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Few studies have quantified the differences between celerity and velocity of hillslope water flow and explained the processes that control these differences. Here, we asses these differences by combining a 24-day hillslope sprinkling experiment with a spatially explicit hydrologic model analysis. We focused our work on Watershed 10 at the H. J. Andrews Experimental Forest in western Oregon. Celerities estimated from wetting front arrival times were generally much faster than average vertical velocities of δ2H. In the model analysis, this was consistent with an identifiable effective porosity (fraction of total porosity available for mass transfer) parameter, indicating that subsurface mixing was controlled by an immobile soil fraction, resulting in the attenuation of the δ2H input signal in lateral subsurface flow. In addition to the immobile soil fraction, exfiltrating deep groundwater that mixed with lateral subsurface flow captured at the experimental hillslope trench caused further reduction in the δ2H input signal. Finally, our results suggest that soil depth variability played a significant role in the celerity–velocity responses. Deeper upslope soils damped the δ2H input signal, while a shallow soil near the trench controlled the δ2H peak in lateral subsurface flow response. Simulated exit time and residence time distributions with our hillslope hydrologic model showed that water captured at the trench did not represent the entire modeled hillslope domain; the exit time distribution for lateral subsurface flow captured at the trench showed more early time weighting.

The Hydropedograph Toolbox has been developed to provide a set of standardized tools for analyzing soil moisture time series in an efficient and consistent manner. This toolbox contains various modules that permit the exploration and visualization of key soil hydrological parameters and processes using multi-depth real-time soil moisture monitoring datasets. This includes statistical summary, soil water release curve, preferential flow occurrence, hydraulic redistribution, and the relationship between soil moisture and soil temperature. After describing this toolbox, this paper demonstrates the utility of this toolbox in a case study from the Shale Hills Critical Zone Observatory in USA. The case study illustrates the topographic impacts on soil moisture dynamics along a hillslope transect, and quantifies the frequency of the occurrence of preferential flow, diel fluxes of water, and seasonal storage dynamics. It is expected that such a toolbox, with continued enhancements in the future and wide applications across diverse landscapes, can facilitate the advancement of comparative hydrology and hydropedology.

The spatial patterns of groundwater recharge on hillslopes with a thin soil mantle overlying bedrock are poorly known. Complex interactions between vertical percolation of water through the soil, permeability contrasts between soil and bedrock and lateral redistribution of water result in large spatial variability of water moving into the bedrock. Here, we combine new measurements of saturated hydraulic conductivity of soil mantle and bedrock of the well-studied Panola Mountain experimental hillslope with previously collected (sub)surface topography and soil depth data to quantify the factors affecting the spatial pattern of bedrock groundwater recharge. We use geostatistical characteristics of the measured permeability to generate spatial fields of saturated hydraulic conductivity for the entire hillslope. We perform simulations with a new conceptual model with these random fields and evaluate the resulting spatial distribution of groundwater recharge during individual rainstorms and series of rainfall events. Our simulations show that unsaturated drainage from soil into bedrock is the prevailing recharge mechanism and accounts for 60% of annual groundwater recharge. Therefore, soil depth is a major control on the groundwater recharge pattern through available storage capacity and controlling the size of vertical flux. The other 40% of recharge occurs during storms that feature transient saturation at the soil-bedrock interface. Under these conditions, locations that can sustain increased subsurface saturation because of their topographical characteristics or those with high bedrock permeability will act as hotspots of groundwater recharge when they receive lateral flow. Copyright © 2015 John Wiley & Sons, Ltd.

The difference between celerity and velocity of hillslope water flow is poorly understood. We assessed these differences by combining a 24-day hillslope sprinkling experiment with a spatially explicit hydrologic model analysis. We focused our work at Watershed 10 at the H. J. Andrews Experimental Forest in western Oregon. δ2H label was applied at the start of the sprinkler experiment. Maximum event water (δ2H labeled water) contribution was 26 % of lateral subsurface flow at 20 h. Celerities estimated from wetting front arrival times were generally much faster (on the order of 10–377 mm h−1) than average vertical velocities of δ2H (on the order of 6–17 mm h−1). In the model analysis, this was consistent with an identifiable effective porosity (fraction of total porosity available for mass transfer) parameter, indicating that subsurface mixing was controlled by an immobile soil fraction, resulting in an attenuated δ2H in lateral subsurface flow. Furthermore, exfiltrating bedrock groundwater that mixed with lateral subsurface flow captured at the experimental hillslope trench caused further reduction in the δ2H input signal. Our results suggest that soil depth variability played a significant role in the velocity-celerity responses. Deeper upslope soils damped the δ2H input signal and played an important role in the generation of the δ2H breakthrough curve. A shallow soil (~ 0.30 m depth) near the trench controlled the δ2H peak in lateral subsurface flow response. Simulated exit time and residence time distributions with the hillslope hydrologic model were consistent with our empirical analysis and provided additional insights into hydraulic behavior of the hillslope. In particular, it showed that water captured at the trench was not representative for the hydrological and mass transport behavior of the entire hillslope domain that generated total lateral subsurface flow, because of different exit time distributions for lateral subsurface flow captured at the trench and total lateral subsurface flow.

Diel fluctuations can comprise a significant portion of summer discharge in small to medium catchments. The source of these signals and the manner in which they are propagated to stream gauging sites is poorly understood. In this work, we analysed stream discharge from 15 subcatchments in Dry Creek, Idaho, Reynolds Creek, Idaho, and HJ Andrews, Oregon. We identified diel signals in summer low flow, determined the lag between diel signals and evapotranspiration demand and identified seasonal trends in the evolution of the lag at each site. The lag between vegetation water use and streamflow response increases throughout summer at each subcatchment, with the rate of increase as a function of catchment stream length and other catchment characteristics such as geology, vegetation and stream geomorphology. These findings support the hypothesis that variations in stream velocity are the key control on the seasonal evolution of the observed lags. Copyright © 2012 John Wiley & Sons, Ltd.

Despite the widespread acceptance of hydrologic importance, controls on the initiation of preferential flow in natural soil profiles and the frequency of its occurrence at different times of year remain elusive. This study determined the controls and frequency of preferential flow occurrence in the Shale Hills Critical Zone Observatory. Soil moisture profiles and precipitation were monitored at 10 sites along a topographic gradient for >3 yr, encompassing 175 precipitation events. For each event and each site, the flow regime was classified as either preferential flow, sequential flow, or nondetectable flow based on the sequence of soil moisture response at various depths within the same site. Preferential flow here specifically refers to out-of-sequence soil moisture response, with a deeper horizon responding to precipitation earlier than a shallower horizon. Indices describing antecedent precipitation, precipitation characteristics, precipitation timing, and initial soil moisture were examined to determine the characteristics of events that resulted in preferential flow vs. those that resulted in sequential flow. Analyses showed that preferential flow was common throughout the catchment, occurring during 17 to 54% of the 175 events at each of the 10 monitored sites. Preferential flow occurred in at least one site during 90% of the 175 events. While the frequency of preferential flow appeared insensitive to topographic position, the controls on preferential flow initiation varied with landscape position. Analysis of subsets of the time series data showed that while the frequency of preferential flow can be determined from 1 yr of real-time monitoring, the controls on preferential flow require much longer (≥3 yr) monitoring to be reliably identified.

Climate change is expected to increase the intensity of precipitation, but our ability to model the consequences for soil respiration are limited by a lack of data from soils that are saturated and draining. In this study, we used large intact soil columns (28 × 30 cm) to 1) quantify changes in CO 2 flux as soils drain from saturated conditions, and 2) to determine which soil water metrics best predict instantaneous maximum CO 2 flux. The columns were from three agricultural landscape positions that vary in soil properties. We simulated water table fluctuations that were observed at the field site (and predicted to increase in future climate scenarios) by flooding the columns from bottom to surface and then allowing the columns to drain for 96 h while monitoring volumetric soil water content (VWC), water filled pore space (WFPS), water content normalized to field capacity, matric potential, and CO 2 flux. Mean cumulative CO 2 flux was 4649 mg CO 2 ―C m − 2 96 h − 1 . Regardless of landscape position, CO 2 flux rates exhibited a single maximum slightly below saturation, near field capacity. This result suggests that many field studies have not captured soil respiration rates when water availability is optimum for heterotrophic respiration. Across landscape positions, matric potential was the most consistent indicator of instantaneous maximum CO 2 flux, with maximum fluxes occurring within the narrow range of − 0.15 to − 4.89 kPa. In contrast, instantaneous maximum CO 2 flux rates occurred between 95 and 131% of water content normalized to field capacity, 72–97% WFPS, and 29–45% VWC. Thus, our data suggest that instantaneous maximum CO 2 flux rates, a key parameter in ecosystem models, can be predicted across an agricultural landscape with diverse soils if matric potential is used as a water scalar.

Deep seepage is a term in the hillslope and catchment water balance that is rarely measured and usually relegated to a residual in the water balance equation. While recent studies have begun to quantify this important component, we still lack understanding of how deep seepage varies from hillslope to catchment scales and how much uncertainty surrounds its quantification within the overall water balance. Here, we report on a hillslope water balance study from the H. J. Andrews Experimental Forest in Oregon aimed at quantifying the deep seepage component where we irrigated a 172-m2 section of hillslope for 24·4 days at 3·6 ± 3 mm/h. The objective of this experiment was to close the water balance, identifying the relative partitioning of, and uncertainties around deep seepage and the other measured water balance components of evaporation, transpiration, lateral subsurface flow, bedrock return flow and fluxes into and out of soil profile storage. We then used this information to determine how the quantification of individual water balance components improves our understanding of key hillslope processes and how uncertainties in individual measurements propagate through the functional uses of the measurements into water balance components (i.e. meteorological measurements propagated through potential evapotranspiration estimates). Our results show that hillslope scale deep seepage composed of 27 ± 17% of applied water. During and immediately after the irrigation experiment, a significant amount of the irrigation water could not be accounted for. This amount decreased as the measurement time increased, declining from 28 ± 16% at the end of the irrigation to 20 ± 21% after 10 days drainage. This water is attributed to deep seepage at the catchment scale. Copyright © 2010 John Wiley & Sons, Ltd.

summary Hillslope hydrological response to precipitation is extremely complex and poorly modeled. One possible approach for reducing the complexity of hillslope response and its mathematical parameterization is to look for macroscale hydrological behavior. Hillslope threshold response to storm precipitation is one such macroscale behavior observed at field sites across the globe. Nevertheless, the relative controls on the precipitation–discharge threshold poorly known. This paper presents a combined model development, calibration and testing experiment study to investigate the primary controls on the observed precipitation–discharge threshold relationship. We focus on the dominant hydrological processes revealed in part one of this two-part paper and with our new numerical model, replicate the threshold response seen in the discharge record and other hydrometric and tracer data available at the site. We then present a series of virtual experiments designed to probe the controls on the threshold response. We show that the threshold behavior is due to a combination of environmental (storm spacing and potential evapotranspiration) and geologic (bedrock permeability and bedrock topography) factors. The predicted precipitation– discharge threshold subsumes the complexity of plot-scale soil water response. We then demonstrate its use for prediction of whole-catchment storm discharge at other first order catchments at Maimai and the HJ Andrews Experimental Forest in Oregon.

summary Hillslope threshold response to storm rainfall is poorly understood. Basic questions regarding the type, location, and flow dynamics of lateral, subsurface flow remain unanswered, even at our most intensively studied field sites. Here we apply a forensic approach where we combined irrigation and excavation experiments at the well studied Maimai hillslope to determine the typology and morphology of the primary lateral subsurface flowpaths, and the control of bedrock permeability and topography on these flowpaths. The experiments showed that downslope flow is concentrated at the soil bedrock interface, with flowpath locations controlled by small features in the bedrock topography. Lateral subsurface flow is characterized by high velocities, several orders of magnitude greater than predicted by Darcy’s Law using measured hydraulic conductivities at the site. We found the bedrock to be moderately permeable, and showed that vertical percolation of water into the bedrock is a potentially large component of the hillslope water balance. Our results suggest that it is the properties of the bedrock (topography and permeability) that control subsurface flow at Maimai, and the soil profile plays a less significant role than previously thought. A companion paper incorporates these findings into a conceptual model of hydrological processes at the site to explore the generalities of whole-hillslope threshold response to storm rainfall.

Anticipated increases in precipitation intensity due to climate change may affect hydrological controls on soil N 2 O fluxes, resulting in a feedback between climate change and soil greenhouse gas emissions. We evaluated soil hydrologic controls on N 2 O emissions during experimental water table fluctuations in large, intact soil columns amended with 100 kg ha -1 -KNO 3 -N. Soil columns were collected from three landscape positions that vary in hydrological and biogeochemical properties (N = 12 columns). We flooded columns from bottom to surface to simulate water table fluctuations that are typical for this site, and expected to increase given future climate change scenarios. After the soil was saturated to the surface, we allowed the columns to drain freely while monitoring volumetric soil water content, matric potential and N 2 O emissions over 96 h. Across all landscape positions and replicate soil columns, there was a positive linear relationship between total soil N and the log of cumulative N 2 O emissions (r 2 = 0.47; P = 0.013). Within individual soil columns, N 2 O flux was a Gaussian function of water-filled pore space (WFPS) during drainage (mean r 2 = 0.90). However, instantaneous maximum N 2 O flux rates did not occur at a consistent WFPS, ranging from 63% to 98% WFPS across landscape positions and replicate soil columns. In contrast, instantaneous maximum N 2 O flux rates occurred within a narrow range (-1.88 to -4.48 kPa) of soil matric potential that approximated field capacity. The relatively consistent relationship between maximum N 2 O flux rates and matric potential indicates that water filled pore size is an important factor affecting soil N 2 O fluxes. These data demonstrate that matric potential is the strongest predictor of the timing of N 2 O fluxes across soils that differ in texture, structure and bulk density.

Recent studies have called for a new unifying hydrological theory at the hillslope and watershed scale, emphasizing the importance of coupled process understanding of the interactions between hydrology, ecology, pedology, geochemistry and geomorphology. The Biosphere 2 Hillslope Experiment is aimed at tackling this challenge and exploring how climate, soil and vegetation interact and drive the evolution of the hydrologic hillslope behavior. A set of three large-scale hillslopes (18 m by 33 m each) will be built in the climate-controlled experimental biome of the Biosphere 2 facility near Tucson, Arizona, USA. By minimizing the initial physical complexity of these hillslopes, the spontaneous formation of flow pathways, soil spatial heterogeneity, surface morphology and vegetation patterns can be observed over time. This paper documents the hydrologic design process for the Biosphere 2 Hillslope Experiment, which was based on design principles agreed upon among the Biosphere 2 science community. Main design principles were that the hillslopes should promote spatiotemporal variability of hydrological states and fluxes, facilitate transient lateral subsurface flow without inducing overland flow and be capable of supporting vegetation. Hydrologic modeling was used to identify a hillslope configuration (geometry, soil texture, soil depth) that meets the design objectives. The recommended design for the hillslopes consists of a zero-order basin shape with a 10 degree overall slope, a uniform soil depth of 1 m and a loamy sand soil texture. The sensitivity of the hydrologic response of this design to different semi-arid climate scenarios was subsequently tested. Our modeling showed that the timing of rainfall in relation to the timing of radiation input controls the spatiotemporal variability of moisture within the hillslope and the generation of lateral subsurface flow. The Biosphere 2 Hillslope Experiment will provide an excellent opportunity to test hypotheses, observe emergent patterns and advance the understanding of interactions.

This chapter investigates soil moisture spatial-temporal patterns and preferential flow dynamics in two contrasting landscapes in the Ridge and Valley Physiographic Region in eastern United States: 1) the Shale Hills is a steep-sloped forestland, 7.9 ha, with a ratio of elevation change over total area (E/A) of 6.8, and 2) the Kepler Farm is a more gently rolling cropland, 19.5 ha, with the E/A ratio of 1.2. Similarities and differences between the two landscapes were examined based on multiple years' soil and hydrologic monitoring. Through a series of statistical analyses, we found that: 1) Soil moisture spatial correlation length was an order of magnitude shorter in the Shale Hills (20-40 m) as compared to the Kepler Farm (100-153 m), suggesting a greater spatial variability within a shorter distance in the Shale Hills that was aligned with topography. Regression kriging with terrain attributes was found optimal for interpolating soil hydrologic properties in the Shale Hills, while ordinary kriging was optimal at the Kepler Farm; 2) Soil moisture spatial variability across the Shale Hills increased exponentially with catchment-wide wetness, whereas that increased linearly at the Kepler Farm. This was in part due to different landscape configurations and the E/A ratios; 3) The mean relative difference (MRD) of soil moisture was smaller at the Kepler Farm (-55 to 56%) than that in the Shale Hills (-80 to 81%) over the same time period analyzed in time stability analysis, reflecting the overall larger spatial variation of soil moisture in the Shale Hills. Inversely, standard deviation of MRD was slightly larger in the Kepler Farm (4-46%) than that in the Shale Hills (3-28%), implying slightly stronger temporal dynamics in the Kepler Farm due to cropping impacts; 4) The primary Empirical Orthogonal Functions explained 76-90% variation of soil moisture in the Shale Hills, but only 31-67% variation at the Kepler Farm, reflecting the dominant control on soil moisture by terrain in the Shale Hills, while soil properties showed stronger influence on soil moisture at the Kepler Farm; 5) The Shale Hills had overall averaged 37% frequency of preferential flow occurrence during 175 rainfall events in 2007-2009, while the Kepler Farm had 19% during 139 rainfall events occurred in 2008-2010. This study illustrated the spatial complexity and temporal dynamics in soil moisture patterns and preferential flow dynamics that were controlled by the interactions of terrain, soil, and vegetation, and that such interactions changed with landscape characteristics, seasonal wetness, and soil depth.

Mechanistic assessment of how transpiration influences subsurface flow is necessary to advance understanding of catchment hydrology. We conducted a 24-day, steady-state irrigation experiment to quantify the relationships among soil moisture, transpiration and hillslope subsurface flow. Our objectives were to: (1) examine the time lag between maximum transpiration and minimum hillslope discharge with regard to soil moisture; (2) quantify the relationship between diel hillslope discharge and daily transpiration; and (3) identify the soil depth from which trees extract water for transpiration. An 8 x 20 m hillslope was irrigated at a rate of 3.6 mm h -1 . Diel fluctuations in hillslope discharge persisted throughout the experiment. Pre-irrigation time lags between maximum transpiration and minimum hillslope discharge were 6·5 h, whereas lags during steady-state and post-irrigation conditions were 4 and 2 h, respectively. The greatest correlation between transpiration and hillslope discharge occurred during the post-irrigation period, when the diel reduction in hillslope discharge totalled 90% of total measured daily transpiration. Daily transpiration of trees within the irrigated area remained relatively constant throughout the experiment. Diel fluctuations in soil moisture were greatest at a depth of 0·9-1·2 m prior to irrigation and became more uniform throughout the soil profile during and post-irrigation. This study clearly demonstrates that when soil moisture is high, hillslope trees can be an important factor in diel fluctuations in stream discharge. We advance a conceptual model for the site whereby the relationship between transpiration and hillslope discharge is a function of soil moisture status and drainable porosity.