Jung-Madsen, S. and Bach H. (red.) 2022. Transport of nitrogen and phosphorus from land to sea around year 1900. Aarhus University, DCE – Danish Centre for Environment and Energy, 192 pp. Scientific Report No. 498 http://dce2.au.dk/pub/SR498.pdf
Background
The current report aims to describe the nutrient loads from land to sea around the year 1900. The nutrient loads are calculated considering the various factors affecting the nutrient inputs and transport based on available data from that time, literature, comparative analysis methods and modelling tools. The main factors investigated are climate, hydrology, land use, agricultural practices and drainage, urban development and landscape (e.g. nutrient retention in groundwater, wetlands, lakes and streams). The ambition was to use as much data and information from the time as possible, taking into consideration the quality and representativeness, and use modelling and GIS tools to provide a geographically distributed estimate of total nitrogen (TN) and total phosphorus (TP) concentrations and loads from the root zone to the sea. To be able to do this, the datasets on climate and hydrology were analysed and expanded as part of the project, and runoff was modelled using the national water resources model (DK- model).
The agricultural practices and land use in different areas of the country were analysed in detail, and the root zone concentration of nitrogen was determined for different land use categories. This was based on old farmland statistics from Denmark and Northern Germany, which at the time included land areas that are now Danish, and root zone nitrogen concentrations extracted from farming experiments resembling year 1900 farm management practices. Phosphorus inputs to the surface waters from the landscape, including agricultural activities, were estimated for the main input categories: soil drainage, land reclamation, grazing animals, soil erosion and manure storage.
Furthermore, a detailed analysis of the contribution of nutrients from cities (point sources) was included. This analysis estimated nitrogen as well as phosphorus inputs.
To model the concentration and transport of nitrogen in the freshwater load to the sea, the national nitrogen model (NMN) was used. The model requires geographically distributed input of climate variables, such as temperature and water runoff, the input of nitrogen from the root zone and point sources as well as data on the depth of groundwater and the proportion and location of wetlands, lakes and streams, to estimate the retention in the surface water system.
The phosphorus analysis used an approach of ”background” or ”nature” water concentration levels of phosphorous, on top of which the relevant additional diffuse and point sources were added and retention in lakes subtracted. The transport of phosphorus through the catchments was simulated using the same water discharge as for the nitrogen modelling, but the model as such is much simpler than the nitrogen model because of the way phosphorus behaves in the environment.
Among other things, the analysis showed that in the year 1900, weather conditions were colder and drier than today, more land was in agricultural use but less was tile drained, and due especially to a larger proportion of wetlands the retention of nutrients in the landscape was higher.
The conclusion from the chapters on climate, hydrology, nutrient input nutrient transport from land to sea and uncertainties are given in the synthesis of results in chapter 11 and are also presented below.
Climate
The climate was colder and drier around year 1900 compared with the present-day. The estimated average annual precipitation around year 1900 was about 60 mm, or 7% lower than today. Digitized climate data from around year 1900 at observation points across the country, including temperature, wind and rainfall, were used to find monthly values of bias-corrected precipitation. The correction approach was evaluated for the period 1917-1950 using water balance modelling of discharge. At national level, a water balance error of 3% indicated reasonable correction estimates, but large regional differences in error level was found. In order to obtain spatially distributed corrected precipitation for the period 1890-1910 a delta change climate factor approach was used. In this approach national monthly correction factors were calculated based on corrected precipitation for 1890-1910 compared with 1989-2010. These national factors were then applied to the present daily corrected precipitation assuming a similar geographical distribution of precipitation around year 1900 as in the present time reference period (1989-2010) to provide a spatially distributed daily time series of precipitation for the period 1890-1910.
To be able to model nitrate leaching, global radiation and potential evapotranspiration must be calculated. By using the measured minimum and maximum air temperatures for 1890-1950, the global radiation and potential evapotranspiration can be calculated and used in the simulation of nitrate leaching in this period. The modelled global radiation and potential evapotranspiration around year 1900 are in good agreement with values measured at Foulum from 1987-2013.
Hydrology
The total discharge based on the precipitation estimates and drainage density estimated for the historic time was in average 292 mm/yr for the period 1890-1910 compared with 333 mm/yr for the present period (1990-2010) as calculated by the hydrological model (DK model, chapter 3). Subsequently, applying a delta change method, the total discharge was recalculated to 335 mm/yr for the present period and 297 mm/yr for the year 1900 period (chapter 8). This means that for the total average, annual discharge was about 11% lower around year 1900 compared with the present time. The change in discharge for the two periods largely reflects the changes in precipitation, but is amplified in some areas by the lower density of drainage in the historical period. The calculated change in discharge when comparing the present time to the time around year 1900 ranged between 0 and 20% for most of the country, which agrees with the trend analysis of long discharge time-series presented in Jensen (ed.) (2017). However, for the western part of Zealand, the data indicate an increase in discharge between the two periods of approximately 30%, while the simulation resulted in a decrease in discharge of approx. 5%. The methodology used implies that the total discharge to the sea may resemble the conditions around year 1900, but it cannot be expected to reproduce the local conditions at that time.
Nutrient inputs
The nutrients reaching the sea from the land mainly originated from agricultural activities and dwellings across the country around year 1900. In addition, runoff from erosion along the streams contributed to the nutrient content in freshwater, particularly phosphorus.
Agriculture
The area in agricultural use increased dramatically during the last half of the 19th century and accounted for close to 3/4 of the area under Danish administration around year 1900. Crop production differed significantly from current agriculture for virtually all growth factors: inferior crop varieties, higher weed pressure, lack of chemical crop protection and inferior plant nutrient supply, including the absence of mineral fertiliser. The main sources of nutrients were solid farmyard manure, liquid manure and nitrogen fixation by legume crops. The number and categories of livestock as well as the farm structure and management practices around year 1900 also differed from today’s practices.
Parish-level statistics from around year 1900 for the area under current Danish administration were unified into eight categories (winter and spring crops, grass, root crops, fallow, nature and forest), and for each category a nitrogen concentration was ascribed to the root zone percolate. The nitrogen root zone concentrations were set using data from studies of organic farming as a proxy for the past time situation. Literature data were found for the remaining categories. These values were applied in the nitrogen modelling. The calculation of the area-weighted average nitrogen concentration for land in agricultural use (78% of the land area) resulted in a value of 12 mg N/l, while the value for the entire land area was 9.6 mg N/l in root zone percolate (inorganic nitrogen).
The estimation of agricultural sources for phosphorus considered factors such as soil drainage, land reclamation, grazing animals, soil erosion and manure storage. These factors were difficult and uncertain to determine, leading to an estimated range from 56 to 196 ton P annually around year 1900.
Point sources
Sewer systems were increasingly implemented in towns, but wastewater treatment did not exist in year 1900. Therefore, towns were significant point sources around year 1900, with 4,261 ton N/yr and 764 ton P/yr emitted in excrements from humans and animals and industrial wastewater. The findings indicate that the majority of the nutrients from point sources discharged directly to receiving waters (55%), but emissions to landfills (20%) and agricultural soil (25%) were significant as well. The total contribution from inland and direct point sources to water was estimated to 471 ton P, about 65–70% of the present-day value (704 ton P, average 2014–2018) and 2,531 ton N, about 47% of the present-day TN point sources (5,400 ton N, 2020).
Other sources, including background nutrient concentration
Nitrogen inputs from the atmosphere were estimated by multiplying EMEP simulations for year 2000 by 0.3 (Jensen (ed.), 2017). Organic nitrogen originating from landscape sources and internal surface water sources was included to be able to calculate total nitrogen concentrations. Estimates based on literature studies assume that the organic nitrogen concentration around year 1900 was about 20% below the current levels. Furthermore, it is assumed that the current geographical distribution of organic N is valid for the time around year 1900.
A literature review and measurements from largely undisturbed streams were used to estimate background TP stream concentrations. An area-weighted TP median value at 0.052 mg/l was estimated.
Nutrient transport to the sea
Nitrogen percolates through the soil and reaches the groundwater where reduction of nitrogen under oxygen-free conditions (retention) can take place before the remaining nitrogen ends up in surface waters (wetlands, lakes, streams). The National Nitrogen Model simulates transport and retention in groundwater based on water discharge and nitrogen percolate input. The surface water component calculates the nitrogen retention in wetlands, streams and lakes, while also considering point source inputs, atmospheric inputs and the contribution of organic nitrogen. Landscape changes between the time around year 1900 and the present time were handled by modifying the current landscape maps based on various information sources on the past landscape related to rivers and lakes. For wetlands, different maps were used.
The phosphorus analysis was based on total phosphorus considerations and used an approach of a ”background” or ”nature” concentration level, on top of which the relevant additional agricultural and point sources were added and retention in lakes was subtracted. The transport and routing of phosphorus through the catchments were simulated using the same water discharge as in the nitrogen modelling.
The nitrogen retention in inland surface water was shown to be higher in the present period (28,000 ton N) than in the 1900 period (26,000 ton N) due to a larger present-day nitrogen load. However, the relative nitrogen retention was higher around the year 1900, as 43% of the load was removed compared with 33% for the present period.
The total nitrogen load is modelled to be approximately 36,000 ton N/yr around year 1900, which is approximately 40% less than for the present period (59,000 ton N/yr). The nitrogen concentration is modelled to be around 2.8 mg N/l around year 1900 compared with 4.1 mg N/l in the present period. The national nitrogen model yields regional results, which are utilised for estimating regional year 1900 nitrogen and freshwater loads.
The estimated average stream water phosphorus concentration around year 1900 was 0.062–0.075 mg P/l, equivalent to 60–70% of the present-day stream water TP concentration (0.1 mg P/l). The TP values were calculated for nine geographical regions across the country. The total TP loading to the sea, including background, other diffuse sources, inland and direct point sources and subtracted phosphorus retention in lakes, was estimated to 1,200–1,340 ton P, 60–65% of (or 35-40% less than) the present-day phosphorus loading (2,021 ton/yr).
Uncertainties
Working with a period 120 years ago naturally makes most aspects of calculating the national nitrogen and phosphorus load more uncertain than when calculating it for the present period. An in-depth analysis of uncertainties of data layers, variables, model and model assumptions was not a part of the present study however, some considerations regarding uncertainty and sensitivity (the effect of a given parameter on the result) have been made.
Most of the parameters used to estimate the nitrogen loads around the year 1900 are considered to have “medium” uncertainty (on a three-step scale from low to high). The uncertainty of the nitrogen loads is influenced by a variety of factors, the most important being the uncertainty of the estimates of precipitation, run-off, root zone concentration of nitrogen and retention in surface and groundwater.
Overall, the model concept used to calculate the year 1900 nitrogen load is considered relatively robust and the overall uncertainty at national scale acceptable. However, the uncertainty increases with decreasing geographical- and timescales.
Most of the parameters used to estimate phosphorus loads are considered “medium” to “highly” uncertain. The uncertainty of phosphorous loads is especially influenced by the uncertainties of the estimates of precipitation, run off, P input from point sources and the background TP concentration. Despite the many uncertainties the results of this study are believed to be the best possible estimate of the year 1900 phosphorous loads. Furthermore, the results are supported by historical lake measurements that also find the historical TP-concentrations to be lower than today but considerably higher than the background concentration, though.
Perspectives
Many of the European studies that are compared with the present study report nitrogen concentrations around the year 1900 that are considerably lower than in the present study. The reasons for this are probably differences in landscape, land use, farming practices and runoff between the investigated areas and Denmark. It probably also reflects the degree to which agricultural practices and nutrient dynamics are included in the studies. In the present study, the year 1900 root zone leaching is calculated, and nitrogen fixation, the main source of nitrogen in Danish agriculture in the year 1900, is considered, which is not the case in most other studies.
