Integrated Surface/subsurface flow modeling in PFLOTRAN

A hybrid coupled model of surface and subsurface flow for surface irrigation

We explore the coupling of surface and subsurface flows on fully unstructured meshes that conform to complex soil structures. To accommodate the distorted meshes that inevitably result from explicit representation of complex soil structures, we leverage the structure of the Mimetic Finite Difference (MFD) spatial discretization scheme to couple surface and subsurface flows. The MFD method achieves second-order accuracy and maintains local mass conservation on distorted meshes. We couple the diffusion wave approximation for surface flows to the Richards equation for subsurface flow, ensuring continuity of both pressure and flux between the surface and subsurface. The MFD method is particularly convenient for this coupling because it uses face-based constraints in the subsurface system that can be expressed as face-pressure unknowns. Those unknowns are coincident with surface cell-based unknowns, thus allowing the discrete surface system to be directly substituted into the subsurface system and solved implicitly as a global system. Robust representation of the transition between wet and dry surface conditions requires upwinding of the relative permeability and is facilitated by globalization in the nonlinear solver. The approach and its implementation in the Advanced Terrestrial Simulator (ATS) are evaluated by comparison to previously published benchmarks. Using runoff from soils with patchy groundcover (duff) as an example, we show that the new method converges significantly faster in mesh convergence tests than the commonly used two-point flux approximation.

This study investigated the efficiency of a sequential system involving surface flow and vegetated sub-surface flow constructed wetland (CW) in the treatment of Brewery Wastewater. Six experimental CW (2 surface and 4 subsurface flow) and control (1 surface and 2 vegetated subsurface flow) with 200 mm depth of 19.05 mm diameter granite and 100 mm depth of sharp sand as substrate were used for the experiment. The CWs were planted with locally available macrophytes: water hyacinth (Eichhornia crassipes), Cattail (Typha latifolia) and Vetiver grass (Vetiveria nigritana). The microcosms were irrigated using wastewater from Brewery Effluent (BE), pollution parameters were measured and treatment efficiency was monitored. The pH, Electrical Conductivity (EC), Total Dissolved Solids (TDS), Dissolved Oxygen (DO) and Biological Oxygen Demand (BOD5) of the BE studied were 6.84, 1189 μs/m, 2998 mg/l, 9.4 mg/l and 1244 mg/l, respectively. Average reductions of 92.53, 48.30 and 67.16 % were observed in the TDS, BOD and Nitrate after treatment. The BOD5 percentage removal was higher in the Subsurface CW than in the Surface flow CW (30.11 and 49.04 % for surface and subsurface flow, respectively). The study showed that CW using surface and subsurface flow constructed wetland with locally available macrophytes is efficient in Brewery wastewater treatment.

Wastewater treatment by constructed wetland is an appropriate technology for tropical developing countries like Indonesia because it is inexpensive, easily maintained, and has environmentally friendly and sustainable characteristics. The aim of the research is to examine the capability of constructed wetlands for treating laboratory wastewater at our Center, to investigate the suitable flow for treatment, namely vertical subsurface or horizontal surface flow, and to study the effect of the seasons. The constructed wetland is composed of three chambered unplanted sedimentation tanks followed by the first and second beds, containing gravel and sand, planted with Typha sp.; the third bed planted with floating plant Lemna sp.; and a clarifier with two chambers. The results showed that the subsurface flow in the dry season removed 95% organic carbon (COD) and total phosphorus (T-P) respectively, and 82% total nitrogen (T-N). In the transition period from the dry season to the rainy season, COD removal efficiency decreased to 73%, T-N increased to 89%, and T-P was almost the same as that in the dry season. In the rainy season COD and T-N removal efficiencies increased again to 95% respectively, while T-P remained unchanged. In the dry season, COD and T-P concentrations in the surface flow showed that the removal efficiencies were a bit lower than those in the subsurface flow. Moreover, T-N removal efficiency was only half as much as that in the subsurface flow. However, in the transition period, COD removal efficiency decreased to 29%, while T-N increased to 74% and T-P was still constant, around 93%. In the rainy season, COD and T-N removal efficiencies increased again to almost 95%. On the other hand, T-P decreased to 76%. The results show that the constructed wetland is capable of treating the laboratory wastewater. The subsurface flow is more suitable for treatment than the surface flow, and the seasonal changes have effects on the removal efficiency.

Agricultural hillslopes present particular challenges for estimating vadose zone dynamics due to a variety of processes, such as surface runoff, vertical flow, erosion, subsurface preferential flow affected by soil structure and layering, non-linear chemical behaviour, evapotranspiration, etc. To investigate these processes and complexity, the SUPREHILL critical zone observatory (CZO) was started in 2020, at vineyard hillslope site in Croatia. The observatory is extensively equipped for the soil-water regime and agrochemical fluxes monitoring, and includes an extensive sensor network, lysimeters (weighing and passive wick), suction probes, surface and subsurface flow and precipitation collection instruments. The main objective of the SUPREHILL observatory is to quantify subsurface lateral and local scale preferential flow processes. Local-scale nonlinear processes in eroded agricultural hillslope sites have large significance on water and solute behaviour within the critical zone and thus need to be researched in depth using combined methods and various approaches. First results from the sensor and lysimeter network, soil-water regime monitoring, isotope analysis, and agrochemical concentrations in 2021 supported the hypothesis of the observatory, that the subsurface flow plays a relevant part in the hillslope soil-water dynamics. In the wick lysimeter network, although the highest cumulative outflow values were found at the hilltop, the highest individual measurements were found at the footslope. During high-intensity rainfall events, there were differences in weighing lysimeters, possibly related to subsurface lateral flow. Based on the isotope analysis, wick lysimeters exhibit a greater variation of d-excess values than suction probes. Agrochemical fluxes confirmed the sloping effect on their transport in soil and demonstrated the favourability of Cu transport by subsurface flow. Using the comprehensive database presented herein, future analyses of this hypothesis will be carried out in more detail using model-based analyses.  

A new model for coupling surface and subsurface water flows: With an application to a lagoon

Data‐based mechanistic (DBM) modeling is an established approach to time series model identification and estimation, which seeks model structures and parameters that are both statistically optimal and consistent with plausible mechanistic interpretations of the system. This paper describes the application of the DBM method to 10 min, relatively high precision, rainfall‐flow data, including observations of both surface flow and subsurface flow. For a generally wet winter period, the preferred surface flow model is nonlinear in flow generation and linear in routing, while the preferred subsurface flow model is linear in flow generation and nonlinear in routing. These models have mechanistic interpretations in terms of mass balance, hydrodynamics, and conceptual flow pathways. The four‐parameter surface and subsurface flow models explain 91% and 96% of the variance of the corresponding observations. Other plausible models were identified but were less parsimonious or were more reliant on prior perceptions. For a wet summer validation period, the models performed as well as in the calibration period; however, when a long dry spell was included, the performance deteriorated. It is speculated that this is because of complex wetting‐drying dynamics and potential nonstationarity of the soil properties that are not sufficiently revealed in the available data. Conceptual models informed by the DBM results matched the DBM model performance for subsurface flow but gave poorer performance for the more complex surface flow responses. It is concluded that the DBM method can identify nonlinearity in both flow generation and routing and provide conceptual insights that can go beyond prior expectations.

Microbial biomass, activity and community composition in constructed wetlands

The frequency of landslides has increased recently due to uneven patterns of weather distribution and climate change. Major landslide predictions are based on rainfall models, and only a few studies consist of hydrological models, leading to low accuracy.  On the other hand, using hydrological parameters has considerably increased the accuracy of landslide predictions. This study focuses on interlinking the climate, surface, and subsurface flows on the landslide events in the Cauvery River basin, Karnataka, India, using the Soil and Water Assessment Tool (SWAT). Relative humidity, solar irradiance, wind speed, temperature, and precipitation are needed for the model, along with slope, land use, and digital elevation model (DEM). Mann-Kendall trend analysis is carried out to detect trends in rainfall and temperature in the study area, as these parameters can directly influence the surface and subsurface flows.  Preliminary results of the study indicate a strong correlation between the surface and subsurface flows on landslides along with the changing climatic conditions. The study region witnessed a series of landslide events in 2018, which, in turn, can be associated with the heavy surface flows observed using the model output for the same period. The derived correlation can further be used to forecast the landslide events with the surface and subsurface flow information. As an outcome, the study also proposes sustainable solutions to mitigate the impact of climate change and land conservation practices to protect the community. 

The interaction between the surface and subsurface flow components plays an important role especially in initial loss and overland flow initiation at the early stage in rainfall events. Therefore, coupling of the surface and subsurface flow submodels is necessary in more comprehensive and sophisticated watershed modeling to deal with the interaction theoretically. We already proposed the conjunctive 2-D surface and 3-D subsurface flow model using an approximate version of the Saint-Venant equations to simulate the two-dimensional unsteady surface flow and a modified version of Richard’s equation for the three-dimensional unsaturated and saturated unsteady subsurface flow [1, 2]. This paper shows a further development of the conjunctive model and focuses on the interaction between surface and subsurface flow components. The interaction of the two components is directly related to the estimation of effective rainfall or initial loss of hyetographs. In the conjunctive model the interaction is formulated using the comparison of the two parameters,“rainwater supply”and“infiltrability (infiltration capacity)”. The model reproduces the initiation of overland flow and initial loss process and contributes to more precise and reliable estimation of effective rainfall than the usual methods using the empirical formulas in watershed modeling.

Headwater streams and wetlands with a combination of surface and subsurface flows are common features of many upland-forested watersheds. Unlike headwater stream reaches with continuous surface flow, the hydrology and ecology of subsurface stream reaches are poorly studied and not factored into existing wetland legislation. We assessed subsurface habitats and associated biota in a 435-m reach of a first-order, intermittent stream draining a riparian zone dominated by eastern hemlock (Tsuga canadensis) in north central Massachusetts. Stream flow was found only in subsurface flowpaths beneath large boulders and surface root mats over approximately 70% of the total stream length at summer base flow. Temperature, specific conductivity, dissolved oxygen, and dissolved organic carbon concentrations of subsurface water were similar to surface water. Macroinvertebrates were found in subsurface habitats but at a lower abundance and richness per unit area compared to surface habitats. Collectors such as Chironomidae, Polycentropodidae, and Ephemerellidae were generally the most abundant families in both surface and subsurface habitats. Our findings indicate that in some glaciated watersheds, intermittent streams with no visual evidence of surface flow may contain subsurface flowpaths with water chemistry and biota comparable to coupled perennial surface flow reaches. The prevalence and importance of subsurface habitats in some headwater streams may warrant review or revision of existing state and local regulatory definitions of intermittent and headwater streams.

The spatial heterogeneity and temporal variability of hillslope subsurface flow (SSF) generation are poorly understood in the alpine critical zone (CZ). Owing to the limitations of observation technology bottlenecks and harsh environments, existing studies lack a holistic and process‐based understanding of SSF generation, particularly for the deep CZ and annual freeze‐thaw (FT) cycle. This study improves the conventional trenching observation scheme and is the first to have successfully monitored SSF processes from the soil to the bedrock layer with high‐resolution from 2017 to 2020 in a headwater catchment of the Qinghai‐Tibetan Plateau. The results show that hillslope runoff generation exhibited similar trends over the years while exhibiting distinct patterns across seasons. Spatially, slope aspects significantly affect the runoff amount and ratio. The south‐facing slope is dominated by overland flow (OF, 84.2%–97.0%), while the north‐facing slope is dominated by SSF (88.9%–97.3%). The runoff depth of the north‐facing slope is 11.0 times that of the south‐facing slope. The SSF in the deep CZ accounts for 20.8%–37.8% of the total runoff along the north‐facing slope. Temporally, runoff generation exhibits distinct patterns across seasons. OF exhibits two different patterns, while SSF exhibits five across seasons. Our results indicate that runoff generation in different FT stages should be studied by the time phase. Therefore, we propose seasonal or FT period‐based classification criteria and divide OF and SSF into two and three consecutive periods throughout the calendar year. We suggest that these inferences and observations should be considered in the framework of hydrological models in cold regions.

The purple soil slope farmland is an important agricultural land in southwest China but is also one of the main sources of agricultural non-point source pollution in the Three Gorges Reservoir area. Taking reasonable measures to control the loss of soil nutrients is of great significance to the treatment of non-point source pollution in the region. Here, a three-year (2018-2020) field runoff experiment was conducted to monitor and evaluate the phosphorus (P) loss in sloping farmland via surface runoff (i.e., surface flow, 0-20 cm) and subsurface runoff (i.e., subsurface flow, 20-60 cm), with five treatments including no fertilization (CK), conventional fertilization (CF), optimal fertilization (OF), biochar combined with 85% of OF (BF), and straw combined with 85% of OF (SF). The results showed that fertilization application reduced the sediment yields and surface runoff flux but increased the subsurface runoff flux. The total loss flux of phosphate (PO3-4-P), total phosphorus (TP), and particulate phosphorus (PP) in surface flow were the highest in the BF treatment and the lowest in the SF treatment. All fertilization applications increased the P loss fluxes in subsurface flow relative to that in CK. The highest PO3-4-P and TP loss flux in subsurface flow was found in the BF (213.88 g·hm-2 and 694.54 g·hm-2, respectively) treatment, followed by that in the OF and SF treatments. Redundancy analysis (RDA) results showed that surface runoff flux and biochar application were the main factors contributing to increased P loss in surface flow, and subsurface runoff flux was the main factor contributing to increased P loss in subsurface flow. In summary, the SF treatment reduced the amount of sediment yield and surface runoff flux in sloping farmland of purple soil and was the most effective for controlling P loss, whereas the risk of subsurface runoff flux requires further attention.

An integrated model of fully coupled two-phase surface and subsurface flow was built by using GETFLOWS (GEneral purpose Terrestrial fluid-FLOW Simulator), a finite difference fluid-flow numerical simulator, to perform quantitative evaluation of exchange of overland water and groundwater, subsurface seepage and surface water runoff on the hillside slope surface. Surface water on hillside slope was modeled as open-channel flows and coupled with air-water two phase Darcy flow for underground permeation by using a generalized flow formula. In this study, a simple verification model was firstly built and validated by the extensively used Abdul and Gillham example with simulating the coupled surface and subsurface flow. Then a three-dimensional integrated model of surface and subsurface flow was built by using the DEM (Digital Elevation Model) of a natural mountain slope in Hokkaido, Japan, where runoff induced several debris flows and slope failures occurred during Typhoon 10 hit Hokkaido in 2016. Finally the results of integrated simulation was compared with the results from two-dimensional (2D) impermeable plane flow simulation using Nays2D Flood solver of the iRIC software in the same region. The results suggest that on the hillside slope upstream of embankment, the infiltration rate is equals to the rainfall intensity at the beginning of the rainfall event. After the rainfall intensity is greater than the soil infiltration capacity, runoff is generated. The runoff from upstream allows more water to infiltrate into the embankment causing the possibility of slope failure at the exit of the valley to be much greater than other locations along the highway.KeywordsIntegrated modelingSurface flowSubsurface flowGETFLOWSInfiltration excess runoff

The Alexandra Wetlands, part of PUB's Active, Beautiful, Clean Waters (ABC Waters) Programme, showcase a surface flow wetland, an aquatic pond and a sub-surface flow wetland on a 200 m deck built over an urban drainage canal. Water from the canal is pumped to a sedimentation basin, before flowing in parallel to the three wetlands. Water quality monitoring was carried out monthly from April 2011 to December 2012. The order of removal efficiency is sub-surface flow (81.3%) >aquatic pond (58.5%) >surface flow (50.7%) for total suspended solids (TSS); sub-surface (44.9%) >surface flow (31.9%) >aquatic pond (22.0%) for total nitrogen (TN); and surface flow (56.7%) >aquatic pond (39.8%) >sub-surface flow (5.4%) for total phosphorus (TP). All three wetlands achieved the Singapore stormwater treatment objectives (STO) for TP removal, but only the sub-surface flow wetland met the STO for TSS, and none met the STO for TN. Challenges in achieving satisfactory performance include inconsistent feed water quality, undesirable behaviour such as fishing, release of pets and feeding of animals in the wetlands, and canal dredging during part of the monitoring period. As a pilot showcase, the Alexandra Wetlands provide useful lessons for implementing multi-objective wetlands in an urban setting.