Conceptualizing Peatlands in a Physically-Based Spatially Distributed Hydrologic Model Charles W. Downer and Mark D. Wahl Coastal and Hydraulics Laboratory, U.S. Army Engineer Research and Development Center HS10.8 A.370. European Geosciences Union General Assembly 2017,Vienna, Austria Moss/Peat as Vegetative Abstract Soils at Caribou/Poker Creeks Research Watershed Land Cover at Caribou/Poker Creeks Research Watershed Roughness 0.18 Vegetation Conceptualization 0.16 As part of a research effort focused on climate change effects on permafrost near • Increased the overland roughness Fairbanks, Alaska, it becam e apparent t hat peat soils, overlain by thick sphagnum values of the peat and sphagnum 0.14 m oss, had a considerable effect on the overall hydrology. Peat lands represent a well beyond the range associat ed 0.12 with the forested landuse. 0.10 confounding mixture of vegetation, soils, and w at er t hat present challenges for Data • Failed to m at ch the timing of the conceptualizing and parametrizing hydrologic models. We employed the Gridded 0.08 peak and overall shape of the Su r f ac e Subsurface Hydrologic Analysis Model (GSSHA) in our analysis of the 0.06 observed hydrograph. Caribou Poker Creek Experimental Watershed (CPCRW). Th e model enables 0.04 simulation of surface w at er and groundwater interactions, as well as soil • Over lan d flow equations with 0.02 temperature and frozen ground effects on subsurface w at er movement. A sit e increased roughness poorly represent the lat eral fluxes 0 visit exposed the presence of surface w at er flows indicating a mixed basin t hat 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Time would require both surface and subsurface simulation capability to properly through the peat . Outlet Hydrograph C2 (GSSHA)Stream flow capture the response. Soils in the watershed are predominately silt loam Moss/Peat as Soil Layer Simulation result for calibration period with Moss/Peat underlain by shallow fractured bedrock. Throughout much of the basin, a thick • Peat is treated as an additional soil conceptualized as a vegetation. layer of live sphagnum m oss and fine peat covers the ground surface. A restrictive layer in Richards Equation layer of permafrost is found on north facing slopes. Th e combination of thick • Su r f ac e roughness were m oss and peat soils presented a challenge in terms of conceptualizing the consistent with normal ranges for 0.18 Soil Conceptualization hydrology and identifying reasonable param et er ranges for physical properties. each landuse type. 0.16 Various combinations of overland roughness, surface retention, and subsurface • Failed to m at ch the timing of the 0.14 flow were used to represent the peatlands. Th e process resulted in som e peak and overall shape of the 0.12 interesting result s t hat m ay shed light on the dominant hydrologic processes observed hydrograph. 0.10 associat ed with peatland, as well as w hat hydrologic conceptualizations, • Simulating the peat/moss as part Data 0.08 simulation tools, and approaches are applicable in modeling peat land hydrology. of the unsaturated zone minimizes the lat eral fluxes 0.06 through the peat . 0.04 Caribou/Poker Creeks Research Watershed Moss/Peat as Wetland 0.02 0 Soil map based on Reiger (1972) at the Caribou/Poker Creek Research Watershed • Fluxes through the peat are 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Time simulated using the three lat eral Land cover map at the Caribou/Poker Creek Research Watershed including extents of the 2004 Boundary Fire. Simulation result for calibration period with Moss/Peat Open and closed designations refer to canopy density. An open canopy permits a view of the sky through it. zones in the GSSHA w et land cells. conceptualized as a soil layer. • Th e resulting hydrograph better m at ches the peak and overall 0.20 shape of the observed 0.18 Wetland Conceptualization Hydrologic Simulation hydrograph. 0.16 Overland Flow • While this simulation can be 0.14 • Alternating direction explicit (ADE) method. improved, the result s indicate t hat • Over lan d roughness values assigned according to land cover within the range all flow domains are critical in 0.12 suggested by Senarat h and Ogden 2000. representing the peat hydrology. 0.10Data • Retention storage accounted for in soils with heavy forest litter. • Ignoring any of the processes (i.e. 0.08 • Over lan d flow routed through snowpack using Darcy’s law. free surface overland flow, mixed 0.06 Channel Routing flux through the vegetation, and 0.04 • A one-dimensional diffusive w ave channel routing schem e is used to simulate Darcian flux through the peat ) 0.02 produces poor result s and 0 st ream flow. 0 200 400 600 800 1000 1200 1400 1600 1800 2000 unrealistic param et er values. Time • Idealized trapezoidal channel cross-sections are assum ed. Simulation result for calibration period with Moss/Peat Infiltration conceptualized as a wetland. • Richards Equation. References • 1D iterative finite-difference solution to describe the movement of w at er Chapin, F. S. S., and Hollingsworth, J. (2010a). “Caribou Poker Creek Research Watershed GIS Data: through an unsaturated soil (Downer and Ogden 2006). Permafrost, Bonanza Creek LTER.” University of Alaska Fairbanks, Fairbanks, Alaska. • Ver t ical discretization is a critical fact or to the accuracy of the solution Chapin, F. S. S., and Hollingsworth, J. (2010b). “Caribou Poker Creek Research Watershed GIS Data: Steam (Downer and Ogden 2004b). Lines, Bonanza Creek LTER.” University of Alaska Fairbanks, Fairbanks, Alaska. • A convergence study identified appropriate sizes of vertical cells for each soil Van Cleve, K., Chapin, F. S. S., Ruess, R. W., and Jones, J. (2011). “Caribou-Poker Creeks Research Watershed: Daily Flow Rates for C2, C3, C4, Bonanza Creek LTER.” University of Alaska Fairbanks, Hydrologic units of the Caribou/Poker Creeks Research Watershed (CPCRW) with locations of layer. Generally the upper m ost soil layer had a vert ical cells less t han 1 cm hydrometeorological stations and permafrost extents indicated (Chapin and Hollingsworth 2010a; b). while cell sizes for the subsequent layers increased from 1-3 cm in m ost cases. Fairbanks, Alaska. Models of the C2 and C3 sub-watersheds compare the hydrologic effects from disparate expanses of Groundwater Interactions Downer, C. W., and Ogden, F. L. (2003). “Prediction of runoff and soil moistures at the watershed scale: permafrost. Effects of model complexity and parameter assignment.” Water Resources Research, 39(3), n/a–n/a. • 2D simulation of lat eral sat urat ed groundwater flow is included in the model. Downer, C. W., and Ogden, F. L. (2004a). “GSSHA: Model to simulate diverse stream flow producing Caribou/Poker Creek Research Watershed • Su b -surface st ream losses and gains governed by a river flux boundary processes.” Journal of Hydrologic Engineering, (June), 161–174. condition. • Lo cat ed in the Yukon-Tanana Uplands of the Northern Plat eaus Physiographic Downer, C. W., and Ogden, F. L. (2004b). “Appropriate vertical discretization of Richards’ equation for Province near Fairbanks Alaska. (Lat / Lo n : 65˚10' N 147˚30' W) Evapotranspiration two-dimensional watershed-scale modelling.” Hydrological Processes, 18(1), 1–22. • Area: 104 km 2 • Penm an Monteith method specified within GSSHA to calculat e ET. Downer, C. W., and Ogden, F. L. (2006). Gridded Surface Subsurface Hydrologic Analysis (GSSHA) User’s • Charact erized by rounded hilltops with gentle slopes and alluvium-floored • Model param et ers assigned according to the vegetation m ap above. Manual; Version 1.43 for Watershed Modeling System 6.1. US Army Corps of Engineers, Engineer valleys having minimal relief (Wahrhaft ig 1965) underlaid by a mica shist of the • Su r f ac e albedo values are based on recommended values from the GSSHA user Research and Development Center,, Vicksburg, MS, 207. Birch Creek formation (Rieger et al. 1972). m anual (Downer and Ogden 2006). Eliáš, P. (1979). “Leaf diffusion resistance pattern in an oak-hornbeam forest.” Biologia Plantarum, 21(1), • Co ld continental climate characterized by short w arm summers and long cold • Canopy t ransm ission coefficients assigned according to light interception 1–8. winters. studies for deciduous (Hutchison and Mat t 1977) and coniferous (Gh o l z and Follum, M. L., and Downer, C. W. (2013). Snow Water Equivalent Modeling Capabilities of the GSSHA Vogel 1991) forest s. Watershed Model Project Report Coastal and Hydraulics Laboratory. Vicksburg, MS, 40. • Canopy st om at al resist ance based on two published studies (Eliáš 1979; Ver m a Gholz, H., and Vogel, S. (1991). “Dynamics of canopy structure and light interception in Pinus elliottii Modeling Approach and Baldocchi 1986). stands, north Florida.” cological Monographs, 61(1), 33–51. • Coupled Gridded Su r f ac e Subsurface • Penm an Monteith method is sensit ive to st om at al resist ance (Lemeur and Jones, J. (2011). “Caribou-Poker Creeks Research Watershed: Daily Flow Rates for C2, C3, C4, Bonanza Hydrologic Analysis (GSSHA) model and soil Zhang 1990) so consideration w as given to st om at al resist ance during Creek LTER.” University of Alaska Fairbanks, Fairbanks, Alaska. t herm al regime model (Marchenko et al. 2008) calibration. Lemeur, R., and Zhang, L. (1990). “Evaluation of three evapotranspiration models in terms of their from the Geophysical Institute Permafrost Lab applicability for an arid region.” Journal of hydrology, 114(3-4), 395–411. (GIPL). Marchenko, S., Romanovsky, V., and Tipenko, G. (2008). “Numerical modeling of spatial permafrost • Subdivided the basin into sub-watersheds for Conceptualization of Peat/Moss Layers dynamics in Alaska.” Proceedings of the Ninth International Conference on Permafrost, D.