Lassi Warsta

Distributed hydrological modeling with channel network flow of a forestry drained peatland site

Peatland drainage has been an important component of forestry management in the boreal zone and the resulting ditch networks are maintained regularly to sustain forest productivity. In Finland, this is recognized as the most detrimental forestry practice increasing diffuse loads of suspended solids. Alongside forestry management on peatlands, interest in peatland restoration has grown lately. Distributed hydrological modeling has the potential to address these matters by recognizing relevant physical mechanisms and identifying most suitable strategies for mitigating undesired outcomes. This study investigates the utility of such a modeling approach in a drained peatland forest environment. To provide a suitable tool for this purpose, we coupled channel network flow to the three-dimensional distributed hydrological model FLUSH. The resulting model was applied to a 5.2 ha drained peatland forest catchment in Eastern Finland. The model was calibrated and validated using field measurements obtained over frost-free periods of five months. The application showed that distributed modeling can disentangle the importance of spatial factors on local soil moisture conditions, which is significant as peatland drainage aims to control these conditions. In our application, we limited the spatial aspect to the topography and the drainage network, and found that the drainage configuration had a clear effect on the spatial soil moisture patterns but that the effect was less pronounced during the wetter summer. Future applications of distributed modeling in this field comprises investigating the impacts of other spatial factors, modeling channel erosion and solid transport to address strategies for their mitigation, and evaluating restoration schemes.

Development and application of an automated subcatchment generator for SWMM using open data

Abstract An open source subcatchment generator program was developed for the Stormwater Management Model (SWMM) to automate tedious stages in the model construction process. The generator divides the investigated area into subcatchments using a uniform computation grid and connects the grid cells together and to the underlying stormwater network. The system was tested by applying it to two small urban catchments with different fractions of impervious surfaces in Helsinki, Finland, using mostly openly available data. The simulated discharge results were compared to measured data and to results obtained from manually built models. The proposed system significantly accelerated the setup of a SWMM modelling project, as the routing between the subcatchments as well as the subcatchment slopes and flow widths were directly derived from the computation grid. Automatically generated and manually constructed SWMM models produced discharge results that differed only slightly from each other.

Spatio-temporal patterns of major ions in urban stormwater under cold climate

Department of Geosciences and Geography, University of Helsinki, Helsinki, Finland Department of Built Environment, Aalto University, Espoo, Finland Department of Environmental Sciences, University of Helsinki, Helsinki, Finland Correspondence Maija Taka, Department of Geosciences and Geography, University of Helsinki, P.O. Box 64, FI‐00014 Helsinki, Finland. Email: maija.taka@helsinki.fi Funding information European Union Life + programthe Academy of Finland, Grant/Award Number: 263335. 263320. 263308.

Simulating water balance and evapotranspiration in a subsurface drained clayey agricultural field in high-latitude conditions

Secondary drainage impact of groundwater outflow can affect drainage design and form a pathway for nutrient loading in agricultural areas. Holistic assessment of water balance and all outflow pathways can benefit design of sustainable drainage in a changing climate. In this study, three-dimensional, hydrological FLUSH model was applied to investigate a field-scale data set and to produce a closure of water balance throughout all seasons in a clayey subsurface drained agricultural field in high-latitude conditions. Description of evapotranspiration (ET)-groundwater interactions using a three-dimensional hydrological model provides a new approach for evaluating standard computational methods to estimate ET with limited crop data. Different ET estimates were tested in the context of total water balance, and the coupling of ET and groundwater outflow was assessed. Comparison of measured and simulated water balance components demonstrated that reference ET (Penman–Monteith method) overestimated ET in the cropped field in high latitude conditions. The FAO-56 single crop coefficient approach was also noted to overestimate ET in the studied conditions. A calibrated constant crop coefficient satisfactorily described ET in spring and in autumn, but underestimated it during summer periods. The results suggest that care should be taken when applying standard methods in high-latitude conditions. Groundwater outflow and ET were shown to be interlinked, but even a relatively high potential ET affected the amount of groundwater outflow only slightly. The results demonstrate that groundwater outflow can form an important component of the water balance in clayey subsurface drained fields. The strength of the 3D model was demonstrated in showing how ET had an impact on all outflow components of drained field sections. Such a modelling tool is useful for generating scenarios that show how changes in climate forcing and thereby ET can alter the partitioning of the field-scale water balance.

Development and application of a solute transport model to describe field-scale nitrogen processes during autumn rains

A new generic, three-dimensional, solute transport component was developed into FLUSH, which is a hydrological model developed for Nordic conditions. Water flow and solute transport descriptions in FLUSH follow the dual-permeability concept, which divides the total soil pore space into mobile soil matrix and macropore systems. The solute transport model was parameterized to simulate the main processes of nitrogen (N) cycle in clayey, subsurface-drained soils during autumn periods after the harvest. The model simulates transport of nitrate and ammonium N, as well as mineralization, nitrification, and denitrification. Reactions in soil are affected by temperature and moisture, as simulated by FLUSH. Ammonium can adsorb on soil particles in both pore systems, while organic N is described in simulations as an immobile solute in the soil matrix. One-dimensional version of the model was applied to two subdrained field sections (1.3 and 3.4 ha) in the Nummela experimental field in southern Finland during two autumn periods (2008 and 2011). The model was able to replicate the measured dynamics of nitrate N concentrations in drain discharge during both the periods. Concentrations were the most dependent on drain discharge dynamics and the rate of nitrification. Measured and simulated ammonium concentrations in drain discharge were about 10 times smaller than nitrate concentrations, even though the levels of N input with initial values and deposition for both inorganic fractions were similar. Successful solute transport simulation results further increase the confidence in the description of the water flow processes in FLUSH.

Modeling sediment transport after ditch network maintenance of a forested peatland

Elevated suspended sediment (SS) loads released from peatlands after drainage operations and the resulting negative effect on the ecological status of the receiving water bodies have been widely recognized. Understanding the processes controlling erosion and sediment transport within the ditch network forms a prerequisite for adequate sediment control. While numerous experimental studies have been reported in this field, model based assessments are rare. This study presents a modeling approach to investigate sediment transport in a peatland ditch network. The transport model describes bed erosion, rain-induced bank erosion, floc deposition, and consolidation of the bed. Coupled to a distributed hydrological model, sediment transport was simulated in a 5.2 ha forestry-drained peatland catchment for two years after ditch cleaning. Comparing simulation results to measured SS concentrations suggested that the loose peat material, produced during excavation, contributed markedly to elevated SS concentrations immediately after ditch cleaning. Both snowmelt and summer rainstorms contributed critically to annual loads. Springtime peat erosion during snowmelt was driven by ditch flow whereas during summer rainfalls, bank erosion by raindrop impact was identified as an important process. Relating modeling results to observed spatial topographic changes in the ditch network was challenging and the results were difficult to verify. Nevertheless, the model has potential to identify risk areas for erosion. The results demonstrate that modeling is effective in separating the importance of different processes and complements pure experimental approaches. Modeling results can aid planning and designing efficient sediment control measures and guide the focus of experimental studies. This article is protected by copyright. All rights reserved.