Lateral Subsurface Flow Research Articles

A top–down soil moisture and sap flux sampling design of a rain–snow transition mountain watershed

AbstractThis paper presents a top–down approach for soil moisture and sap flux sampling design with the goal of understanding ecohydrologic response to interannual climate variation in the rain–snow transition watersheds. The design is based on a priori estimates of soil moisture and transpiration patterns using a physical distributed model, Regional Hydro‐Ecologic Simulation System (RHESSys). RHESSys was initially calibrated with existing snow depth and streamflow data. Calibrated model estimates of seasonal trajectories of snowmelt, root‐zone soil moisture storage, and transpiration were used to develop five hydrologic similarity indicators and map these at (30 m) patch scale across the study watershed. The partitioning around medoids‐clustering algorithm was then used to define six distinctive spatially explicit clusters based on the five hydrologic similarity indictors. A representative site within each cluster was identified for sampling. For each site, soil moisture sensors were installed at the 30‐ and 90‐cm depths and at the five soil pits and a sap flux sensor at the averaged‐size white fir tree for each site. The model‐based cluster analysis suggests that the elevation gradient and topographically driven flow drainage patterns are the dominant drivers of spatial patterns of soil moisture and transpiration. The comparison of model‐based calculated hydrological similarity indicators with measured‐data‐based values shows that spatial patterns of field‐sampled soil moisture data typically fell within uncertainty bounds of model‐based estimates for each cluster. There were however several notable exceptions. The model failed to capture the soil moisture and sap flux dynamics in a riparian zone site and in a site where lateral subsurface flow may not follow surface topography. Results highlight the utility of using a hypothesis driven sampling strategy, based on a physically based model, for efficiently providing new information that can drive both future measurements and strategic refinements to model inputs, parameters, or structure that might reduce these errors. Future research will focus on strategies for using of finer scale representations of microclimate, topography, vegetation, and soil properties to improve models.

Varying Influence of Dairy Manure Injection on Phosphorus Loss in Runoff over Four Years.

Surface application of manure on no-till farms can exacerbate P losses in runoff, contributing to the eutrophication of surface waters. We monitored 12 400-m field plots over 4 yr to compare P losses in surface runoff and lateral subsurface flow with shallow disk injection and broadcast application of dairy manure. Given the substantial variability in annual P losses, as well as a gradual, annual buildup of residual soil test P, significant differences in runoff P losses were detected in only 1 of 4 yr: in 2014, total P losses in runoff were 68% greater from broadcast manure plots than injected manure plots. Dissolved and particulate P were roughly even in their contribution to runoff. Even so, there were significant relationships between annual dissolved P losses and P in the soil surface, which pointed to soils as a regular source of P in runoff. Overall, results confirm the potential for injection to reduce P loss in runoff relative to broadcast application, but because in a few sampling dates injection resulted in greater losses, this study also highlights the importance of assessing mitigation benefits of manure application practices over longer timeframes.

Eogenetic Karst in Interbedded Carbonates and Evaporites and Its Impact on Hydrocarbon Reservoir: A New Case from Middle Triassic Leikoupo Formation in Sichuan Basin, Southwest China

Karst in interbedded carbonates and evaporites has been reported to have important and complex impacts on reservoir. It is significant for exploration and karst geology. Here, we report such a new case from Middle Triassic Leikoupo Formation of Sichuan Basin, Southwest China. Stratigraphic incompleteness and the occurrence of unconformity provide evidence for the presence of eogenetic karst. Under the impact of this eogenetic karst, residual weathered and solution-collapse breccia, solution pores and silicification and dedolomitization have been observed. Classic stratigraphic zonation of karst is not readily distinguishable, which is ascribed to the stratigraphic collapse of carbonate rocks resulting from the dissolution of evaporites by lateral subsurface fluid flow. In terms of impact on reservoir quality, karst can generally improve the initial physical property of the porous layers in theory. However, subsurface fluid flow dissolved the evarporitic beds and facilitated the collapse of overlying strata. As a consequence, the lateral continuity of the reservoirs would be destroyed, and relatively high-quality reservoirs can only be developed with little collapse of overlying strata, reflecting reservoir heterogeneities. This may be a general feature of reservoir formation under the impact of karst in interbedded carbonates and evaporites.

Effectiveness of water-oriented thinning in two semiarid forests: The redistribution of increased net rainfall into soil water, drainage and runoff

Water is the key element that modulates the provision of goods and services together with global/climate stressors affecting semiarid forests. In this sense, there is a need to improve the understanding and quantification of forest and water relationships as affected by forest management. This work addresses this issue by comparing net rainfall (Pn) redistribution into different belowground hydrological processes (BHP) in two forest types after a thinning treatment: a holm oak coppice (HU) and a post-fire Aleppo pine regeneration (CAL). The relative contribution (RI) of forest structure, antecedent soil moisture (θst), rainfall and meteorological conditions on the BHP was assessed through boosted regression trees models. In both sites, the RI of the forest structure itself was limited (<10%). However, θst, which clearly increased significantly with thinning, received an average RI of 29%. Surface and subsurface lateral flows showed values <1% of gross rainfall (Pg) in either site and were not significantly affected by thinning. On the other hand, soil moisture and drainage were affected by the thinning treatment, although with different extent depending on the site: in the drier site (CAL), the increased Pn in the thinning was mainly allocated into increased soil water content, with very limited improvement in drainage (<10 mm/year); in contrast, in the wetter continental site of HU, drainage to deeper soil layers was the most remarkable effect of thinning (50 mm/year higher than in control), given the higher θst and hence the lower soil water storage available. Thinning also improved the response of BHP during drought, making these processes more elastic and less vulnerable to climatic extremes. The results presented here complement those previously reported on rainfall partitioning in these sites and all together provide a comprehensive understanding of the short-term effect (3–4 years) of water-oriented silviculture in Quercus ilex and Pinus halepensis low-biomass semiarid forests. Questions such as the long-term effects of thinning remain open for these ecosystems.

Simulation of Flow and Agricultural Non-Point Source Pollutant Transport in a Tibetan Plateau Irrigation District

Flow and transport processes in soil and rock play a critical role in agricultural non-point source pollution (ANPS) loads. In this study, we investigated the ANPS load discharged into rivers from an irrigation district in the Tibetan Plateau and simulated ANPS load using a distributed model. Experiments were conducted for two years to measure soil water content and nitrogen concentrations in soil and the quality and quantity of subsurface lateral flow in the rock and at the drainage canal outlet during the highland barley growing period. A distributed model, in which the subsurface lateral flow in the rock was described using a stepwise method, was developed to simulate flow and ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3−-N) transport processes. Sobol’s method was used to evaluate the sensitivity of simulated flow and transport processes to the model inputs. The results showed that with a 21.2% increase of rainfall and irrigation in the highland barley growing period, the average NH4+-N and NO3−-N concentrations in the soil layer decreased by 10.8% and 14.3%, respectively, due to increased deep seepage. Deep seepage of rainfall water accounted for 0–52.4% of total rainfall, whereas deep seepage of irrigation water accounted for 36.6–45.3% of total irrigation. NH4+-N and NO3−-N discharged into the drainage canal represented 19.9–30.4% and 19.4–26.7% of the deep seepage, respectively. The mean Nash–Sutcliffe coefficient value, which was close to 0.8, and the lowest values of root mean square errors, the fraction bias, and the fractional gross error indicated that the simulated flow rates and nitrogen concentrations using the proposed method were very accurate. The Sobol’s sensitivity analysis results demonstrated that subsurface lateral flow had the most important first-order and total-order effect on the simulated flow and NH4+-N and NO3−-N concentrations at the surface drainage outlet.

How Meaningful are Plot‐Scale Observations and Simulations of Preferential Flow for Catchment Models?

Core Ideas We tested a range of dual‐permeability parameterizations at plot and catchment scale. Well‐performing parameters at plot scale did not clearly improve catchment simulation. Vertical preferential flow was important for simulating plot‐scale observations. At catchment scale, it appeared more important to consider fast lateral subsurface flow. This showed that different nonuniform flow processes are critical at different scales. Despite ubiquitous field observations of nonuniform flow processes, preferential flow paths are rarely considered in hydrological models, especially at catchment scale. In this study, we investigated the extent to which plot‐scale observations of preferential flow paths are informative for rainfall–runoff simulations at larger scales. We used data from three plot‐scale irrigation experiments in the Weierbach catchment (Luxembourg) to identify preferential flow parameterizations via a Monte Carlo simulation with HydroGeoSphere. Subsequently, we tested whether these parameter sets could be used directly to simulate the hydrological response of the Weierbach headwater with a HydroGeoSphere catchment model. The Monte Carlo simulations showed that the different depth profiles of Br− tracer observed in irrigation experiments could be reproduced when vertical preferential flow was simulated with a dual‐permeability approach. However, it was not possible to identify unique parameter values for preferential flow. The direct transfer of a range of different dual‐permeability parameter sets to the catchment model revealed that the variability of simulated hydrometric catchment responses (discharge and soil moisture over 18 mo) was independent of the variability among the three irrigation experiments. More importantly, the dual‐permeability approach did not improve the match between simulated and observed discharge and soil moisture responses compared with the single‐domain reference model, where multiple soil layers with differing hydraulic conductivities had already been implemented. This suggests that including structures that allow nonuniform lateral flow was more important for reproducing the hydrological response in the Weierbach catchment than the vertical preferential flow observed at plot scale.

Water budget and run‐off response of a tropical multispecies “reforest” and effects of typhoon disturbance

AbstractTo examine claims that reforesting degraded Imperata grassland in Leyte (Philippines) made streamflow perennial again, we studied the hydrological behaviour of a 23‐year‐old mixed‐species “reforest” between June 2013 and May 2014. Typhoon Haiyan, one of the strongest events ever, caused major damage to the site in November 2013. Average daily apparent water use (ET) was 5.0 mm day−1 pretyphoon and 3.2 mm day−1 after disturbance. Corresponding ratios of period total quickflow Qq to precipitation were 16% and 44%. Quickflow volume and peak discharge increased rapidly once a threshold value of ~250 mm for soil water storage in the top 60 cm was exceeded. Before disturbance, quickflow consisted predominantly of lateral subsurface flow due to high soil hydraulic conductivities down to 60 cm. After disturbance, shallow groundwater rose regularly to within 10 cm of the surface on foot slopes, and saturation overland flow was observed during several large storms. Comparing estimated annual ET and Qq for undisturbed conditions for the reforest and a nearby degraded Imperata grassland microcatchment suggested that the extra infiltration following reforestation (~240 mm year−1) exceeded the extra ET by the reforest (100–185 mm year−1), implying a net positive trade‐off (55–140 mm year−1) and tentatively confirming local claims of improved dry‐season flow.

Controls on gross production in an aspen–sagebrush vegetation mosaic

AbstractThe critical zone (CZ; defined as the zone between the top of the vegetation canopy and the groundwater) mediates the impact of precipitation amount and timing on water availability and plant productivity. However, CZ structure, including soil and subsurface properties, are almost always unknown, leading to considerable uncertainty in the links between precipitation, plant water availability, and gross production. Using multiyear records of gross ecosystem CO2 exchange (GEE), micrometeorology, and streamflow, we examined the sensitivity of GEE to environmental controls in a sagebrush shrubland and an aspen forest. The sites were within 500 m of each other and had similar precipitation and temperature but marked differences in CZ structure. Cumulative growing season GEE was approximately two times greater at the aspen compared with the sagebrush, underscoring the importance of CZ properties in structuring spatial “hot spots” for carbon cycling. Larger soil water holding capacity and topography that produced lateral subsurface flow to the aspen enabled marked difference in plant functional type. Annual variability in growing season GEE within each site was not driven by annual precipitation; this lack of relationship was attributed to the CZ's limited ability to store water and an associated increase in water yield. Instead, both seasonal timing and cumulative growing season GEE for the sagebrush varied with spring and summer rain, whereas aspen GEE responded to spring snowpack conditions. These results emphasized how mapping and understanding CZ structure will help predict the spatial variability in plant functional type and temporal sensitivity of growing season GEE.

Occurrence of subsurface lateral flow in the Shale Hills Catchment indicated by a soil water mass balance method

Subsurface lateral flow (SLF) contributes substantially to hillslope runoff. However, because of the lack of appropriate methods, field investigation of SLF at the hillslope and catchment scales has been limited. Recently, high‐frequency soil moisture monitoring has been tested to characterize SLF. This study presented a simple approach to determine SLF based on soil water mass balance and compared this method with an established approach that used the depth‐specific soil moisture response time to precipitation to identify preferential flow. The new method defined the occurrence of SLF when the increase in soil water storage was greater than the accumulated quantity of effective precipitation during a rain event. We applied this method to a 10‐minute resolution soil moisture dataset collected over 3 years from nine soil profiles along a concave hillslope. We found that (i) SLF frequency derived by the proposed method matched well with preferential flow frequency obtained by the established method (r &gt; 0.9), (ii) precipitation and initial soil wetness together controlled the generation of SLF, precipitation intensity determined whether or not the hillslope produced SLF and the quantity of precipitation governed the spatial extent of SLF, and (iii) topographic and small‐scale soil features led to spatially different frequencies of SLF during small storms. Identifying SLF from a soil moisture time series over an entire rain event complements the established method based on soil moisture dynamics at the beginning of an event. Applying both methods creates the potential to measure SLF frequency and vertical preferential flow frequency, respectively. Soil moisture sensor networks have improved the large‐scale investigation of such preferential flows.Highlights How can the occurrence of subsurface lateral flow (SLF) be detected by soil moisture time series? A simple approach is proposed to detect SLF in a forested hillslope using soil water mass balance. Three‐year SLF frequency obtained by the new method agreed with that from an established method. Rain, topography and soil attributes control the generation and spatial distribution of SLF.

Multimodel assessment of climate change-induced hydrologic impacts for a Mediterranean catchment

Abstract. This work addresses the impact of climate change on the hydrology of a catchment in the Mediterranean, a region that is highly susceptible to variations in rainfall and other components of the water budget. The assessment is based on a comparison of responses obtained from five hydrologic models implemented for the Rio Mannu catchment in southern Sardinia (Italy). The examined models – CATchment HYdrology (CATHY), Soil and Water Assessment Tool (SWAT), TOPographic Kinematic APproximation and Integration (TOPKAPI), TIN-based Real time Integrated Basin Simulator (tRIBS), and WAter balance SImulation Model (WASIM) – are all distributed hydrologic models but differ greatly in their representation of terrain features and physical processes and in their numerical complexity. After calibration and validation, the models were forced with bias-corrected, downscaled outputs of four combinations of global and regional climate models in a reference (1971–2000) and future (2041–2070) period under a single emission scenario. Climate forcing variations and the structure of the hydrologic models influence the different components of the catchment response. Three water availability response variables – discharge, soil water content, and actual evapotranspiration – are analyzed. Simulation results from all five hydrologic models show for the future period decreasing mean annual streamflow and soil water content at 1 m depth. Actual evapotranspiration in the future will diminish according to four of the five models due to drier soil conditions. Despite their significant differences, the five hydrologic models responded similarly to the reduced precipitation and increased temperatures predicted by the climate models, and lend strong support to a future scenario of increased water shortages for this region of the Mediterranean basin. The multimodel framework adopted for this study allows estimation of the agreement between the five hydrologic models and between the four climate models. Pairwise comparison of the climate and hydrologic models is shown for the reference and future periods using a recently proposed metric that scales the Pearson correlation coefficient with a factor that accounts for systematic differences between datasets. The results from this analysis reflect the key structural differences between the hydrologic models, such as a representation of both vertical and lateral subsurface flow (CATHY, TOPKAPI, and tRIBS) and a detailed treatment of vegetation processes (SWAT and WASIM).

Role of underground fissure flow in near-surface rainfall-runoff process on a rock mantled slope in the karst rocky desertification area

Because of the complex near-surface double-layer geologic structure (i.e., surface and underground fissures) of slopes in karst regions, it is difficult to collect data on all flows, especially the flows generated in underground fissures. This lack of information leads to a poor understanding of the rainfall-runoff process associated with soil loss in karst landscapes. The goal of this study was to determine the output characteristics of surface, subsurface and underground fissure flows and their conversion relationships, as well as the role of underground fissure flow in the near-surface rainfall-runoff process, using rainfall experiments. The results revealed the following findings. (і) Light rainfall intensities (15 and 30 mm/h) generated subsurface lateral flow and underground fissure flow, whereas greater rainfall intensities generated surface runoff in addition to subsurface and underground flows. (ii) The runoff/drainage depths of all the above flows increased with increasing rainfall intensity, while the runoff/drainage coefficient exhibited no obvious regular change, except for the surface runoff. Moreover, the distribution ratio of surface runoff to total flow also increased with increasing rainfall intensity but decreased with increasing underground fissure degree, while the distribution ratio of underground fissure flow exhibited the opposite trend. (iii) Underground fissure flow, with distribution ratios of 27.8–78.0%, dominated the total flow yield. This study has significance for controlling soil erosion and water shortage in the karst rocky desertification region and also improves the understanding of the near-surface rainfall-runoff process in this region

Simulating crop yield, surface runoff, tile drainage and phosphorus loss in a clay loam soil of the Lake Erie region using EPIC

Useful in evaluating best management practices and nutrient management planning, the prediction of phosphorus (P) transfer from agricultural lands to water bodies via surface runoff and tile drainage remains as a challenge, as few models can provide reasonably accurate P loss simulations. The EPIC (Environmental Policy Integrated Climate) model was firstly applied to simulate crop yields, surface runoff, tile drainage, and dissolved reactive P (DRP) losses under a corn-soybean rotation grown on a “cracking” Brookston clay loam soil (Vertisol) in the Lake Erie basin, Ontario, Canada. Different potential evapotranspiration and curve number equations were compared to determine the most suitable equations (Penman-Monteith equation and variable Daily Curve Number with soil moisture index) for this region. A crack flow coefficient was used to deal with inflow partitioned to cracks. A soil layer below tile drain with low saturated hydraulic conductivity was employed in simulating the experimental site, where most water was leaving the field through tile drain. Lateral subsurface flow was used to substitute for drainage. Annual simulations of crop grain yield, cumulative surface runoff, and cumulative drainage closely matched observed data. Over shorter periods (months), surface runoff (NSE = 0.78), tile drainage (NSE = 0.57), and relevant DRP loss (NSE > 0.5) simulations were satisfactory, except for two periods of DRP loss in surface runoff, where most DRP moved downward through lateral flow and deep percolation due to limitations in the crack flow coefficient. For this vertic soil, EPIC generally simulated crop yields and flow volumes well, while DRP losses were only adequately simulated.

Intrinsic vulnerability assessment of shallow aquifers of the sedimentary basin of southwestern Nigeria.

The shallow groundwater of the multi-layered sedimentary basin aquifer of southwestern Nigeria was assessed based on its intrinsic vulnerability property. The vulnerability evaluation involves determining the protective cover and infiltration condition of the unsaturated zone in the basin. This was achieved using the PI (P stands for protective cover effectiveness of the overlying lithology and I indicates the degree of infiltration bypass) vulnerability method of the European vulnerability approach. The PI method specifically measures the protection cover and the degree to which the protective cover is bypassed. Intrinsic parameters assessed were the subsoil, lithology, topsoil, recharge and fracturing for the protective cover. The saturated hydraulic conductivity of topsoil, infiltration processes and the lateral surface and subsurface flow were evaluated for the infiltration bypassed. The results show moderate to very low vulnerability areas. Low vulnerability areas were characterised by lithology with massive sandstone and limestone, subsoils of sandy loam texture, high slopes and high depth to water table. The moderate vulnerability areas were characterised by high rainfall and high recharge, low water table, unconsolidated sandstones and alluvium lithology. The intrinsic vulnerability properties shown in vulnerability maps will be a useful tool in planning and monitoring land use activities that can be of impact in groundwater pollution.

Effects of horizontal grid resolution on evapotranspiration partitioning using TerrSysMP

Biotic leaf transpiration (T) and abiotic evaporation (E) are the two major pathways by which water is transferred from land surfaces to the atmosphere. Earth system models simulating the terrestrial water, carbon and energy cycle are required to reliably embed the role of soil and vegetation processes in order to realistically reproduce both fluxes including their relative contributions to total evapotranspiration (ET). Earth system models are also being used with increasing spatial resolutions to better simulate the effects of surface heterogeneity on the regional water and energy cycle and to realistically include effects of subsurface lateral flow paths, which are expected to feed back on the exchange fluxes and their partitioning in the model.Using the hydrological component of the Terrestrial Systems Modeling Platform (TerrSysMP), we examine the uncertainty in the estimates of T/ET ratio due to horizontal model grid resolution for a dry and wet year in the Inde catchment (western Germany). The aggregation of topography results in smoothing of slope magnitudes and the filtering of small-scale convergence and divergence zones, which directly impacts the surface-subsurface flow. Coarsening of the grid resolution from 120 m to 960 m increased the available soil moisture for ground evaporation, and decreased T/ET ratio by about 5% and 8% for dry and wet year respectively. The change in T/ET ratio was more pronounced for agricultural crops compared to forested areas, indicating a strong local control of vegetation on the ground evaporation, affecting the domain average statistics.

Quantifying Subsurface Lateral Flow along Sloping Horizon Boundaries in Soil Profiles of a Hummocky Ground Moraine

Core Ideas Hydropedologic factors control subsurface lateral flow in eroded hillslope soils. Depth to C horizon, hydraulic properties, and initial conditions are most sensitive. Virtual 2D model experiment predicts effect of tracer injection on lateral movement. Lateral flow at sloping C horizon surface locally separated from tracer plume. Erosion‐induced feedback on subsurface lateral flow by varying depths to C horizon. Subsurface lateral flow in hillslope soils depends on lower permeability or texture‐contrasting soil horizons. In the arable hummocky soil landscape, erosion processes caused glacial till appearance closer to the soil surface at upslope positions. The objective of this work was to quantify the potential for subsurface lateral flow depending on the erosion‐affected spatial hydropedological complexity. The eroded Haplic Luvisol profile was studied due to the presence of a relatively dense C horizon that varied in depth, thickness, and sloping angle. A two‐dimensional numerical modeling and sensitivity analysis for the saturated hydraulic conductivity (Ks) of the C horizon and the depth to C horizon (i.e., soil solum thickness) was performed for rainstorms in 2011 and 2012 using HYDRUS‐2D. A Ks value of ≤2.5 cm d−1 for the C horizon was required for lateral flow initiation. Lateral flow was (i) increasing with decreasing solum thickness, indicating an erosion‐induced feedback on subsurface lateral flow, and (ii) dependent on the soil moisture regime prior to rainstorms. The effect of lateral flow on the movement of a conservative tracer was simulated in the form of a “virtual experiment”. Simulation scenarios revealed only a relatively small lateral shift of the tracer plume caused by a local decoupling of water (already lateral) from subsequent tracer movement (still vertical). Longer term simulations suggested that, for the present conditions, lateral flow was limited mostly to occasional summer storm events. Even without considering preferential flow contribution to lateral flow, highly complex hydropedologic interactions are present in erosion‐affected heterogeneous soil profiles.

A Simulation Study to Estimate Effects of Wildfire and Forest Management on Hydrology and Sediment in a Forested Watershed, Northwestern U.S.

Abstract. Suitable fuel reduction treatments are needed in the Colville National Forest, Washington, to reduce the risk of severe wildfire. This study aimed to identify high-risk erosion hillslopes following wildfire to aid in forest fuel reduction planning and to evaluate the effects of fuel treatments on the watershed hydrological response. The specific objectives were (1) to assess the soil burn severity associated with wildfires and use that information to identify critical hillslopes for forest fuel treatments, and (2) to evaluate the potential changes in water yield and peak flows from pre-treatment (undisturbed forest) to post-treatment (thinning and prescribed burn) conditions, in the East Deer Creek Watershed (EDCW), a subwatershed of the Colville National Forest. Assessments were made using a modeling approach for hypothetical wildfire and fuel treatment scenarios. FlamMap, a fire behavior model, was used to predict the spatial distribution of wildfire intensity for a hypothetical event under current vegetation conditions. WEPP simulations were subsequently completed to obtain sediment and water yields based on fire intensity and topography. WEPP erosion estimations following a simulated wildfire showed hillslope sediment yield varying from 0 to 49.4 Mg ha-1 year-1 from the 777 hillslopes, which were ranked in descending order of sediment yield to identify critical hillslopes for fuel treatments. The WEPP model calibrated for a nearby gauged watershed was then applied to the EDCW for pre-treatment and post-treatment conditions. At the watershed scale, the increase in water yield from pre-treatment to post-treatment conditions ranged from 0.7% to 5.6% on hillslopes delivering 10% to 50% of the predicted post-fire sediment. Simulated water balance components at the treated hillslopes showed substantial changes. Surface runoff, subsurface lateral flow, and deep percolation increased 150% (5 mm), 50% (9 mm), and 40% (41 mm), respectively, whereas evapotranspiration (ET) decreased 23% (124 mm). The relative differences between pre- and post-harvest peak flows showed no clear trends as treatment area increased. The results suggest that thinning and prescribed burns to treated hillslopes in the EDCW may lead to an increase in water yield and significant alterations in hydrological processes. Keywords: Fuel treatments, Modeling, Peak flows, Sediment, Water yield, Wildfire.

Testing the Fill‐and‐Spill Model of Subsurface Lateral Flow Using Ground‐Penetrating Radar and Dye Tracing

Core Ideas Lateral flow patterns revealed by dye tracing agreed with time‐lapse GPR data. Lateral flow downslope varied from &lt;1 to 1 m at adjacent sites. 3D radar detected banding in the strike direction but dye fingering did not. Fractured saprock modifies the fill‐and‐spill model by facilitating vertical flow. Preferential flow (PF), which bypasses large portions of the soil or subsurface matrix, is critical in the transport of water and dissolved constituents in the unsaturated zone. To test the “fill‐and‐spill” model of hillslope hydrology that describes the generation and pattern of downslope lateral PF after storms, we used dye tracer and time‐lapse, ground‐penetrating radar (GPR) on a forested hillslope in the Susquehanna–Shale Hills Critical Zone Observatory. We injected 50 L of water mixed with Brilliant Blue dye (4 g L−1) into a shallow trench cut perpendicular to the slope and used GPR to monitor the tracer downslope across a 1.0‐ by 2.0‐m grid. The site was then excavated to the soil–saprock interface and photographed to document the dye pathways. We observed vertical dye fingering near the infiltration trench. Downslope lateral PF at the soil–saprock boundary was limited to ~0.40 m, which is evidence that the soil–saprock interface did not fill‐and‐spill. The extent, depth, and direction of the downslope PF indicated by GPR generally matched the dye staining patterns in the excavation, but the resolution of the 800‐MHz GPR antenna was insufficient to distinguish small fingers of dye. A revised fill‐and‐spill model was proposed for this site that incorporates the PF through fractured saprock before water encounters fresh bedrock surface. This study demonstrates that GPR integrated with dye tracer infiltration can provide a useful means of testing hillslope hydrological hypotheses and unraveling the complexity of PF at the hillslope scale in a field setting.

Coupling a three-dimensional subsurface flow and transport model with a land surface model to simulate stream–aquifer–land interactions (CP v1.0)

Abstract. A fully coupled three-dimensional surface and subsurface land model is developed and applied to a site along the Columbia River to simulate three-way interactions among river water, groundwater, and land surface processes. The model features the coupling of the Community Land Model version 4.5 (CLM4.5) and a massively parallel multiphysics reactive transport model (PFLOTRAN). The coupled model, named CP v1.0, is applied to a 400 m × 400 m study domain instrumented with groundwater monitoring wells along the Columbia River shoreline. CP v1.0 simulations are performed at three spatial resolutions (i.e., 2, 10, and 20 m) over a 5-year period to evaluate the impact of hydroclimatic conditions and spatial resolution on simulated variables. Results show that the coupled model is capable of simulating groundwater–river-water interactions driven by river stage variability along managed river reaches, which are of global significance as a result of over 30 000 dams constructed worldwide during the past half-century. Our numerical experiments suggest that the land-surface energy partitioning is strongly modulated by groundwater–river-water interactions through expanding the periodically inundated fraction of the riparian zone, and enhancing moisture availability in the vadose zone via capillary rise in response to the river stage change. Meanwhile, CLM4.5 fails to capture the key hydrologic process (i.e., groundwater–river-water exchange) at the site, and consequently simulates drastically different water and energy budgets. Furthermore, spatial resolution is found to significantly impact the accuracy of estimated the mass exchange rates at the boundaries of the aquifer, and it becomes critical when surface and subsurface become more tightly coupled with groundwater table within 6 to 7 meters below the surface. Inclusion of lateral subsurface flow influenced both the surface energy budget and subsurface transport processes as a result of river-water intrusion into the subsurface in response to an elevated river stage that increased soil moisture for evapotranspiration and suppressed available energy for sensible heat in the warm season. The coupled model developed in this study can be used for improving mechanistic understanding of ecosystem functioning and biogeochemical cycling along river corridors under historical and future hydroclimatic changes. The dataset presented in this study can also serve as a good benchmarking case for testing other integrated models.

A sprinkling experiment to quantify celerity-velocity differences at the hillslope scale.

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.

Do Lateral Flows Matter for the Hyperresolution Land Surface Modeling?

AbstractHyperresolution land surface modeling provides an unprecedented opportunity to simulate locally relevant water and energy cycle, but lateral surface and/or subsurface flows that are essential at fine scale are often neglected by most one‐dimensional land surface models (LSMs). To analyze effects of lateral flows across scales, a Conjunctive Surface‐Subsurface Process model, which considers soil moisture‐surface flow interaction and quasi‐three‐dimensional subsurface flow, is implemented over a mountainous HyperHydro test bed in southwestern USA at different resolutions. Validation over more than 70 International Soil Moisture Network stations shows that there are significant improvements in soil moisture simulations from 30 km to 4 km as finer soil property and precipitation data are used, with correlation increased by 5%–16% and error decreased by 5%. Lateral surface flow has a significant influence on surface soil moisture and ground evaporation even at coarse resolution. Effect of lateral subsurface flow on soil moisture is nontrivial at 1 km or finer resolution especially over wet areas. At 100 m resolution, topography‐induced lateral subsurface flow causes drier peaks and wetter valleys, decreases latent heat by 8% at peaks, while increases it by 12% at valleys. Furthermore, influences of lateral subsurface flow on ground evaporation and vegetation transpiration are more significant during dry season due to a stronger coupling between soil moisture and evapotranspiration. Therefore, it is worthy to incorporate lateral flow processes in hyperresolution LSMs to better represent water and energy heterogeneity even with limited hyperresolution meteorological and surface data.