Impacts of annual weather conditions and available soil data on modelling nitrate leaching from farmland at regional and national scale.

Nitrate leaching from farmland varies because of numerous hydrological and biological<br/>processes driven by farm management, soil characteristics, and weather conditions. These<br/>processes affect both water and nutrient transport and chemical and biological<br/>transformation processes. Climatic conditions significantly influence nitrate leaching<br/>processes by affecting soil temperature and moisture, which in turn impact biological and<br/>chemical transformation processes and play crucial roles in crop growth and nutrient uptake.<br/>Soil water surplus creates percolations out of the root zone, which, combined with the nitrate content in the pore water, leads to nitrate leaching out of the root zone.<br/>The variation in the physical properties of the soil affects both the natural drainage and<br/>aeration and thereby also the availability of nutrients. An essential tool for understanding soil<br/>variability is the existence of soil maps. These soil maps include important features such as<br/>soil texture, soil organic matter, bulk density, pH, and soil hydraulic properties. Variations or<br/>uncertainties in these parameters can have a high impact when modelling water transport<br/>and availability, crop growth, and nitrate leaching.<br/>This study compares the impacts of newly generated three-dimensional soil maps and<br/>associated soil characteristics using previously used soil maps and soil data on modelling<br/>water balance and nitrate leaching. We aimed to investigate the variability of the soil maps<br/>and their resulting impact on model outputs of water transport predicted using the Daisy<br/>water transport- and balance- model and NLES5-predicted nitrate leaching. National<br/>datasets for fields and farm data were used to predict nitrate leaching using the empirical<br/>Danish nitrate leaching model NLES5. The results of 32 years of modelling nitrate leaching<br/>using daily weather data (1990 – 2021) on a ten km grid scale and data from Danish farmers<br/>for five years (2017- 2021) were used as inputs in the analysis.<br/>Modelling results showed some low regional impacts on both water balance and nitrate<br/>leaching of the updated soil data description, but on a national scale, the impacts are minor.<br/>On the other hand, the year-to-year weather variations had a significant impact on nitrate<br/>leaching levels. Changes in agricultural policies over the five years with new targeted cover<br/>crops have a low impact on nitrate leaching levels in the aquatic environment compared to<br/>the year-to-year effects of weather, which means that the effects of new measures need to<br/>be monitored for long periods to obtain the full effect.