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.