A modelling framework to simulate field‐scale nitrate response and transport during snowmelt: The WINTRA model

Abstract

Modelling nutrient transport during snowmelt in cold regions remains a major scientific challenge.A key limitation of existing nutrient models for application in cold regions is the inadequate representation of snowmelt, including hydrological and biogeochemical processes. This brief period can account for more than 80% of the total annual surface runoff in the Canadian Prairies and Northern Canada and processes such as atmospheric deposition, overwinter redistribution of snow, ion exclusion from snow crystals, frozen soils, and snow-covered area depletion during melt influence the distribution and release of snow and soil nutrients, thus affecting the timing and magnitude of snowmelt runoff nutrient concentrations.Research in cold regions suggests that nitrate (NO 3 ) runoff at the field-scale can be divided into 5 phases during snowmelt. In the first phase, water and ions originating from ion-rich snow layers travel and diffuse through the snowpack. This process causes ion concentrations in runoff to gradually increase. The second phase occurs when this snow ion meltwater front has reached the bottom of the snowpack and forms runoff to the edge-of-the-field. During the third and fourth phases, the main source of NO 3 transitions from the snowpack to the soil. Finally, the fifth and last phase occurs when the snow has completely melted, and the thawing soil becomes the main source of NO 3 to the stream.In this research, a process-based model was developed to simulate hourly export based on this 5-phase approach. Results from an application in the Red River Basin of southern Manitoba, Canada, shows that the model can adequately capture the dynamics and rapid changes of NO 3 concentrations during this period at relevant temporal resolutions. This is a significant achievement to advance the current nutrient modelling paradigm in cold climates, which is generally limited to satisfactory results at monthly or annual resolutions. The approach can inform catchment-scale nutrient models to improve simulation of this critical snowmelt period.

Using an inverse modelling approach with equifinality control to investigate the dominant controls on snowmelt nutrient export

“…Five‐phase conceptual model used in WINTRA (adapted from Costa et al, ) and schematic view of the sources of dissolved nutrients: snow (L s n o w ) and soil (L s o i l )…”

“…It has been extensively applied in the Canadian Prairies (Fang & Pomeroy, ; Fang et al, ; Mahmood, Pomeroy, Wheater, & Baulch, ). The reader is referred to Costa et al () for more details about the WINTRA model.…”

“…Costa et al () showed that this model could adequately capture hourly NO 3 dynamics using two calibration parameters: one coefficient k d [ ML −3 T −1 ] to account for the depletion of readily available soil‐pore NO 3 via run‐off transport and denitrification and one dimensionless leaching coefficient k l for the computation of preferential ion elution from melting snowpacks due to snow ion exclusion processes.…”

“…Table summarizes the tillage practices, planted crops, and fertilizer use between 2008 and 2011 in the six subwatersheds used in this study (F3, F4, F7, F9, F10, and F11). The reader is referred to Li et al (), Liu, Elliott, Lobb, Flaten, and Yarotski (), and Costa et al () for more information about the basin and the experimental set‐up.…”

“…Finally, rapidity of snowmelt and its importance to nutrient budgets may necessitate the use of high‐resolution temporal scales for modelling. Specifically, snowmelt occurs during a short period but has a major impact on the annual nutrient export (e.g., Costa et al, ; Han et al, ), suggesting that subdaily simulations may be necessary, and practices such as using cumulative monthly export as a test of model fit is likely to overlook important temporal dynamics. Several catchment‐scale models exist that allow for process‐based nutrient simulation in cold climates (e.g., HYdrological Predictions for the Environment, Arheimer, Dahné, Donnelly, Lindström, & Strömqvist, ; Hydrological Simulation Program‐Fortran, Duda, Hummel, Donigian, & Imhoff, ; Integrated Model of Nitrogen in Catchments, Arnold et al, ; and Soil and Water Assessment Tool, Borah & Bera ).…”