Soil Water Residence Time Research Articles

Study of groundwater recharge using combined unsaturated‐and saturated‐zone chloride mass balance methods

AbstractUnderstanding groundwater recharge mechanisms is essential for reliable groundwater recharge estimation for sustainable groundwater resource assessment and management, especially in arid and semiarid areas. In this study, chloride, together with stable isotopes (18O and 2H), in 1‐year rainfall, soil water from representative profiles, and groundwater samples, collected in the Ordos Plateau, northern China were analysed. Groundwater recharge was estimated using unsaturated‐ and saturated‐zone chloride mass balance (CMB). An equation that describes the relationship between chloride concentrations in saturated zone water (Csz) and soil water residence time (τ) was derived. With the help of the relationship, chloride concentrations in unsaturated zone water (Cuz) and in groundwater (Csz) can be used to uncover water flow and chloride transport in the unsaturated zone. The relationship between Csz and Cuz indicates that, the groundwater was recharged mainly by intense rainfall events. Small rainfall events did not lead to groundwater recharge but contributed to the flux of chloride to soil surface. Stable isotopic compositions of 18O and 2H provided corroborative evidence of the recharge processes. The relationship between Csz and τ indicates further that there is no source of chloride in the groundwater other than that from precipitation. Thus, Csz can give more reliable recharge estimates. Cuz was influenced by heavy rainfall‐induced runoff, run‐on and bypass flow events and cannot give reliable recharge estimates. However, if used jointly with Csz, Cuz can help to gain insights into recharge processes and yield groundwater recharge estimates with higher certainty. The outcomes of this study can assist for groundwater recharge investigation and assessment in regions where the assumptions and boundary conditions necessary for the correct application of the CMB method may not be met.

Water Budget, Biological Water Use, and the Soil Hydrological Cycle across Typical Ecosystems of the Heihe River Basin

Quantification of the water budget of an arid inland river ecosystem is essential but still a challenge for the sustainable development of water resources. In situ observed data were used to analyze the monthly and annual water budgets and the soil hydrological cycle for six typical ecosystems in the Heihe River Basin (HRB). The two-source model was used to partition evapotranspiration (ET) into transpiration (T) and evaporation, after which the validated model was applied to quantitatively analyze the biological water use fraction [T/Ecosystem Water Supply (WS)] for different ecosystems. There were differences in the water budgets of the different ecosystems due to differences in climate, vegetation, soil, and external inputs. Precipitation in the HRB decreased from upstream to downstream, whereas there was a gradual increase in ET. External sources of water (e.g., natural runoff from upstream, irrigation in the middle reaches, and groundwater recharge in the lower reaches) to soil layers played an important role in regulating the water budgets of HRB ecosystems. Cropland obtained the maximum biological water use fraction (0.50), followed by Populus euphratica (0.49), alpine meadow (0.49), alpine swamp meadow (0.44), Tamarix ramosissima (0.42), and Kalidium foliatum (0.4). The soil water residence time (at a depth of 40 cm) varied from 14 d to 97 d (average of 60 d). The order of plant species in terms of soil water residence time was: K. foliatum (88 d) > T. ramosissima (72 d) > alpine meadow (68 d) > alpine swamp meadow (63 d) > cropland (53 d) > P. euphratica forest (20 d). Differences in the biological water use fraction and soil water residence time could be attributed to the characteristics of the water budget for each ecosystem. This study quantified the water budget, biological water use, and soil hydrological cycle across typical ecosystems in HRB, and can act as a reference for ecosystem management of the arid inland river basin.

Using Machine Learning to Predict Inland Aquatic CO2 and CH4 Concentrations and the Effects of Wildfires in the Yukon‐Kuskokwim Delta, Alaska

AbstractClimate change is causing an intensification in tundra fires across the Arctic, including the unprecedented 2015 fires in the Yukon‐Kuskokwim (YK) Delta. The YK Delta contains extensive surface waters (∼33% cover) and significant quantities of organic carbon, much of which is stored in vulnerable permafrost. Inland aquatic ecosystems act as hot‐spots for landscape CO2 and CH4 emissions and likely represent a significant component of the Arctic carbon balance, yet aquatic fluxes of CO2 and CH4 are also some of the most uncertain. We measured dissolved CH4 and CO2 concentrations (n = 364), in surface waters from different types of waterbodies during summers from 2016 to 2019. We used Sentinel‐2 multispectral imagery to classify landcover types and area burned in contributing watersheds. We develop a model using machine learning to assess how waterbody properties (size, shape, and landscape properties), environmental conditions (O2, temperature), and surface water chemistry (dissolved organic carbon composition, nutrient concentrations) help predict in situ observations of CH4 and CO2 concentrations across deltaic waterbodies. CO2 concentrations were negatively related to waterbody size and positively related to waterbody edge effects. CH4 concentrations were primarily related to organic matter quantity and composition. Waterbodies in burned watersheds appeared to be less carbon limited and had longer soil water residence times than in unburned watersheds. Our results illustrate the importance of small lakes for regional carbon emissions and demonstrate the need for a mechanistic understanding of the drivers of greenhouse gasses in small waterbodies.

Denitrification in the vadose zone: Modelling with percolating water prognosis and denitrification potential

Transport and transformation processes of nitrogen in the soil are an essential part of understanding the relationship between agricultural input and nitrate (NO3−) concentrations in groundwater. The presented study describes these transformation processes around NO3− degradation at a water catchment in the Lower Rhine Embayment, Germany. Despite intensive agriculture, extracted groundwater at a depth of 21 to 22 m shows unexpectedly very low NO3− levels, below 3 mg/L NO3− for all wells. The local water supplier therefore carried out investigations in this area and generated soil data from 22 representative areas (142 soil samples from 82 drilling meters from the surface to a max. depth of 5.5 m) and groundwater analyses from 17 groundwater monitoring wells (from 3 to 5 m below ground surface). Soil types are predominantly luvisol and gleysol. The substrate in the topsoil is mainly clayey silt; underneath there are mostly medium-grained sands with partial silt intercalations which appear as a separate layer. Based on this dataset, the percolating water residence times and the NO3− leaching potential were calculated in this study. Together with the nitrogen surplus and with the help of reactive transport modelling, the denitrification potential in the vadose zone was simulated. The comparison of simulation results with laboratory-measured data shows a high correlation. Substantial NO3− reduction in the vadose zone was observed: dependent on soil type, reduction capacity and water residence time, up to 25% of the NO3− was reduced here. The applied modelling is considered an improvement in NO3− degradation potential assessment because it considers many relevant variables such as precipitation, soil parameters (grain size, field capacity, available water capacity, coarse fragments) and nitrogen input. Therefore, a transfer to other sites with comparable hydro(geo)logical conditions is possible, also due to relatively easily determinable input data. This assessment of nitrogen degradation in the vadose zone will be a useful tool for NO3− levels forecast in groundwater

Comparison of the oxygen isotope signatures in speleothem records and iHadCM3 model simulations for the last millennium

Abstract. Improving the understanding of changes in the mean and variability of climate variables as well as their interrelation is crucial for reliable climate change projections. Comparisons between general circulation models and paleoclimate archives using indirect proxies for temperature or precipitation have been used to test and validate the capability of climate models to represent climate changes. The oxygen isotopic ratio δ18O, a proxy for many different climate variables, is routinely measured in speleothem samples at decadal or higher resolution, and single specimens can cover full glacial–interglacial cycles. The calcium carbonate cave deposits are precisely dateable and provide well preserved (semi-)continuous albeit multivariate climate signals in the lower and mid-latitudes, where the measured δ18O in the mineral does not directly represent temperature or precipitation. Therefore, speleothems represent suitable archives to assess climate model abilities to simulate climate variability beyond the timescales covered by meteorological observations (101–102 years). Here, we present three transient isotope-enabled simulations from the Hadley Center Climate Model version 3 (iHadCM3) covering the last millennium (850–1850 CE) and compare them to a large global dataset of speleothem δ18O records from the Speleothem Isotopes Synthesis and AnaLysis (SISAL) database version 2 (Comas-Bru et al., 2020b). We systematically evaluate offsets in mean and variance of simulated δ18O and test for the main climate drivers recorded in δ18O for individual records or regions. The time-mean spatial offsets between the simulated δ18O and the speleothem data are fairly small. However, using robust filters and spectral analysis, we show that the observed archive-based variability of δ18O is lower than simulated by iHadCM3 on decadal and higher on centennial timescales. Most of this difference can likely be attributed to the records' lower temporal resolution and averaging or smoothing processes affecting the δ18O signal, e.g., through soil water residence times. Using cross-correlation analyses at site level and modeled grid-box level, we find evidence for highly variable but generally low signal-to-noise ratios in the proxy data. This points to a high influence of cave-internal processes and regional climate particularities and could suggest low regional representativity of individual sites. Long-range strong positive correlations dominate the speleothem correlation network but are much weaker in the simulation. One reason for this could lie in a lack of long-term internal climate variability in these model simulations, which could be tested by repeating similar comparisons with other isotope-enabled climate models and paleoclimate databases.

Recognizing the role of plant species composition in the modification of soil nutrients and water in rubber agroforestry systems

Reliable guidance for crop selection and related management to achieve sustainable soil resource use in rubber agroforestry systems is limited. One important reason for this limited guidance is that our understanding of the effects of different plant functional groups on soil resources is still insufficient. Here, to examine the effects of the species composition of trees, shrubs and herbs on soil nutrients and soil water with increases in the complexity of the plant community structure, we measured the soil nutrient concentrations (i.e., C, N, P, K, Ca and Mg), soil water content and soil water residence time (with stable hydrogen and oxygen isotope tracers) at six soil depths in a monoculture rubber plantation, four multi-species rubber agroforestry systems, and a tropical rainforest. As the plant species composition increased, the soil C and N increased. The soil water content also increased with increases in soil C and N. However, the effects of plant species composition on the soil water content gradually changed from positive to negative, especially under the effects of herb species, which could accelerate soil water drainage and hence shorten the soil water residence time. Therefore, the faster water infiltration and potentially higher flow of soil water in complex plant communities increased the risk and magnitude of mineral nutrient leaching. In addition, as the plant composition increased, plant competition decreased the concentration of soil nutrients, especially soil P, K and Ca. In general, plant interspecific interactions definitively decreased soil mineral nutrients as the plant composition increased, and the effects of tree, shrub and herb species on soil nutrients and soil water differed and sometimes appeared contradictory. However, the effects of plant species composition on soil gradually weakened with increases in soil depth.

Groundwater recharge under irrigated agro-ecosystems in the North China Plain: From a critical zone perspective

From a critical zone perspective, the present paper aims to present the magnitude of groundwater recharge under different agricultural land-use types, reveal the process of water and solute transport in thick vadose zone, evaluate the “time lag” effect of recharge, and underscore the role of thickening vadose zone in recharge. The results indicated that different agricultural land-use types need to be further considered in recharge rate estimate. Under the typical irrigation condition in the piedmont plain, the recharge rate under flood irrigated winter wheat and summer maize (W/M_F), maize (M), non-cultivation (NC), native vegetation (NV), vegetables (V), and orchards (O) is 206.4, 149.7, 194.1, 46.4, 320.0, and 48.6 mm/yr, respectively. In the central plain, the value under W/M_F, M, NC, V, and cotton (C) is 92.8, 50.8, 85.0, 255.5, and 26.5 mm/yr, respectively. Soil water residence time (several years) and groundwater level response time (several months) should be distinguished to further understand the processes of groundwater recharge, because the soil water displacement velocities range from 0.2 to 2.2 m/yr while the rate of wetting front propagation is approximately 47 m/yr in the piedmont plain. The thickening vadose zone would prolong residence time of soil water and contaminant, which could postpone the time of or alleviate groundwater pollution, but have no significant influence on the magnitude of recharge in a long time scale. Recharge coefficient based on shorter time span (e.g. 2 or 3 years) should be used with caution as a parameter for groundwater resources evaluation, because it varies with total water input and target soil depth. Uncertainties in evapotranspiration and other water balance components should be evaluated in recharge estimation and the impact of land-use types on recharge should be emphasized. The critical zone science would greatly improve the understanding of groundwater recharge processes. The results of the present study will be helpful in sustainable groundwater resources management.

Characterizing the Fluxes and Age Distribution of Soil Water, Plant Water, and Deep Percolation in a Model Tropical Ecosystem

AbstractRecent field observations indicate that in many forest ecosystems, plants use water that may be isotopically distinct from soil water that ultimately contributes to streamflow. Such an assertion has been met with varied reactions. Of the outstanding questions, we examine whether ecohydrological separation of water between trees and streams results from a separation in time, or in space. Here we present results from a 9‐month drought and rewetting experiment at the 26,700‐m3mesocosm, Biosphere 2‐Tropical Rainforest biome. We test the null hypothesis that transpiration and groundwater recharge water are sampled from the same soil volume without preference for old nor young water. After a 10‐week drought, we added 66 mm of labeled rainfall with 152‰ δ2H distributed over four events, followed by background rainfall (−60‰ δ2H) distributed over 13 events. Our results show that mean transit times through groundwater recharge and plant transpiration were markedly different: groundwater recharge was 2–7 times faster (~9 days) than transpired water (range 17–62 days). The “age” of transpired water showed strong dependence on species and was linked to the difference between midday leaf water potential and soil matric potential. Moreover, our results show that trees used soil water (89% ±6) and not the “more mobile” (represented by “zero tension” seepage) water (11% ±6). The finding, which rejects our null hypothesis, is novel in that this partitioning is established based on soil water residence times. Our study quantifies mean transit times for transpiration and seepage flows under dynamic conditions.

Patterns in Soil Chemical Weathering Related to Topographic Gradients and Vegetation Structure in a High Andean Tropical Ecosystem

AbstractAlthough climate exerts a major control on mineral weathering and soil formation processes, the combined effect of vegetation and topography can influence the rate and extent of chemical weathering at the hillslope scale. In this paper, we examined spatial patterns in volumetric strain and soil weathering extent associated with topographic gradients and vegetation patterns. In a high Andean catchment, we selected 10 soil toposequences on andesitic flows: 5 under tussock grasses, 3 under cushion forming plants, and 2 under native forest. Along each toposequence, one pit was excavated at the shoulder, backslope, and toeslope resulting in 30 soil profiles. Depth‐weighted total soil porosity of the 30 soil profiles averaged 64 ± 6%. The association between volumetric strain and soil organic C indicates that biotic agents can be effective in dilating the regolith during weathering. The young, postglacial volcanic soils were depleted in mono‐divalent and divalent cations, with total mass losses ranging between 793 and 1610 kg/m2. The accumulation of Al‐humus complexes in the soil matrix plays an essential role in chemical transformation of the nonallophanic soils. Beyond the marginally significant topographic control on chemical weathering extent, our data show highly significant differences in chemical weathering extent between vegetation communities with total mass losses in forest soils being, respectively, 19% and 22% higher than in grasslands and cushion‐forming plants. The vegetation mosaic in alpine ecosystems might therefore provide essential clues to understand soil chemical weathering patterns caused by spatially varying soil particle and water residence times.

Data-model comparison of soil–water δ18O at a temperate site in N. Spain with implications for interpreting speleothem δ18O

Summary An understanding of how seasonal and longer-term δ 18 O signals in meteoric precipitation (δ 18 O p ) are modified by percolation through soils is essential to link temporal changes in speleothem δ 18 O to surface climatic conditions. This study focuses on modifications that occur in a relatively thick soil above a temperate cave site (La Garma, N. Spain). Monthly soil–water δ 18 O (δ 18 O sw ) values at a depth of 60 cm through the year are only 14% of the range in δ 18 O p , implying substantial homogenisation and attenuation of seasonal signals. A striking feature is that δ 18 O sw values at 60 cm depth are lowest in summer and highest in winter, the opposite (anti-phase) to that observed in rainfall. Soil–water residence times of up to circa 6 months in the upper 60 cm of soil, and a matrix flow, piston-type infiltration behaviour with mixing are inferred. Evaporative effects on recovered soil–water δ 18 O are minimal at this wet temperate site, in contrast with published results from arid and semi-arid sites. A soil–water model is presented to estimate monthly δ 18 O sw as a function of air temperature and δ 18 O p , incorporating effects such as variations in the amount of infiltrated water, changes in the ratio between evaporation and transpiration, mixing with antecedent soil moisture and small enrichments in 18 O linked to evaporation and summer moisture deficits. Our model reproduces the observed δ 18 O sw results, and produces δ 18 O outputs in excellent agreement with δ 18 O data for two monitored drip-water sites at La Garma cave that exhibit seasonal δ 18 O variability. We conclude that simple evapotranspiration models that permit infiltration during months that have a positive hydrological balance only, tend to underestimate summer rainfall contributions. Overall, the study provides an improved framework for predicting δ 18 O sw trends at temperate sites such as La Garma that have a relatively thick soil cover, as well as for understanding seasonal ranges and trends in δ 18 O in cave drip-sites.

Flow pathways and nutrient transport mechanisms drive hydrochemical sensitivity to climate change across catchments with different geology and topography

Abstract. Hydrological processes determine the transport of nutrients and passage of diffuse pollution. Consequently, catchments are likely to exhibit individual hydrochemical responses (sensitivities) to climate change, which are expected to alter the timing and amount of runoff, and to impact in-stream water quality. In developing robust catchment management strategies and quantifying plausible future hydrochemical conditions it is therefore equally important to consider the potential for spatial variability in, and causal factors of, catchment sensitivity, as it is to explore future changes in climatic pressures. This study seeks to identify those factors which influence hydrochemical sensitivity to climate change. A perturbed physics ensemble (PPE), derived from a series of global climate model (GCM) variants with specific climate sensitivities was used to project future climate change and uncertainty. Using the INtegrated CAtchment model of Phosphorus dynamics (INCA-P), we quantified potential hydrochemical responses in four neighbouring catchments (with similar land use but varying topographic and geological characteristics) in southern Ontario, Canada. Responses were assessed by comparing a 30 year baseline (1968–1997) to two future periods: 2020–2049 and 2060–2089. Although projected climate change and uncertainties were similar across these catchments, hydrochemical responses (sensitivities) were highly varied. Sensitivity was governed by quaternary geology (influencing flow pathways) and nutrient transport mechanisms. Clay-rich catchments were most sensitive, with total phosphorus (TP) being rapidly transported to rivers via overland flow. In these catchments large annual reductions in TP loads were projected. Sensitivity in the other two catchments, dominated by sandy loams, was lower due to a larger proportion of soil matrix flow, longer soil water residence times and seasonal variability in soil-P saturation. Here smaller changes in TP loads, predominantly increases, were projected. These results suggest that the clay content of soils could be a good indicator of the sensitivity of catchments to climatic input, and reinforces calls for catchment-specific management plans.

Effects of a plastic cover on soil moisture change in a Mediterranean climatic regime

AbstractWater deficit is a serious problem for most agricultural crops, especially in arid–semiarid regions, and limits sustainable development. Production can be improved by reducing evapotranspiration and loss of infiltrated water by the use of a plastic cover. We monitored soil moisture fortnightly over 1 yr using a neutron probe near four trees in an olive grove (Olea europaea, var. Arbequina), two of which had a plastic cover buried in the topsoil around them. These trees were monitored using three neutron probe access tubes per tree to compare the behaviour of soil moisture over time with two other identically instrumented and nearby trees with no cover. Analysis was based on the resultant moisture profiles. The plastic cover retained moisture and increased soil water residence time. During the dry season, the amount of water retained in the plots was at most 15–20% greater in the mid‐season and at least 5–6% greater at the end of season than in the central part of the plots near the trunk. The plastic cover was effective to ca. 50 cm with maximum water content near the soil surface. During the wet season, the cover did not affect soil water. Soil moisture was greater near the tree trunk as a result of stemflow and throughfall.

Factors influencing the residence time of catchment waters: A virtual experiment approach

Estimates of mean residence time (MRT) are increasingly used as simple summary descriptors of the hydrological processes involving storage and mixing of water within catchment systems. Current understanding of the physical controls on MRT remains limited, and various hypotheses have been proposed to explain its variability between catchments. We present a series of virtual experiments to investigate different hypotheses regarding the significance of different hydrological processes and geographical controls in determining the MRT of catchment waters. The experiments were undertaken using a semidistributed conceptual hydrological model, applied to the Maimai experimental catchment in New Zealand. Our results show that in this small steep catchment, with largely impermeable bedrock, the primary control on the stream water mean residence time is storage within the unsaturated zone. The physical location on the hillslope had only a small influence on soil water residence time. Stream water mean residence time was very sensitive to small additional amounts of deep groundwater in the model. Overall, our results suggest that stream water MRT is additive. The component residence times of stream water MRT appear relatable to characteristic properties of the catchment. Through this mechanism there is future potential for extrapolating MRT data from experimental catchments to other areas.

Chloroform in runoff water—a two-year study in a small catchment in Southeast Sweden

Chloroform concentrations were observed and input and output fluxes estimated over a 2-yr period in a small coniferous catchment (0.22 km2) in southeast Sweden. Water discharge was measured daily, and runoff water was sampled bi-weekly for chloroform analysis. An approximate chloroform budget was calculated, which indicated that the annual output of 6 μg m−2 yr−1 was approximately six times higher than the input, inferring an internal source of chloroform in the catchment. To the best of our knowledge, neither flux estimates nor mass balances have previously been made for chloroform on a catchment scale, nor have data regarding natural runoff variation with time been gathered. Concentrations of chloroform in runoff were found to be generally high during wet periods, such as spring, but also peaked during summer rain events. The observed pattern suggests that chloroform is formed in surface soil layers and transported to the outlet under high-flow conditions and during dry-period rain events; it is lost through degradation or evaporation during drier periods due to longer soil water residence times. The data suggest that the variation among replicates increases with concentration; this emphasizes the need to know what the degree of on-site variation is, so one can collect a sufficient number of replicates to permit detection of spatial or temporal changes.

Chloride Retention and Release in a Boreal Forest Soil: Effects of Soil Water Residence Time and Nitrogen and Chloride Loads

The common assumption that chloride (Cl-) is conservative in soils and can be used as a groundwater tracer is currently being questioned, and an increasing number of studies indicate that Cl- can be retained in soils. We performed lysimeter experiments with soil from a coniferous forest in southeast Sweden to determine whether pore water residence time and nitrogen and Cl- loads affected Cl- retention. Over the first 42 days there was a net retention of Cl- with retention rates averaging 3.1 mg CI- m(-2) d(-1) (68% of the added Cl- retained over 42 days). Thereafter, a net release of Cl- at similar rates was observed for the remaining experimental period (85 d). Longer soil water residence time and higher Cl- load gave higher initial retention and subsequent release rates than shorter residence time and lower Cl- load did. Nitrogen load did not affect Cl transformation rates. This study indicates that simultaneous retention and release of Cl- can occur in soils, and that rates may be considerable relative to the load. The retention of Cl- observed was probably due to chlorination of soil organic matter or ion exchange. The cause of the shift between net retention and net release is unclear, but we hypothesize that the presence of O2 or the presence of microbially available organic matter regulates Cl- retention and release rates.

Plant-cover effects on hydrology and pedogenesis in a sandy vadose zone

The hydrology of three field lysimeters was monitored after 15 years of plant growth to examine pedogenic impacts of plant-induced changes to water budgets, flow, and chemical denudation. Between 70% and 87% of annual water discharge from all lysimeters (“sandboxes”) was generated from short (usually ≤5 days) precipitation events and longer (usually ≤25 days) snowmelt events. These infiltration events occur only 10–23% of the time throughout the year. Average annual evapotranspiration (ET) from the red pine sandbox was 73% of precipitation and 1.4–2.6 times larger than ET from the grass and moss/lichen boxes, respectively. In situ unsaturated hydraulic conductivity estimates, based on Darcy's law, are on the order of 10 −8 m/s and did not appear to vary significantly with plant cover. The upper zone of each sandbox is characterized by rapid, large changes in the magnitude of pressure head (−700 to −6 cm), water content (1 to 9%), and magnitude and direction of the hydraulic gradient. The vertical extent of this upper zone varies with plant cover; it extends to a depth between 30 and 70 cm beneath moss/lichen and grass whereas the upper zone beneath red pine extends to at least 120 cm during the growing season. Synoptic field measurements of water content and pressure head were very different from laboratory measurements of hanging-column moisture-retention curves. The red pine sandbox consistently exhibited the smallest water contents and the most negative pressure heads. Upper-zone soil water samples, collected at small and large tension during events, had different silica concentrations, which supports the existence of a dynamic flow system consisting of water residing in large and small pore spaces, respectively. However, large-tension silica concentrations increased systematically with depth in all sandboxes, indicating chemical reaction progresses in bulk soil water along flow paths. A conservative estimate of the shortest mean soil water residence time in the sandboxes is 9 days, which is sufficient time for exchange and weathering reactions to occur. Silica concentrations at 95 cm beneath red pine were twice as large as those beneath the moss/lichen cover. However, larger water fluxes from the moss/lichen sandbox produced an annual silica denudation flux of 910 mol Si/ha/year, whereas silica denudation from the pine sandbox was only 470 mol Si/ha/year. These results suggest that water and solute uptake by rooting vegetation may actually decrease chemical denudation from young, soil-building ecosystems.

The contribution of old and new water to a storm hydrograph determined by tracer addition to a whole catchment

Two tracer experiments have been carried out at an enclosed catchment in southern Norway. The catchment was brought to steady state with respect to rainfall and runoff prior to the tracer addition. A known concentration of lithium bromide was then added to the rainfall for the duration of each event. The tight control on tracer concentration and rainfall amount enabled assessment of the contribution of old and new water to runoff, the dominant flow pathways and soil water residence times during a storm event. A significant volume of ‘old’ water contributes to runoff despite the hydrologically responsive nature of the catchment and several hours of tracer injected rainfall are required before ‘new’ water becomes the dominant runoff source. After 34 h of tracer injection, ‘new’ water apparently contributes c. 83% to instantaneous flow and c. 55% of the total tracer input to the catchment has been lost in runoff. Recovery of the tracer from soil water indicates that the organic soil surface layer is the dominant flow pathway for rainwater through the catchment and that a significant pathway also exists at the soil–bedrock interface. New water is retained in deep pockets of soil for several days. Assessment of the conservative behaviour of the tracer suggests that 10–14% of the input Br− is retained in the soil and the tracer is not conservative. Laboratory experiments indicate that sorption of Br− to organic soil is the likely mechanism of retention. This process is probably concentration dependent and will have occurred predominantly during the initial period of tracer application. Copyright © 2000 John Wiley & Sons, Ltd.

Estimating Rainfall Recharge and Soil Water Residence Times in Pukekohe, New Zealand, by Combining Geophysical, Chemical, and Isotopic Methods

AbstractThe combined use of geophysical (time domain reflectometry [TDR]), isotopic (δ18 O and δ2H) and chemical (NO 3 ‐N, Cl) techniques indicates that residence times in the unsaturated zone of a fractured basalt aquifer in the Pukekohe region of the North Island, New Zealand, are at least six months. Each technique provides useful information on specific aspects of recharge and residence times, but combined they provide a basis for determining the mechanisms of water movement in the unsaturated zone. TDR soil moisture measurements provide similar rainfall recharge estimates as model calculations over longer time periods (monthly or seasonal) but may be more accurate for shorter time scales (weekly or daily).The TDR measurements indicate that recharge occurs mainly in the late autumn and winter months. Measurements of nitrate concentrations in the soil profile over this same period, suggest that nitrate is mineralized in the soil during the summer and is flushed past the root zone at the beginning of the recharge period, in early winter. Nitrate concentrations in the soil profile do not increase in concentration later in the recharge period, even after significant recharge events have occurred, because all stored nitrate has already been released. This pattern of nitrate movement indicates that fertilizer applications in spring and early summer will leach less to ground water and applications made in autumn and winter will leach more. Estimations of yearly recharge from mean monthly chloride concentrations (730 mm/yr) are roughly in agreement with the TDR estimates (680 mm/year). However, there is a large range in the chloride estimates (>100%) because monthly chloride concentration measurements in both the rain and soil water vary due to the proximity of the site to the ocean.

A spatial study of the relationships between streamwater acidity and geology, soils and relief (Vosges, northeastern France)

We have used a geographic information system (GIS) to study the relationships between streamwater pH and alkalinity at base flow with geology, soils and relief in 100 forested catchments located in the sandstone portion of the Vosges mountains, where atmospheric deposition and forest cover can be considered homogeneous. At base flow, streamwater acidity depends primarily on bedrock and soil content of weatherable minerals whose dissolution neutralizes acidity. Catchments are developed on three main stratigraphic levels, consisting of two sandstone layers rich in weatherable minerals, called “rich”, at the upper and lower extremity of the stratigraphic sequence, and a quartzitic sandstone, called “poor”, in between. Catchments were classified into three major groups in relation to the stratigraphic sequence: (1) only “poor” bedrock, (2) “rich” upstream and “poor” downstream, (3) “poor” upstream and “rich” downstream. Results showed that streamwater pH differed depending on the group. Within each group, the pH was related to bedrock, soil and relief characteristics of the catchments. The relative surface covered by the bedrock located in the lower part of the catchment explained more than 50% of the variability of pH. More than 20% of variability could be explained by soil types in catchments composed of “poor” bedrock only. Soil type apparently did not influence streamwater pH in the other groups. The catchment area, probably related to the increase of soil volume and water residence time, explained up to 20% of pH variability, depending on the catchment type.

Modeling Base Flow Soil Water Residence Times From Deuterium Concentrations

Three approaches to determining mean soil water residence times in a steep headwater catchment were investigated. The deuterium concentrations of soil water collected from 11 suction cup samplers at the Maimai M8 catchment were determined weekly for 14 weeks and the results compared with those of rainfall in the same period. Deuterium variations in the suction samples were considerably delayed and diminished compared with the rainfall, indicating significant storage times and mixing with soil water. Soil matrix water at shallow levels (∼200 mm depth) in unsaturated soils was relatively responsive to fresh input, but deeper water and water near the stream subject to occasional water table rises showed much less variation. Steady state and non‐steady state exponential models gave similar mean residence times, ranging from 12 to more than 100 days for different locations. Three groups of soil water response were defined, comprising shallow, medium and deep (near‐stream) soil locations based on the mean residence times. The nonsteady models revealed considerable week‐to‐week and longer variations in mean residence time for shallow soil (SL4), but indicated that steady state models could adequately represent the system in the overall period investigated. In the third approach, model types and parameters that gave the best fits to the soil water deuterium concentrations were determined. Exponential and especially dispersion models were the most satisfactory. Weighting the input (rainfall δD) partially or fully with the amount of rainfall gave much worse fits than with the unweighted input, showing that much of the rainfall bypasses the soil matrix. The best fitting dispersion model (designated DM2) yielded the most accurate mean residence times: 13 days for shallow soil (SL4), 42 days for soil at 400 mm depth (SL5), both at midslope locations, and 63 days for soil at 800 mm depth near the stream (SL2). Capillary flow was important for the unsaturated shallow soil (SL4), while advection and hydrodynamic dispersion (mixing) were more dominant for the periodically saturated (SL5) and the generally saturated (SL2) soils.