5 Conclusions
Wetlands play a crucial role in Earth systems for their climaticand
hydrologicalfunctions. However, reasonably representing the spatial
extent and dynamics of small-scale wetlands has been challenging to LSMs
and coupled ESMs. This is particularly important and urgent in the PPR
as the wetlands are critical to the region’s ecology and the
hydrological conditions are complex. In this research, we developed a
wetland scheme with two modifications to represent wetland dynamics in
the Noah-MP LSM. One is modifying the sub-grid saturation fraction to
indicate the spatial wetland extent based on grid cell soil moisture.
Another is incorporating a dynamic surface water storage scheme to
represent the hydrological processes in wetlands. This new wetland
scheme is incorporated in single-point, offline regional simulation, and
coupled WRF simulation in the PPR.
The single-point simulation showed that the modified sub-grid\(F_{\text{sat}}\) using the first layer soil saturation reasonably
mimics the magnitude and seasonality of surface saturation condition in
the PPR, compared to the default TOPMODEL-based formula. The modified
increased \(F_{\text{sat}}\) formula partitions more water to surface
runoff than infiltration to soil moisture. The enhanced surface inflow
is then collected by the surface storage, mimicking the capacity of
wetland depressions, with water exceeding the capacity as fill and spill
outflow. The single-point simulation also shows that the wetland
modification of surface energy and water balance depends on its maximum
capacity. For shallow-storage wetlands, both spring inflow and summer
evaporation demand would exceed their maximum capacity, limiting their
function in energy and water exchange with the atmosphere.
In the offline regional simulation in the PPR, two spatially varied
parameters are incorporated, the maximum \(F_{\text{satmx}}\) fraction
and maximum storage capacity \(W_{\text{cap}}\). The results show that
the model-simulated wetlands are located in the Northeast portion of the
PPR domain, where \(F_{\text{satmx}}\) is high but maximum capacity is
shallow. By incorporating the wetland scheme, the summertime evaporation
and latent heat fluxes are evidently increased, with decreasing surface
runoff and sensible heat fluxes.
Finally, we examine the wetlands’ feedback to regional temperature and
precipitation in the coupled WRF-NoahMP-Wetland model. A cooling effect,
induced by the presence of wetlands, is evident in all three years’
summer for about 0.5~1℃ in a dry year (2006) and
0.7~1.5℃ in a wet year (2005), occurring where the
wetland fraction is high. This cooling is the result of wetlands
altering energy balance partitioning, increasing latent heat fluxes
while reducing sensible heat fluxes. The cooling effect is strongest in
July and weakest in May, consistent with the theory of evaporation being
energy limited in early summer but transitioning to water limited in
mid-summer. In the summer of 2006, when an extreme heatwave hit the
Central U.S. and Southern Canada, the presence of wetlands could
profoundly reduce the number of extreme hot days by more than 10 during
the summer period, effectively reducing the heat stress to human
comfort. On the other hand, wetland scheme impacts on regional
precipitation do not manifest in an obvious spatial pattern, including
both positive and negative effects on precipitation.
Our results show that the presence of wetlands could be beneficial to
many sectors by regulating surface runoff during flooding and cooling
atmospheric temperatures during heatwaves. These highlights should
inspire future studies to understand wetlands’ value in regional
environments and the Earth system, especially those that have been
neglected at the cost of human expansion.
Acknowledgments
Z. Zhang, Z. Li and Y. Li acknowledge the financial support from the
Natural Sciences and Engineering Research Council of Canada (NSERC)
Discovery Grant, and Global Water Futures Program, Canada First Research
Excellence Fund and Global Institute for Water Security (GIWS). Z. Zhang
was funded by a Mitacs Accelerate Fellowship funded by Ducks Unlimited
Canada’s Institute for Wetland and Waterfowl Research. This project was
supported by grants from Wildlife Habitat Canada, Bass Pro Shops
Cabela’s Outdoor Fund, and the Alberta NAWMP Partnership.