soil moisture and soil nutrients. Though species variance could only be partially explained, a clear ranking in the relative importance of environmental variables emerged. Tree basal area of formerly cultivated Castanea sativa (Mill.) was the most important factor accounting for up to 50% of species' variation. Influence of topographic attributes was minor, restricted to profile curvature, and partly contradictory in response. Our results suggest the importance of biotic factors and soil properties for small-scale variation in tree species composition and need for further investigations in the study area on the ecological requirements of tree species in the early growing stage.
In undisturbed tropical montane rainforests massive organic layers accommodate the majority of roots and only a small fraction of roots penetrate the mineral soil. We investigated the contribution of vegetation to slope stability in such environments by modifying a standard model for slope stability to include an organic layer with distinct mechanical properties. The importance of individual model parameters was evaluated using detailed measurements of soil and vegetation properties to reproduce the observed depth of 11 shallow landslides in the Andes of southern Ecuador. By distinguishing mineral soil, organic layer and above-ground biomass, it is shown that in this environment vegetation provides a destabilizing effect mainly due to its contribution to the mass of the organic layer (up to 973 t ha-1 under wet conditions). Sensitivity analysis shows that the destabilizing effect of the mass of soil and vegetation can only be effective on slopes steeper than 37.9 degrees. This situation applies to 36% of the study area. Thus, on the steep slopes of this megadiverse ecosystem, the mass of the growing forest promotes landsliding, which in turn promotes a new cycle of succession. This feedback mechanism is worth consideration in further investigations of the impact of landslides on plant diversity in similar environments.
Spatio-temporal patterns of throughfall and solute deposition in an open tropical rain forest
(2008)
The influence of land-use changes on soil hydraulic properties : implications for runoff generation
(2006)
Phosphorus availability is commonly assumed to limit productivity in lowland tropical rainforests, yet there is relatively little information on the chemical forms of soil phosphorus in such ecosystems. We used the Hedley sequential fractionation scheme to assess phosphorus chemistry in five soils supporting tropical rainforest on Barro Colorado Island, Republic of Panama. The soils represented a range of orders (Inceptisols, Alfisols, and Oxisols) formed on contrasting geological substrates and topography, but under uniform climate and vegetation. Total phosphorus in surface horizons ranged between 315 and 1114 mg P kg(-1), being lowest on a soil derived from marine sediments and highest on soils derived from andesite. The majority of the phosphorus occurred in recalcitrant forms, although between 14% and 39% occurred as organic phosphorus. Readily-available phosphate, as extracted by anion-exchange membranes, occurred in small concentrations (4-13 mg P kg(-1)), although labile phosphorus, defined as phosphate extracted by anion-exchange membrane plus inorganic and organic phosphorus extracted by 0.5 M NaHCO3, accounted for between 4.7% and 11.4% of the total soil phosphorus. Our results indicate a strong control of geology and topography on soil phosphorus in tropical rainforests, which may have important implications for understanding the diversity and distribution of plant species in such ecosystems. Further, some of the most common soils on Barro Colorado Island, including those on the 50 ha forest dynamics plot, are rich in phosphorus despite their relatively advanced stage of pedogenesis.
The expansion and intensification of soya bean agriculture in southeastern Amazonia can alter watershed hydrology and biogeochemistry by changing the land cover, water balance and nutrient inputs. Several new insights on the responses of watershed hydrology and biogeochemistry to deforestation in Mato Grosso have emerged from recent intensive field campaigns in this region. Because of reduced evapotranspiration, total water export increases threefold to fourfold in soya bean watersheds compared with forest. However, the deep and highly permeable soils on the broad plateaus on which much of the soya bean cultivation has expanded buffer small soya bean watersheds against increased stormflows. Concentrations of nitrate and phosphate do not differ between forest or soya bean watersheds because fixation of phosphorus fertilizer by iron and aluminium oxides and anion exchange of nitrate in deep soils restrict nutrient movement. Despite resistance to biogeochemical change, streams in soya bean watersheds have higher temperatures caused by impoundments and reduction of bordering riparian forest. In larger rivers, increased water flow, current velocities and sediment flux following deforestation can reshape stream morphology, suggesting that cumulative impacts of deforestation in small watersheds will occur at larger scales.
Prolonged dry periods, and increasingly the generation of smoke and dust in partially-deforested regions, can influence the chemistry of rainfall and throughfall in moist tropical forests. We investigated rainfall and throughfall chemistry in a palm-rich open tropical rainforest in the southwestern Brazilian Amazon state of Rondonia, where precipitation averages 2300 mm year(-1) with a marked seasonal pattern, and where the fragmentation of remaining forest is severe. Covering the transition from dry to wet season (TDWS) and the wet season (WS) of 2004-2005, we sampled 42 rainfall events on event basis as well as 35 events on a within-event basis, and measured concentrations of DOC, Na+, K+, Ca2+, Mg2+, NH4+ , Cl-, SO42- , NO3- and pH in rainfall and throughfall. We found strong evidence of both seasonal and within-event solute rainfall concentration dynamics. Seasonal volume-weighted mean (VWMS) concentrations in rainfall of DOC, K+, Ca2+, Mg2+, NH4+ , SO42- and NO3- were significantly higher in the TDWS than the WS, while VWMS concentrations in throughfall were significantly higher for all solutes except DOC. Patterns were generally similar within rain events, with solute concentrations declining sharply during the first few millimeters of rainfall. Rainfall and throughfall chemistry dynamics appeared to be strongly influenced by forest and pasture burning and a regional atmosphere rich in aerosols at the end of the dry season. These seasonal and within-event patterns of rainfall and throughfall chemistry were stronger than those recorded in central Amazonia, where the dry season is less pronounced and where regional deforestation is less severe. Fragmentation and fire in Rondonia now appear to be altering the patterns in which solutes are delivered to remaining moist tropical forests.
The investigation of throughfall patterns has received considerable interest over the last decades. And yet, the geographical bias of pertinent previous studies and their methodologies and approaches to data analysis cast a doubt on the general validity of claims regarding spatial and temporal patterns of throughfall. We employed 220 collectors in a 1-ha plot of semideciduous tropical rain forest in Panama and sampled throughfall during a period of 14 months. Our analysis of spatial patterns is based on 60 data sets, whereas the temporal analysis comprises 91 events. Both data sets show skewed frequency distributions. When skewness arises from large outliers, the classical, nonrobust variogram estimator overestimates the sill variance and, in some cases, even induces spurious autocorrelation structures. In these situations, robust variogram estimation techniques offer a solution. Throughfall in our plot typically displayed no or only weak spatial autocorrelations. In contrast, temporal correlations were strong, that is, wet and dry locations persisted over consecutive wet seasons. Interestingly, seasonality and hence deciduousness had no influence on spatial and temporal patterns. We argue that if throughfall patterns are to have any explanatory power with respect to patterns of near-surface processes, data analytical artifacts must be ruled out lest spurious correlation be confounded with causality; furthermore, temporal stability over the domain of interest is essential.
Soils play a crucial role in biogeochemical cycles as spatially distributed sources and sinks of nutrients. Any spatial patterns depend on soil forming processes, our understanding of which is still limited, especially in regards to tropical rainforests. The objective of our study was to investigate the effects of landscape properties, with an emphasis on the geometry of the land surface, on the spatial heterogeneity of soil chemical properties, and to test the suitability of soil-landscape modeling as an appropriate technique to predict the spatial variability of exchangeable K and Mg in a humid tropical forest in Panama. We used a design-based, stratified sampling scheme to collect soil samples at 108 sites on Barro Colorado Island, Panama. Stratifying variables are lithology, vegetation and topography. Topographic variables were generated from high-resolution digital elevation models with a grid size of 5 m. We took samples from five depths down to I m, and analyzed for total and exchangeable K and Mg. We used simple explorative data analysis techniques to elucidate the importance of lithology for soil total and exchangeable K and Mg. Classification and Regression Trees (CART) were adopted to investigate importance of topography, lithology and vegetation for the spatial distribution of exchangeable K and Mg and with the intention to develop models that regionalize the point observations using digital terrain data as explanatory variables. Our results suggest that topography and vegetation do not control the spatial distribution of the selected soil chemical properties at a landscape scale and lithology is important to some degree. Exchangeable K is distributed equally across the study area indicating that other than landscape processes, e.g. biogeochemical processes, are responsible for its spatial distribution. Lithology contributes to the spatial variation of exchangeable Mg but controlling variables could not be detected. The spatial variation of soil total K and Mg is mainly influenced by lithology.
Changes in soil hydraulic properties following ecosystem disturbances can become relevant for regional water cycles depending on the prevailing rainfall regime. In a tropical montane rainforest ecosystem in southern Ecuador, plot- scale investigations revealed that man-made disturbances were accompanied by a decrease in mean saturated hydraulic conductivity (Ks), whereas mean Ks of two different aged landslides was undistinguishable from the reference forest. Ks spatial structure weakened after disturbances in the topsoil. We used this spatial-temporal information combined with local rain intensities to assess the probability of impermeable soil layers under undisturbed, disturbed, and regenerating land-cover types. We furthermore compared the Ecuadorian man-made disturbance cycle with a similar land-use sequence in a tropical lowland rainforest region in Brazil. The studied montane rainforest is characterized by prevailing vertical flowpaths in the topsoil, whereas larger rainstorms in the study area potentially result in impermeable layers below 20 cm depth. In spite of the low frequency of such higher-intensity events, they transport a high portion of the annual runoff and may therefore significant for the regional water cycle. Hydrological flowpaths under two studied landslides are similar to the natural forest except for a somewhat higher probability of impermeable layer formation in the topsoil of the 2-year-old landslide. In contrast, human disturbances likely affect near-surface hydrology. Under a pasture and a young fallow, impermeable layers potentially develop already in the topsoil for larger rain events. A 10-year-old fallow indicates regeneration towards the original vertical flowpaths, though the land-use signal was still detectable. The consequences of land-cover change on near-surface hydrological behaviour are of similar magnitude in the tropical montane and the lowland rainforest region. This similarity can be explained by a more pronounced drop of soil permeability after pasture establishment in the montane rainforest region in spite of the prevailing much lower rain intensities.
[1] Observations of hydrological response often exhibit considerable scatter that is difficult to interpret. In this paper, we examine runoff production of 53 sprinkling experiments on the water-repellent soils in the southern Alps of Switzerland; simulated plot scale tracer transport in the macroporous soils at the Weiherbach site, Germany; and runoff generation data from the 2.3-km(2) Tannhausen catchment, Germany, that has cracking soils. The response at the three sites is highly dependent on the initial soil moisture state as a result of the threshold dynamics of the systems. A simple statistical model of threshold behavior is proposed to help interpret the scatter in the observations. Specifically, the model portrays how the inherent macrostate uncertainty of initial soil moisture translates into the scatter of the observed system response. The statistical model is then used to explore the asymptotic pattern of predictability when increasing the number of observations, which is normally not possible in a field study. Although the physical and chemical mechanisms of the processes at the three sites are different, the predictability patterns are remarkably similar. Predictability is smallest when the system state is close to the threshold and increases as the system state moves away from it. There is inherent uncertainty in the response data that is not measurement error but is related to the observability of the initial conditions.
Influence of land-use change on near-surface hydrological processes : undisturbed forest to pasture
(2010)
Soil compaction that follows the clearing of tropical forest for cattle pasture is associated with lower soil hydraulic conductivity and increased frequency and volume of overland flow. We investigated the frequency of perched water tables, overland flow and stormflow in an Amazon forest and in an adjacent 25-year-old pasture cleared from the same forest. We compared the results with the frequencies of these phenomena estimated from comparisons of rainfall intensity and soil hydraulic conductivity. The frequency of perched water tables based on rainfall intensity and soil hydraulic conductivity was expected to double in pasture compared with forest. This corresponded closely with an approximate doubling of the frequency of stormflow and overland flow in pasture. In contrast, the stormflow volume in pasture increased 17-fold. This disproportional increase of stormflow resulted from overland flow generation over large areas of pasture, while overland flow generation in the forest was spatially limited and was observed only very near the stream channel. In both catchments, stormflow was generated by saturation excess because of perched water tables and near-surface groundwater levels. Stormflow was occasionally generated in the forest by rapid return flow from macropores, while slow return flow from a continuous perched water table was more common in the pasture. These results suggest that deforestation for pasture alters fundamental mechanisms of stormflow generation and may increase runoff volumes over wide regions of Amazonia.
The Amazon Basin is the world's largest tropical forest region and one where rapid human changes to land cover have the potential to cause significant changes to hydrological and biogeochemical processes. The Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA) is a multidisciplinary, multinational research program led by Brazil. The goal of LBA is to understand how the Amazon Basin functions as a regional entity in the earth system and how these functions are changing as a result of ongoing human activity. This compilation of nine papers focuses on a central LBA question in the area of nutrient dynamics and surface water chemistry-how do changes in land use alter fluxes of dissolved and particulate materials from uplands across riparian zones and down the channels of river corridors? These papers cover work conducted in small watersheds on a wide range of topics within the spirit and geographical focus area of LBA: water balance and runoff generation, nutrient transformations in riparian zones and stream channels, carbon fluxes in water moving from land to water and the influence of soils on flowpath structure and stream chemistry. Important new insights can be gained from these and other studies. Forest clearing for pastures results in a decrease in soil hydraulic conductivity that forces water into surficial flowpaths throughout most of the rainy season across wide regions of the Amazon. Riparian zones along small forest streams appear to be very effective in removing nitrate arriving from the uplands, while forest streams take up nitrate at very low rates, allowing them to travel downstream for long distances. Although substantial, the contribution of dissolved organic C (DOC) to the carbon flux from forests to streams appears to be lower than the flux of dissolved inorganic C that is subsequently outgassed as CO2. Remaining key challenges within LBA will be to synthesize existing data sets on river networks, soils, climate, land use and planned infrastructure for the Amazon to develop models capable of predicting hydrologic and biogeochemical fluxes at a variety of scales relevant to the development of strategies for sustainable management of the Amazon's remarkable forest, soil and freshwater resources.
Nitrogen transformations in flowpaths leading from soils to streams in Amazon forest and pasture
(2009)
The replacement of undisturbed tropical forest with cattle pasture has the potential to greatly modify the hydrology of small watersheds and the fluxes of solutes. We examined the fluxes of water, Cl-, NO3--N: SO42---S, NH4+-N, Na+, K+, Mg2+ and Ca2+ in different flow paths in similar to 1 ha catchments of undisturbed open tropical rainforest and a 20 year-old pasture established from forest in the southwestern Brazilian Amazon state of Rondonia. Storm flow discharge was 18% of incident rainfall in pasture, but only 1% in forest. Quickflow predominated over baseflow in both catchments and in both wet and dry seasons. In the pasture, groundwater and quickflow were important flow paths for the export of all solutes. In the forest, quickflow was important for NO3--N export, but all other solutes were exported primarily by groundwater outflow. Both catchments were sinks for SO42--S and Ca2+, and sources of Na+. The pasture catchment also lost K+ and Mg2+ because of higher overland flow frequency and volume and to cattle excrement. These results show that forest clearing dramatically influences small watershed hydrology by increasing quickflow and water export to streams. They also indicate that tropical forest watersheds are highly conservative for most solutes but that pastures continue to lose important cations even decades after deforestation and pasture establishment.
Disproportionate single-species contribution to canopy-soil nutrient flux in an Amazonian rainforest
(2012)
Rainfall, throughfall and stemflow were monitored on an event basis in an undisturbed open tropical rainforest with a large number of palm trees located in the southwestern Amazon basin of Brazil. Stemflow samples were collected from 24 trees with a diameter at breast height (DBH) > 5 cm, as well as eight young and four full-grown babassu palms (Attalea speciosa Mart.) for 5 weeks during the peak of the wet season. We calculated rainfall, throughfall and stemflow concentrations and fluxes of Na+, K+, Ca2+, Mg2+,, Cl-, SO42-, NO3- and H+ and stemflow volume-weighted mean concentrations and fluxes for three size classes of broadleaf trees and three size classes of palms. The concentrations of most solutes were higher in stemflow than in rainfall and increased with increasing tree and palm size. Concentration enrichments from rainfall to stemflow and throughfall were particularly high (81-fold) for NO3-. Stemflow fluxes of NO3- and H+ exceeded throughfall fluxes but stemflow fluxes of other solutes were less than throughfall fluxes. Stemflow solute fluxes to the forest soil were dominated by fluxes on babassu palms, which represented only 4% of total stem number and 10% of total basal area. For NO3-, stemflow contributed 51% of the total mass of nitrogen delivered to the forest floor (stemflow + throughfall) and represented more than a 2000-fold increase in NO3- flux compared what would have been delivered by rainfall alone on the equivalent area. Because these highly localized fluxes of both water and NO3- persist in time and space, they have the potential to affect patterns of soil moisture, microbial populations and other features of soil biogeochemistry conducive to the creation of hotspots for nitrogen leaching and denitrification, which could amount to an important fraction of total ecosystem fluxes. Because these hotspots occur over very small areas, they have likely gone undetected in previous studies and need to be considered as an important feature of the biogeochemistry of palm-rich tropical forest.