Advances in the simulation of nutrient dynamics in cold climate
agricultural basins: developing new N and P modules for the Cold Regions
Hydrological Modelling Platform
Abstract
Excess nutrients in aquatic ecosystems is a major water quality problem
globally. Worsening eutrophication issues are notable in cold temperate
areas, with pervasive problems in many agriculturally dominated
catchments. Predicting nutrient export to rivers and lakes is
particularly difficult in cold agricultural environments because of
challenges in modelling snow, soil, frozen ground, climate, and
anthropogenic controls. Previous research has shown that the use of many
popular small basin nutrient models can be problematic in cold regions
due to poor representation of cold region hydrology. In this study, the
Cold Regions Hydrological Modelling Platform (CRHM), a modular modelling
system, which has been widely deployed across Canada and cold regions
worldwide, was used to address this problem. CRHM was extended to
simulate biogeochemical and transport processes for nitrogen and
phosphorus through a complex of new process-based modules that represent
physicochemical processes in snow, soil and freshwater. Agricultural
practices such as tillage and fertilizer application, which strongly
impact the availability and release of soil nutrients, can be explicitly
represented in the model. A test case in an agricultural basin draining
towards Lake Winnipeg shows that the model can capture the extreme
hydrology and nutrient load variability of small agricultural basins at
hourly time steps. It was demonstrated that fine temporal resolutions
are an essential modelling requisite to capture strong concentration
changes in agricultural tributaries in cold agricultural environments.
Within these ephemeral and intermittent streams, on average, 30%, 31%,
20%, and 16% of the total annual load of NO3,
NH4, SRP and partP occurred during the episodic snowmelt
freshet ~9 days, accounting for 21% of the annual
flow), but shows extreme temporal variation. The new nutrient modules
are critical tools for predicting nutrient export from small
agricultural drainage basins in cold climates via better representation
of key hydrological processes, and a temporal resolution more suited to
capture dynamics of ephemeral and intermittent streams.