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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.

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.

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.

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.