Improving and Testing the Prairie Hydrological Model at Smith Creek Research BasinJ. Pomeroy, K. Shook, X. Fang, S. Dumanski, C. Westbrook, T. Brown

Centre for Hydrology, University of saskatCHewan

Introduction & Background

The relationship between wetland drainage and flooding has always been suggested, but now there is research to support it. A long-term monitoring and computer modeling study by the University of Saskatchewan (U of S) Centre for Hydrology has revealed that wetland drainage is a major factor in increasing prairie streamflows and increasing flooding in wet years.

The University of Saskatchewan established a Canada Research Chair in Water Resources and Climate Change (CRC) in 2003, directed towards improved understanding and model- ling of hydrology in prairie, mountain and northern environments. One of the research activities to support this CRC was the development of the Cold Regions Hydrological Modelling Platform (CRHM).

The Prairie Hydrological Model (PHM) was developed using the CRHM to understand the relationship between wetland storage and contributing area to predict the impacts of intact wetlands and wetland drainage on the five sub-basins and main basin of Smith Creek Research Basin (SCRB), Saskatchewan, located approximately 60 kilometres southeast of Yorkton. The drainage basin of Smith Creek has undergone substantial drainage. In 1958 there were 96 square kilometres of wetlands (24% of the basin area) and now there are 43 square kilometres (11% of the basin area).

Snowmelt and surface runoff on the Canadian Prairies typically drains into depressions, forming wetlands or sloughs that act as closed basins. These wetlands store surface water and, under natural conditions, do not contribute to downstream flows. Draining wetlands adds permanent surface connections, reducing the ability for wetlands to store surface water and increasing the land area contributing to streamflow when snowmelt runoff is generated. The land area contributing to streamflow increases in wet conditions and decreases in droughts because of the changing storage of water in wetlands.

Wetland storage in the SCRB has been strongly affected by drainage, and it is critical to understand how these changes have affected the hydrology of the watershed.

Purpose of this Study

The purpose of this study was to further develop the PHM so that it can realistically describe the hydrology of wetland complexes including drained wetlands by employing high-quality hydrological data from the SCRB.

The SCRB was established, instrumented with a high quality weather, snowpack and soil moisture measurement station, and 10 more rainfall measurement stations, and had extensive

Figure 1. Change in the extent of wetland drainage networks within the Smith Creek Research Basin from 1958 to 2009 (Ducks Unlimited Canada (2013).

1958 2009

Stream

Drainage Network

0 10 km

baseline measurements of soil type, wetland hydrography, wetland level, streamflows, snow- pack, fall soil moisture, land cover, precise topographic elevation and drainage collected from 2007 to 2013.

Agriculture (cropland and pasture) is the dominant land use, encompassing approximately 48% of the basin. The SCRB has been partially drained, with the extent of wetland area estimated to have decreased from 24% to 11% of the basin area from 1958-2001 (figure 1).

Methods

The PHM was set up for the SCRB and run over six years (2007-2013) using data from U of S weather and rainfall stations. These years included the largest flood on record for the region in 2011. The PHM was tested against its ability to simulate measured snow-packs, snowmelt and streamflow in Smith Creek and was found to be a useful hydrological simulation model. It was then manipulated to decrease wetland volume (drainage) or increase wetland volume (restoration). The limit for restoration was the measured wetland extent in 1958; the limit for drainage was complete wetland removal.

Historical wetland storage volumes and drainage networks were analyzed using air photo based wetland mapping from 1958 and 2000 to compare with the detailed wetland hydro- graphy analysis possible using the 2008 LiDAR-measured DEM.

Scenarios were run using a maximum number and area of wetlands (circa 1958) through to a hypothetical highly drained condition. These scenarios simulated various wetland storage capacities and connectivities for drained to restored conditions.

Conclusions

• Wetland drainage has a strong impact on streamflow in flood conditions. Wetland drainage from 1958-2008 increased the 2011 flood peak by 32% and the 2011 yearly streamflow volume by 29%.

• Continued wetland drainage will have a very strong impact on streamflow in flood con- ditions. For the flood of 2011, complete drainage of the existing wetlands would increase the 2011 flood peak by 78% and the 2011 yearly volume of streamflow by 32%.

• Drainage has an even stronger impact on streamflow in normal to dry years than it does in flood years. For these moderate years, streamflow volumes increase by from 200% to 300% with drainage of existing wetlands and the yearly peak flow rate increases by from 150% to 350%.

• Over six years of computer model simulation (2007-2013), total streamflow volumes increase by 55% with complete drainage of existing wetlands and decrease by 26% with restoration of existing wetlands to their state in 1958.

• These results are from model simulations that are by far the most reliable and sophisticated estimate of the hydrological response to wetlands and drainage in the Canadian Prairies.

• This model simulation exercise shows wetland drainage increases the contributing area of wetland-dominated prairie drainage basins, and can increase annual and peak daily flows substantially, with notable increases in the peak daily flows of the flood of record.

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