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Laine-Kaulio H.,Aalto University | Backnas S.,Geological Survey of Finland | Karvonen T.,Waterhope | Koivusalo H.,Aalto University | And 2 more authors.
Water Resources Research | Year: 2014

Preferential flow dominates water movement and solute transport in boreal forest hillslopes. However, only a few model applications to date have accounted for preferential flow at forest sites. Here we present a parallel and coupled simulation of flow and transport processes in the preferential flow domain and soil matrix of a forested hillslope section in Kangaslampi, Finland, using a new, three-dimensional, physically based dual-permeability model. Our aim is to simulate lateral subsurface stormflow and solute transport at the slope during a chloride tracer experiment, and to investigate the role of preferential flow in the tracer transport. The model was able to mimic the observed tracer transport during tracer irrigation, but overestimated the dilution velocity of the tracer plume in the highly conductive soil horizons near the soil surface after changing the irrigation to tracer-free water. According to the model, 140 times more chloride was transported downslope in the preferential flow domain than in the soil matrix during the tracer irrigation. The simulations showed, together with reference simulations with a traditional one pore domain model, that a two pore domain approach was required to simulate the observed flow and transport event. The event was characterized by the transmissivity feedback phenomenon and controlled by preferential flow mechanisms, in particular by lateral by-pass flow. According to our results, accounting for the slow-flow and fast-flow domains of soil, as well as the water and solute exchange between the domains, is essential for a successful simulation of flow and solute transport in preferential flow dominated hillslopes. Key Points Preferential flow mechanisms control lateral subsurface stormflow in forest till A two pore domain approach is needed to model the coupled flow and transport Tracer data facilitate the model parameterization and testing © 2014. American Geophysical Union. All Rights Reserved.

Warsta L.,Aalto University | Taskinen A.,Finnish Environment Institute | Paasonen-Kivekas M.,Sven Hallin Research Foundation | Karvonen T.,Waterhope | Koivusalo H.,Aalto University
Soil and Tillage Research | Year: 2014

Runoff and sediment transport are distinctively three-dimensional (3D) processes and occur through overland, tillage layer and subsurface pathways. The objective was to quantify water balance and sediment concentrations in runoff waters and to assess sediment loads via surface runoff and drainflow in a clayey, subsurface drained field section using the FLUSH model. The model can simulate field scale two-dimensional overland flow and 3D unsaturated and saturated subsurface flow, including preferential flow in macropores. The erosive processes, comprised of hydraulic and raindrop splash erosion, occur in the overland domain while suspended sediment is conveyed from the field surface to subsurface drains via preferential transport in macropores in the subsurface domain. The study site, located in southern Finland, is a clayey, subsurface drained field section with an area of 12ha and an average slope of 2.8%. The growing seasons and the following autumns of two years with distinctly different rainstorm characteristics were modelled. The simulated sediment loads via tillage layer runoff and drainflow were 85 and 117kgha-1, respectively in 1988, and 63 and 189kgha-1 in 1984. Despite the high precipitation in October 1984 (143mm), erosion in the field area was low due to minimal surface runoff and consequently minimal hydraulic erosion. The suspended sediment at the site was generated by raindrop impacts and the eroded material was transported to the open ditch through tillage layer flow and subsurface drainflow. The model was able to reproduce the measured sediment concentrations in the main open ditch into which both tillage layer runoff and drainflow are discharged from the field. Spatially distributed erosion simulations facilitate the detection of net erosion and deposition locations in the field and could be used to design intensive measurement campaigns and guide erosion control practices in the future. © 2014 Elsevier B.V.

Warsta L.,Aalto University | Taskinen A.,Finnish Environment Institute | Koivusalo H.,Aalto University | Paasonen-Kivekas M.,Sven Hallin Research Foundation | Karvonen T.,Waterhope
Journal of Hydrology | Year: 2013

Soil erosion is an important environmental issue in agricultural areas of northern Europe where clayey soils are prevalent. Clayey soils are routinely subsurface drained to accelerate drainage which creates an additional discharge route for suspended sediment. Previously, assessment of the sediment load from clayey fields has been difficult, because process-based models were only able to simulate sediment loads via surface runoff. A new distributed, process-based erosion model was developed and incorporated into the FLUSH modelling system to fulfil this void. The model facilitates simulation of spatially distributed soil erosion on the field surface and sediment loads via surface runoff and subsurface drainflow. Soil erosion on the field surface is simulated with the two-dimensional sediment continuity equation coupled with hydraulic and rain drop splash erosion, sediment settling, and transport capacity processes. Subsurface sediment transport in macropores is described with the three-dimensional advection-dispersion equation. The model was applied to a clayey, subdrained field section (~3.6ha) in southern Finland. The results demonstrated the capability of the model to simulate soil erosion and sediment transport in terms of the match between the measured (2669kgha-1) and modelled (2196kgha-1) sediment loads via surface runoff and the measured (2937kgha-1) and modelled (2245kgha-1) loads via drainflow during the validation period of 7months. The model sensitivity analysis pointed out the importance of the flow model parameters in simulation of soil erosion through their control on the division of total runoff into surface runoff and drainflow components. The key parameters in the erosion model were those that affected hydraulic and splash erosion rates. The model application in the experimental field suggested that both hydraulic and splash erosion were the factors behind the sediment losses during the growing season and early autumn, whereas high sediment loads in late autumn were caused by hydraulic erosion due to overland flow in tilled soil. © 2013 Elsevier B.V.

Warsta L.,Aalto University | Karvonen T.,Waterhope | Koivusalo H.,Aalto University | Paasonen-Kivekas M.,Sven Hallin Research Foundation | Taskinen A.,Finnish Environment Institute
Journal of Hydrology | Year: 2013

Water flow is a key component in the evaluation of soil erosion and nutrient loads from agricultural fields. Field cultivation is the main non-point pollution source threatening water quality of surface waters in Nordic and many other countries. Few models exist that can describe key hydrological processes in clayey soils, i.e. overland flow, preferential flow in macropores and soil shrinkage and swelling. A new three-dimensional (3-D) distributed numerical model called FLUSH is introduced in this study to simulate these processes. FLUSH describes overland flow with the diffuse wave simplification of the Saint Venant equations and subsurface flow with a dual-permeability approach using the Richards equation in both macropore and matrix pore systems. A method based on the pentadiagonal matrix algorithm solves flow in both macropore and matrix systems directly in a column of cells in the computational grid. Flow between the columns is solved with iteration accelerated with OpenMP parallelisation. The model validity is tested with data from a 3-D analytical model and a clayey subsurface drained agricultural field in southern Finland. According to the simulation results, over 99% of the drainflow originated from the macropore system and drainflow started in some cases within the same hour when precipitation started indicating preferential flow in the profile. The moisture content of the clay soil had a profound effect on runoff distribution between surface runoff and drainflow. In summer, when the soil was dry and cracked, drainflow dominated the total runoff, while in autumn, when the shrinkage crack network had swollen shut, surface runoff fraction clearly increased. Observed differences in surface runoff fraction before and after tillage indicated that the operation decreased hydraulic conductivity of the profile. © 2012 Elsevier B.V.

Lappalainen M.,Finnish Forest Research Institute | Koivusalo H.,Aalto University | Karvonen T.,WaterHope | Lauren A.,Finnish Forest Research Institute
Boreal Environment Research | Year: 2010

The objective of this study was to describe erosion, sedimentation and transportation processes of erodible material in peatland forests after ditch network maintenance. A sediment transportation model was developed to simulate bed elevation changes in ditches and concentration of suspended solids in water. The model was suitable for simulating short-term effects (the first two years) of ditch network maintenance. The modelled spatial differences in sediment concentration were related to variation in the ditch bottom slope. The temporal variability in concentration was influenced by the water discharge rate. Other factors controlling the erosion and sedimentation in the model were the particle size of the material in bed and in suspension, the roughness height of the bed, and the Manning coefficient. Further development of the model calls for testing against comprehensive field measurements of sediment load and changes in channel dimensions. © 2010 Helsiniki 30December 2010.

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