Small Forested Catchment Research Articles

Investigating short-term dynamics and long-term trends of SO 4 in the runoff of a forested catchment using artificial neural networks

The impact of long-lasting non-point emissions on groundwater and streamwater in remote watersheds has been studied at numerous sites. In spite of substantially decreasing emissions in the last decade, recovery has not yet been observed in all cases. This trend might be masked by the considerable short-term variability of the chemical hydrographs. In this study, artificial neural networks are applied to investigate the SO 4 dynamics in the runoff of a small forested catchment susceptible to SO 4 deposition. Empirical models are fitted to the short-term dynamics at a time step of one day. About 75% of the variance of the SO 4 data is explained by the instantaneous discharge, short-term history of discharge and the moving average of SO 4 concentration in throughfall. In contrast, neither air temperature as an indicator for biological activity nor a snowmelt indicator based on the temperature sum increase the performance of the model. The model is used to investigate long-term trends in sub-regions of the phase space spanned by the identified input variables. According to the model, decreasing emissions have a significant effect on runoff SO 4 concentration only during the first severe storms at the end of the vegetation period. This suggests to focus on these events as indicators for recovery of the topsoil layers.

Investigating trends of hydrochemical time series of small catchments by artificial neural networks

Abstract The short-term variation of discharge and solute concentration of the runoff of small catchments generally reflects the interplay of a variety of different processes. This makes the investigation of anthropogenic impacts on the catchment's runoff often rather difficult. On the other hand, short-term dynamics at the output boundary provide information about the system. This information can be used, in principle at least, to assess its long-term behaviour more precisely. In this paper examples of time series of sulphate and nitrate in the runoff of two small forested catchments are presented. To minimise the danger of over-parametrisation, the objective was to find a very simple empirical model to map a substantial portion of the observed variance (daily values). Here artificial neural networks were applied. They yield an efficiency of more than 0.7 for the solutes investigated, based on discharge depth and air temperature as input variables only. As a next step, the invariance of these relationships was investigated. In the case of sulphate, a significant trend is observed. However, it differs considerably for different subregions of the regression plane. Thus the neural network approach reveals a much more detailed insight into temporal shifts of the dynamics than an overall trend analysis.

Impact of ditching in a small forested catchment on concentrations of suspended material, organic carbon, hydrogen ions and metals in stream water

Stream-water samples were collected during a 4-year-period in twosmall streams, one whose catchment was ditched for forestry halfway through the sampling period and another nearby reference stream whose catchment was not ditched during this period. The main aim was to study the impact of forest ditching on stream-water quality. Whereas the artificial drainage did not change the hydrograph pattern, it had a large effect on stream hydrochemistry: the concentrations of suspended material, Mn, Ca, Mg and Al increased, theconcentrations of total organic carbon decreased, and pH increased by approximately one unit, from an average of 4.4 to 5.4. The increase in suspended material, Mn and Al concentrations is explainedby the physical mobilisation of mineral particulates/colloids from mineral soils (till) exposed on the ditch slopes beneath the peat layer, while the increase in Ca and Mg loads is explained by the release of Ca2+ and Mg2+ in exchange reactions in this same soil layer. The increase in pH and decrease in TOC concentrations after ditching are related to changes in hydrological flow paths in the catchments, and most likely to immobilisation of both hydrogen ions and humic substances in the near neutral till horizon exposed beneath the peat layer. Only the aquatic abundance of Fewas not significantly affected by the ditching.

Recovery of15N-labelled nitrate applied to a small upland boreal forest catchment

Nitrate labelled with15N was added for 2 years to a small Boreal Shield forested catchment to evaluate where new nitrogen (N) inputs are stored in this system in the short term. The landscape in the study catchment is a mosaic of Pinus banksiana Lamb. - Picea mariana (Mill.) BSP "forest islands" surrounded by "pillows" of lichens, mosses, and grasses on exposed bedrock surfaces. After the second year of N addition, 10% of the added15N was found in aboveground biomass, 27% in organic horizons, 6% in forest island mineral soil, 3% in "soil" underneath pillows, and 16% was lost through runoff. Despite covering a smaller proportion of the catchment and having only slightly larger N stocks, forest islands stored more15N (31%) than pillows (15%). Preferential routing of water towards forest islands and a greater potential for biotic and abiotic N immobilization are hypothesized to favor N retention in forest islands. Thirty-eight percent of the15N could not be accounted for. The comparison of a mass-balance analysis with the15N recovery suggests a missing sink for the15N in forest islands, possibly woody detritus. In the short term (~2 years), organic horizons are the main sinks for elevated N inputs in the boreal forest.

Recovery of <sup>15</sup>N-labelled nitrate applied to a small upland boreal forest catchment

Nitrate labelled with 15N was added for 2 years to a small Boreal Shield forested catchment to evaluate where new nitrogen (N) inputs are stored in this system in the short term. The landscape in the study catchment is a mosaic of Pinus banksiana Lamb. - Picea mariana (Mill.) BSP surrounded by of lichens, mosses, and grasses on exposed bedrock surfaces. After the second year of N addition, 10% of the added 15N was found in aboveground biomass, 27% in organic horizons, 6% in forest island mineral soil, 3% in soil underneath pillows, and 16% was lost through runoff. Despite covering a smaller proportion of the catchment and having only slightly larger N stocks, forest islands stored more 15N (31%) than pillows (15%). Preferential routing of water towards forest islands and a greater potential for biotic and abiotic N immobilization are hypothesized to favor N retention in forest islands. Thirty-eight percent of the 15N could not be accounted for. The comparison of a mass-balance...

Hierarchical testing of three rainfall–runoff models in small forested catchments

This study examined whether the quasi-distributed response function used in TOPMODEL provides superior performance for event simulation in small, temperate forested catchments, compared to lumped reservoir representations of runoff routing similar to those employed in many catchment hydrology models. The alternatives were a two-reservoir black-box model and a three-reservoir model structured to represent our perceptual model of runoff processes based on field observations. A second objective was to test the statistical significance of differences in model performance using a new approach that combines the Jackknife with analysis of variance (ANOVA). The models were tested against streamflow data from two small forested catchments using Klemes’ hierarchical scheme for operational validation. At levels 1 (split-sample test) and 2 (proxy-basin test) there were no statistically significant differences in model performance when expressed using the entire hydrographs. However, TOPMODEL did appear to provide superior fits to the peak flows. At level 3 (differential split-sample test), TOPMODEL performed statistically significantly better than both lumped models in both catchments. The statistical analysis for level 4 (proxy-basin differential split-sample test) indicates that TOPMODEL performed significantly better than the lumped models at one catchment but not at the other. The statistical design combining ANOVA with the Jackknife shows promise as a workable method for establishing statistical significance of differences in model performance indices based on the entire hydrograph. However, performance indices based only on the peak flows exhibited extreme skew that was not amenable to normalization by transformation. Further research should investigate alternative, robust methods for assessing statistical significance of model performance indices.

The CLIMEX soil-heating experiment: soil response after 2 years of treatment

Most model predictions concerning the response of boreal forest ecosystems to climate change are inferred from small-scale experiments on artificial, simplified systems. Whole-ecosystem experiments designed to validate these models are scarce. We experimentally manipulated a small forested catchment in southern Norway by increasing soil temperature (+3 °C in summer to +5 °C in winter) using heating cables installed at 1 cm depth in the litter layer. Especially nitrification in the 0 to 10-cm soil layer increased as a result of the climate manipulation. Betula litter, produced after exposing trees for 2 years to ambient and elevated CO2 in greenhouses, was incubated for 1 year in the manipulated catchment. Exposure to elevated CO2 did not affect the C/N ratio or decomposition of the Betula litter, but lignin content decreased by 10%. We found no effect of elevated temperature on litter decomposition, probably due to desiccation of the litter. The heating cables caused a permanent increase in soil temperature in this soil layer, but when soils were dry, the temperature difference between control and heated plots decreased with increasing distance from the cables. When soils were wet, no gradients in temperature increase occurred.

Comparison of hydrochemical tracers to estimate source contributions to peak flow in a small, forested, headwater catchment

Three‐component (throughfall, soil water, groundwater) hydrograph separations at peak flow were performed on 10 storms over a 2‐year period in a small forested catchment in north‐central Maryland using an iterative and an exact solution. Seven pairs of tracers (deuterium and oxygen 18, deuterium and chloride, deuterium and sodium, deuterium and silica, chloride and silica, chloride and sodium, and sodium and silica) were used for three‐component hydrograph separation for each storm at peak flow to determine whether or not the assumptions of hydrograph separation routinely can be met, to assess the adequacy of some commonly used tracers, to identify patterns in hydrograph‐separation results, and to develop conceptual models for the patterns observed. Results of the three‐component separations were not always physically meaningful, suggesting that assumptions of hydrograph separation had been violated. Uncertainties in solutions to equations for hydrograph separations were large, partly as a result of violations of assumptions used in deriving the separation equations and partly as a result of improper identification of chemical compositions of end‐members. Results of three‐component separations using commonly used tracers were widely variable. Consistent patterns in the amount of subsurface water contributing to peak flow (45–100%) were observed, no matter which separation method or combination of tracers was used. A general conceptual model for the sequence of contributions from the three end‐members could be developed for 9 of the 10 storms. Overall results indicated that hydrochemical and hydrometric measurements need to be coupled in order to perform meaningful hydrograph separations.

Nitrogen budgets of two small experimental forested catchments at Alptal, Switzerland

Two small experimental headwater catchments (approx. 1500 m 2) were set up in an alpine spruce forest in the Alptal valley (central Switzerland). They are located at an altitude of 1200 m and receive an average of 2300 mm precipitation a year. The site is moderately impacted by atmospheric nitrogen deposition (12 kg N ha −1 yr −1 bulk deposition of inorganic N). Two different gley soils occur as patches atop a Flysch substratum. The spatial heterogeneity of the soil is related with both the microtopography of the site and the vegetation. The catchments were made by digging 80-cm deep trenches into the shallow gleyic soils. Because of the low permeability of the clay-rich subsoil, these trenches collect the lateral water flow generated in the plots. The catchments were equipped with gauging stations (V-notch weirs) and automatic runoff-proportional water samplers. Meteorological parameters, deposition, water and solute discharge were monitored during one year. A correct water balance was obtained from the experimental catchments. The leaching of inorganic nitrogen was estimated to be 4 kg N ha −1 yr −1, mainly as nitrate. It is, however, not clear if this nitrate is leached because the ecosystem is nitrogen saturated (N in surplus of the maximal ecosystem retention). Quick preferential water flow occurs through the soil, and this may prevent nitrate from rain or snowmelt to be removed by the soil matrix before entering the water runoff pathways. Both mini-catchments showed similar biogeochemical cycles, especially for nitrogen. The time series obtained will be used as reference period for a paired-catchment experiment simulating an increased nitrogen (ammonium nitrate) deposition.

Episodic behaviour of nitrate in runoff during six years of nitrogen addition to the NITREX catchment at Gårdsjön, Sweden

Since 1991 nitrogen deposition to a small forested catchment has been experimentally increased from ambient 11 to a total of 50 kg N ha −1 year −1 by sprinkling small portions of dissolved ammonium nitrate on about 30 occasions annually. Concentrations of inorganic N in runoff (streamwater) were measured by flow-proportional sampling and during 6 shorter periods by frequent sampling and continuous monitoring. Shortly after the onset of the N additions, nitrate (NO 3) began to appear in the runoff (streamwater), after 2 years in soil solution at shallow depths and after 4 years at all depths. N concentrations in runoff were highly variable, and most of the N lost from the catchment occurred during a few episodes. Flux of N from the catchment in runoff was much larger than that calculated from concentrations in lysimeter samples and discharge from groundwater levels. The annual inorganic N loss was only about 5% of the incoming N. Episodic behaviour of NO 3 in runoff was studied by monitoring discharge episodes after 2, 3, 4 and 6 years of N addition. The nature of NO 3 concentrations in runoff during these episodes changed as the treatment proceeded. Factors such as season, hydrology, timing and size of N additions govern the loss of NO 3 in runoff during episodes. Nitrogen saturation at this catchment may proceed first in the discharge areas near the outlet to uphill soils on the slopes.

Changes in Mass Element Fluxes and Their Importance for Critical Loads: Geomon Network, Czech Republic

Three years (1994-1996) of runoff, bulk deposition, and throughfall S and N data obtained from GEOMON, a network of 14 small forest catchments in the Czech Republic, are presented. In this paper, we assess regional differences and temporal trends in S and N fluxes under changing regional burdens. Decreases in S deposition have occurred in the emission-burdened northern and western parts of the Czech Republic, where remedial measures have been applied. For example, at the Uhliřska catchment (Jizerske hory Mtns., northern Czech Republic), throughfall S fluxes decreased from 72.7 to 48.5 kg ha-1 yr-1. In contrast, the relatively clean central and southern parts of the Czech Republic showed an increase in S deposition; for example at the Na lizu catchment (Sumava Mtns., southern Czech Republic) throughfall S fluxes increased from 9.0 to 18.8 kg ha-1 yr-1. In burdened catchments with damaged forests, high runoff of N was observed. Using fluxes of S and N measured at the GEOMON sites, we calculated critical loads and exceedances for both S and N in the catchments. The critical loads of S ranged from 505 to 3,631 mol L-1 yr-1 during 1994-1996; in about 80% of the catchments the critical loads were exceeded by atmospheric deposition; half of the catchments showed an exceedance for N.

Temporal and spatial scale of response area and groundwater variation in Till

The main objective of this study has been to test the theories describing the pathways and the processes generating runoff from rainfall. A small forest catchment in south-eastern Norway has been used as a research area. The results of this study agree with previous work (Myrabo, 1986), suggesting that the response area concept gives a good description of the behaviour of the processes in small catchments in moraine terrain in Norway. Maps of the areal distribution of groundwater levels at different stream discharges are used, derived from measurements at more than 100 observational sites. The saturated area and response area in the catchment are controlled by the position of the groundwater level. The variations of the measured groundwater level, obtained automatically each hour, and at various stream discharges, are therefore thoroughly investigated. Correlation analysis and comparisons of discharge–groundwater level relationships, show distinct hysteresis effects. Threshold groundwater levels that indicate the limit for quick runoff from particular parts of the catchment have been identified. The behaviour of the discharge–groundwater relationship is dependent on location, antecedent conditions and size of response area before each precipitation event. © 1997 John Wiley & Sons, Ltd.

Storage-outflow modelling of streamflow recessions, with application to a shallow-soil forested catchment

The objectives of this study were to introduce an alternative to the matching strip, correlation and parameter-averaging methods for deriving master recession characteristics from a set of recession segments, and to apply the method to streamflow recession segments for a small forested catchment in which baseflow is provided by drainage of the saturated zone in the shallow permeable soil. The method involves fitting conceptual storage-outflow models using an iterative algorithm. Diagnostic plots revealed that the recessions were non-linear and that the recessions did not follow a common single-valued storage-outflow relation. A model involving two linear reservoirs provided substantially better fit than three single-reservoir models, indicating that the form of the recession curve probably depends not just on the volume of subsurface storage, but also on its initial distribution amongst reservoirs.

Methane uptake in Swedish forest soil in relation to liming and extra N-deposition

Methane uptake to soil was examined in individual chambers at three small forest catchments with different treatments, Control, Limed and Nitrex sites, where N-deposition was experimentally increased. The catchments consisted of both well-drained forest and wet sphagnum areas, and showed uptake of CH4 from the ambient air. The lowest CH4 uptakes were observed in the wet areas, where the different treatments did not influence the uptake rate. In the well-drained areas the CH4 uptakes were 1.6, 1.4 and 0.6 kg ha–1 year–1 for the Limed, Control and Nitrex sites, respectively. The uptake of methane at the well-drained Nitrex site was statistically smaller than at the other well-drained catchments. Both acidification and increase in nitrogen in the soil, caused by the air-borne deposition, are the probable cause for the reduction in the methane uptake potential. Uptake of methane was correlated to soil water content or temperature for individual chambers at the well-drained sites. The uptake rate of methane in soil cores was largest in the 0- to 10-cm upper soil layer. The concentration of CH4 in the soil was lower than the atmospheric concentration up to 30 cm depth, where methane production occurred. Besides acting as a sink for atmospheric methane, the oxidizing process in soil prevents the release of produced methane from deeper soil layers reaching the atmosphere.

Soil water storage and groundwater behaviour in a catenary sequence beneath forest in central Amazonia. II. Floodplain water table behaviour and implications for streamflow generation

Abstract. Valley floor groundwater level data collected during the ABRACOS project (Gash et al. 1996), and published streamflow data from small forested catchments in geomorphologically similar areas nearby have been analysed to improve the understanding of the processes of streamflow generation. Early in the wet season, the floodplain water table is typically at 0.8 m depth, or less, and receives only local, vertical recharge. Large storms may create a groundwater ridge beneath the floodplain, temporarily creating a gradient in the direction of the hilislope. Later in the wet season, floodplain water levels are controlled primarily by the discharge of groundwater which maintains the dry season streamflow. The groundwater is recharged by deep drainage from beneath the plateau and slope areas once the dry season soil water deficit has been overcome. In the late wet season, the water level is almost at the floodplain surface and may create seeps on the lower slopes in very wet years. For the period 1966-1989, the recharge was estimated to range from 290 mm to 1601 mm with a mean of 1087 mm. Published data show that baseflow is 91% of annual runoff. Stormflow is generated on the floodplain, and water table recessions after rainfall events show that the runoff response depends on the depth to the water table. These results are from areas with deeply weathered and permeable soils; in areas of Amazonia with shallower soils, the predominant flow generation processes will differ (Elsenbeer and Lack, 1996).

Export of DOC from forested catchments on the Precambrian Shield of Central Ontario: Clues from13C and 14C

Export of dissolved organic carbon (DOC) from forested catchmentsis governed by competing processes of production, decomposition, sorptionand flushing. To examine the sources of DOC, carbon isotopes (14Cand 13C) were analyzed in DOC from surface waters, groundwatersand soils in a small forested catchment on the Canadian Shield in centralOntario. A significant fraction (greater than 50%) of DOCin major inflows to the lake is composed of carbon incorporated into organicmatter, solubilized and flushed into the stream within the last 40 years. Incontrast, 14C in groundwater DOC was old indicating extensiverecycling of forest floor derived organic carbon in the soil column beforeelution to groundwater in the lower B and C soil horizons. A small uplandbasin had a wide range in 14C from old groundwater values atbaseflow under dry basin conditions to relatively modern values during highflow or wetter antecedent conditions. Wetlands export mainly recently fixedcarbon with little seasonal range. DOC in streams entering the small lakemay be composed of two pools; an older recalcitrant pool delivered bygroundwater and a young labile pool derived from recent organic matter.The relative proportion of these two pools changes seasonally due thechanges in the water flowpaths and organic carbon dynamics. Althoughchanges in local climate (temperature and/or precipitation) may alterthe relative proportions of the old and young pools, the older pool islikely to be more refractory to sedimentation and decomposition in thelake setting. Delivery of older pool DOC from the catchment andsusceptibility of this older pool to photochemical decomposition mayconsequently be important in governing the minimum DOC concentrationlimit in lakes.

Variabilité des processus podzoliques en fonction de l'état hydrique des profils et de leur position sur les versants : le cas d'un petit bassin-versant forestier du centre du Nouveau-Brunswick (Canada)

Variability of podzolic processes as a function of soil water saturation and hillslope position in a small forested catchment of central New Brunswick, Canada. A soil solution study in a forested subcatchment of central New Brunswick helped to verify the theoretical basis of dynamic podzolization models and variability of podzolic processes as a function of soil water saturation and hillslope position. Soil solution chemistry indicated that chelation is active in the top mineral horizons (A). Organo-metallic complexes precipitate mainly in the B horizons. However, these podzolic processes are not active all year. Well-drained podzols are developing mainly during spring and fall when water percolates abundantly through the soil. During summer, translocation of iron and aluminum combined with dissolved organic matter is rare ; it is only active when a long and intense rain event will permit water to percolate as far as the B horizons. In imperfectly drained sites, translocation of podzolic materials is active throughout the frost-free season. Under these soil conditions, water coming from the top soil layers easily reaches the B horizons. In cases where the B horizons are affected by hydromorphic conditions, podzolic processes are modified and precipitated iron in the B horizon is reduced because of the presence of groundwater.

Interpretations of δ34S and sulfate concentration variations over the year in runoff and groundwater in a small forested catchment in western Sweden; evidence for oxidation and reduction

(1996). Interpretations of δ34S and sulfate concentration variations over the year in runoff and groundwater in a small forested catchment in western Sweden; evidence for oxidation and reduction. GFF: Vol. 118, No. sup004, pp. 88-89.

The hydrological response of catchments to simulated changes in climate

The Large Scale Catchment Model has been developed to predict the responses in stream yield and salinity to changes in land use and climate in southwestern Western Australia. In this paper it is used to simulate, for one small forested catchment, the hydrological consequences that might be associated with a doubling of the atmospheric carbon dioxide concentration. The simulations assume that the region will experience a decrease in the amount of winter rainfall (with an increase in rainfall intensity) and an increase in potential evaporation. The results suggest that the assumed change in climate has the potential to lead to a 45% decrease in stream runoff in this catchment. About two-thirds of this decrease is associated with the reduction in rainfall; the remainder being associated with the increased potential evaporation. Furthermore, stream salinity is predicted to increase by about 8%, mostly in response to the enhanced evaporation regime.

Variation of nitrogen concentration in forest streams — influences of flow, seasonality and catchment characteristics

Short-term variation of nitrogen concentrations must be considered when assessing longterm changes in forest ecosystems, and calculating accurate estimates of nitrogen export. In this study, 10 years of monthly concentration and daily flow measurements from 20 small forested catchments in Sweden and Finland were used to link concentrations to flow dynamics and/or seasonal variations, and to relate the detected links to catchment characteristics. The methods used included regression analyses and the non-parametric Wilcoxon test. Significant correlations between different nitrogen fractions and flow volume were found for most of the catchments, but in general flow volume only explained 20% of the variation in N03N and organic-N, and 10% of the variation in NH4N concentrations. Both positive and negative correlations between concentration and flow were detected for the inorganic-N fractions, whereas for organic-N only positive correlations were recorded. Increasing flow conditions were often found to be associated with twofold median inorganic-N concentrations, especially during the spring. However, the most extreme seasonal variation was associated with the influence of the growing season, showing up to 20 times lower inorganic-N concentrations. Regarding NO3N concentrations, high atmospheric nitrogen deposition appeared to moderate this seasonal influence whereas it was highest for catchments including lakes. Intensively drained catchments showed the quantitatively largest difference between growing and dormant seasons for NH4N concentrations. In general, links between nitrogen concentrations and flow/seasons could be related to annual mean temperature, annual mean concentrations and the water regime of the catchments. The geographical heterogeneity of detected links and the still rather low explanation level for the concentration variation, show the importance of a more dynamic approach to nitrogen leakage where also catchment characteristics are included.