DHSVM Distributed Hydrology Soil Vegetation Model Dennis P. Lettenmaier Department of Civil and Environmental Engineering University of Washington Workshop on hyper-resolution land surface modeling Princeton University March 15-17 2010

Distributed hydrology-soil-vegetation model (DHSVM) Physically based hydrologic model that represents the effects of Topography Soil Vegetation Solves the energy and water balance at each grid cell at each timestep

Water and Energy Balances From Pascal Storck presentation ( “kingco.ppt’ 02/13/2002)

Snow Accumulation and Melt Model From Pascal Storck presentation ( “kingco.ppt’ 02/13/2002)

Runoff Generation and Routing Channel pixel - Surface runoff is produced via saturation excess (6 and 7) and infiltration excess (3) based on a user-specified static maximum infiltration capacity (pixel 3) - Table depth computation based on : percolation, reinfiltration of incoming surface runoff (4 and 6), incoming saturated and unsaturated flow from uphill pixels - Surface, saturated and unsaturated subsurface flows routed to downslope neighbors one pixel (150m)/time step (1 hour) - Channel network segments (6): intercept subsurface flow, intercept all surface flow, route water between segments using linear reservoir scheme Hourly time step Typical spatial resolution used in our lab is 150 meters. Others ( USDA forest service, OSU Corvallis, ..) use 30 meters.

Runoff Generation – Dynamic Infiltration Excess Calculation of maximum infiltration capacity: The first timestep there is surface water on the pixel, all surface water infiltrates. If there is surface water in the next timestep, the maximum infiltration capacity is calculated based on the amount previously infiltrated. Dominant form of runoff generation on unpaved roads and post burn land surfaces N. Voisin

Kinematic Runoff Routing Pixel to pixel overland flow routed using an explicit finite difference solution of the kinematic wave approximation to the Saint-Venant equations Manning’s equation is used to solve for flow area in terms of discharge Per DHSVM timestep, a new solution sub-timestep is calculated satisfying the Courant condition, which is necessary for solution stability. L. Bowling

Road-cut flow interception Interception of shallow groundwater Flows through the road-side ditch network Discharges from culvert Subsurface flow interception by a logging road in a model grid cell. Flow interception occurs when the watertable in the grid cell rises above the road drainage ditch. Subsurface flow below the road-cut continues downstream to the adjacent grid cells.

Puget Sound hydrology, past present, and future: The effects of 125 years of land cover and climate change, and projections for the next century

Understanding hydrologic change: The Puget Sound basin as a case study

Topography of the Puget Sound basin

The role of changing land cover – 1880 v. 2002

Modifications of DHSVM for urban areas For pixels with land cover category “urban”, a fraction of impervious surface area is specified. For the fraction that is not impervious, DHSVM handles infiltration using the same parameterizations as for non-urban pixels. A second parameter, the fraction of water stored in flood detention, was also added. Runoff generated from impervious surfaces is assumed to be diverted to detention storage. The runoff diverted to detention storage is allowed to drain as a linear reservoir, and re-enters the channel system in the pixel from which it is diverted. Surface runoff that is not diverted is assumed to enter the channel system directly, i.e., all urban channels are connected directly to the channel system We assume that the natural channel system remains intact, and we retain the support area concept that defines the connectivity of pixels to first order channels. However, impervious surface runoff (and drainage from detention reservoirs) is assumed to be connected to the nearest stream channel directly Once impervious surface runoff has entered a stream channel, it follows the “standard” DHSVM channel flow routing processes.

Springbrook Creek catchment

Springbrook Creek simulations (left) and errors (right) with and without urban module No impervious or detention Impervious, no detention impervious and detention

Forest cover change effects Measurement of Canopy Processes via two 25 m2 weighing lysimeters (shown here) and additional lysimeters in an adjacent clear-cut. Direct measurement of snow interception

Calibration of an energy balance model of canopy effects on snow accumulation and melt to the weighing lysimeter data. (Model was tested against two additional years of data)

Model Calibration

Land cover change effects on seasonal streamflow for eastern (Cascade) upland gages

Land cover change effects on seasonal streamflow at selected eastern lowland (Greater Seattle area) gages

Predicted temperature change effects on seasonal streamflow at western (Olympic) upland gages

Predicted temperature change effects on seasonal streamflow at selected eastern lowland gages (greater Seattle area)

Puget Sound basin land cover projections, 2027 and 2050

Mid-century seasonal mean streamflow projections averaged over 20 GCMs, 2040s (current land cover)

Run time (hours per basin, single processor): Cedar    11 Green    19 Snohomish    70 Tolt    1 Lowland_west    28 Nisqually    40 Lowland-east    130 Stillaguamish    48 Puyallup    58 Deschutes    14 Dosewallips    7 Skokomish    12 Skagit     230 Hamma Hamma    5 Duckabush    6 Quilcene    4