Elisabet Lewan

Effects of a catch crop on leaching of nitrogen from a sandy soil: Simulations and measurements

The effects of an undersown catch crop on the dynamics and leaching of nitrogen in cropping systems with spring cereals were investigated in southern Sweden. Field measurements of soil mineral nitrogen and nitrogen concentrations in drainage water were made for 4 years in a sandy soil. The experiment was performed on four tile-drained field plots sown with spring cereals. On two of the plots, Italian rye grass was undersown and ploughed down the following spring during three of the years. The other two plots were treated in a conventional way and served as controls. Soil nitrate levels were substantially reduced in the catch-crop treatment, but increased during the fourth year when no catch crop was grown. The differences between the treatments in soil nitrate were reflected in the nitrate concentrations measured in the drainage water. A mathematical model was used to simulate nitrogen dynamics in corresponding treatments. There was good agreement between measurements and simulations with regard to patterns of change in soil mineral nitrogen and nitrate concentrations in drainage water for each treatment. Simulated leaching of nitrate in the conventional treatment was 1.9–3.9 g N m−2 y−1 during the first three years while calculated leaching based on the measurements was 2.7–4.4 g N m−2 y−1. In the catch-crop treatment leaching of nitrate was reduced by 1.4–2.6 g m−2 y−1 according to the simulations and by 2.2–4.1 g m−2 y−1 according to calculations based on the measurements. Measurements showed that leaching of nitrogen compounds other than nitrate was hardly affected by the catch crop. In the simulations the ploughed-down catch crop resulted in temporary increases of the litter pool, a net increase of the humus pool and a reduced C-N ratio of the litter pool. Simulated net mineralization from the litter pool was substantially higher in the catch-crop treatment compared with the conventional treatment. In the fourth year, the yield of the main crop was 20–25% higher in the catch-crop treatment, and leaching was higher than in the conventional treatment.

Impact of Spatial Soil and Climate Input Data Aggregation on Regional Yield Simulations

We show the error in water-limited yields simulated by crop models which is associated with spatially aggregated soil and climate input data. Crop simulations at large scales (regional, national, continental) frequently use input data of low resolution. Therefore, climate and soil data are often generated via averaging and sampling by area majority. This may bias simulated yields at large scales, varying largely across models. Thus, we evaluated the error associated with spatially aggregated soil and climate data for 14 crop models. Yields of winter wheat and silage maize were simulated under water-limited production conditions. We calculated this error from crop yields simulated at spatial resolutions from 1 to 100 km for the state of North Rhine-Westphalia, Germany. Most models showed yields biased by <15% when aggregating only soil data. The relative mean absolute error (rMAE) of most models using aggregated soil data was in the range or larger than the inter-annual or inter-model variability in yields. This error increased further when both climate and soil data were aggregated. Distinct error patterns indicate that the rMAE may be estimated from few soil variables. Illustrating the range of these aggregation effects across models, this study is a first step towards an ex-ante assessment of aggregation errors in large-scale simulations.

Simulations of soil carbon and nitrogen dynamics during seven years in a catch crop experiment

Abstract This study aimed, with the use of simulation models, to quantify the effect of several years incorporation of catch crop material into the soil on soil organic matter storage, N mineralisation capacity and risk for N leaching. C and N dynamics in crop and soil were simulated with the SOILN model (Version 9.2). The simulated results were compared with measurements of crop and soil N, crop biomass and N-leaching from a catch crop field experiment situated in southwestern Sweden. The generality of parameter values determining the long-term turnover of soil organic matter was tested. To reproduce measured soil mineral N and crop N, mineralisation had to be favoured by high C mineralisation. After 6 years of catch crop treatment, simulated soil organic matter content had increased by less than 2%, but the N-mineralisation capacity had increased by 25%, corresponding to 37 kg N ha−1. With a continuous annual use of catch crops only a few per cent of the extra mineralised N was leached. Without a succeeding catch crop, however, 30% of N from the increased mineralisation was leached. The results indicated that the decomposition rate increased immediately after frost events.

Modeling Water and Heat Balance of the Boreal Landscape—Comparison of Forest and Arable Land in Scandinavia

The water and heat balances of an arable field and a forest in the boreal zone in Scandinavia were explored using 3 yr of observations and simulations with two different soil‐vegetation‐atmosphere transfer (SVAT) models over a 30-yr period. Results from a detailed mechanistic model [coupled heat and mass transfer model (COUP)] were compared with those obtained with a large-scale type of SVAT model used in the weather prediction model at the European Centre for Medium-Range Weather Forecasts [ECMWF tiled land surface scheme (TESSEL)]. The COUP model simulations agreed well with the observations from a seasonal perspective. The TESSEL model differed significantly from the measurements when standard operational parameter values were used. The

Direct and indirect effects of climate change on herbicide leaching - A regional scale assessment in Sweden

Climate change is not only likely to improve conditions for crop production in Sweden, but also to increase weed pressure and the need for herbicides. This study aimed at assessing and contrasting the direct and indirect effects of climate change on herbicide leaching to groundwater in a major crop production region in south-west Sweden with the help of the regional pesticide fate and transport model MACRO-SE. We simulated 37 out of the 41 herbicides that are currently approved for use in Sweden on eight major crop types for the 24 most common soil types in the region. The results were aggregated accounting for the fractional coverage of the crop and the area sprayed with a particular herbicide. For simulations of the future, we used projections of five different climate models as model driving data and assessed three different future scenarios: (A) only changes in climate, (B) changes in climate and land-use (altered crop distribution), and (C) changes in climate, land-use, and an increase in herbicide use. The model successfully distinguished between leachable and non-leachable compounds (88% correctly classified) in a qualitative comparison against regional-scale monitoring data. Leaching was dominated by only a few herbicides and crops under current climate and agronomic conditions. The model simulations suggest that the direct effects of an increase in temperature, which enhances degradation, and precipitation which promotes leaching, cancel each other at a regional scale, resulting in a slight decrease in leachate concentrations in a future climate. However, the area at risk of groundwater contamination doubled when indirect effects of changes in land-use and herbicide use, were considered. We therefore concluded that it is important to consider the indirect effects of climate change alongside the direct effects and that effective mitigation strategies and strict regulation are required to secure future (drinking) water resources.

Evaporation and discharge from arable land with cropped or bare soils during winter. Measurements and simulations

Abstract Evaporation from cropping systems with and without crop cover during winter (spring cereals with and without undersown Italian rye grass) was studied. Measured daily discharge from field lysimeters and tile-drained plots was used to investigate to what extent discharge measurements could reveal differences in evaporation between the cropping systems. The identified differences were interpreted using a physically based model for simulating water and heat flows in the soil. Measurements were made between 1 April 1988 and 1 April 1991 on a sandy loam in southwestern Sweden. Annual precipitation (1 April–31 March) was 910 mm, 697 mm and 677 mm respectively. Total annual discharge showed a large degree of inconsistent variation between field plots within the same year and treatment, whereas the discharge dynamics showed more consistent differences between treatments. Standard meteorological variables and data on soil properties and crop development were used as input to the model. Calculations of soil evaporation, transpiration and evaporation of intercepted water were based on the Penman-Monteith equation. The model was calibrated against the actual date on which discharge started, which consistently occurred later in the autumn in the cropped soil than in the bare soil. Simulated annual total evaporation amounted to 505 mm, 470 mm and 396 mm from the system with bare soil during winter, and 552 mm, 510 mm and 423 mm from the system with cropped soil during winter. However, during periods with low temperatures and frequent precipitation, total evaporation rates for the bare soil were as high as or even exceeded those for cropped soil. Simulated annual soil evaporation from the system with cropped soil during winter constituted about 38% of the total annual evaporation, whereas it varied between 55 and 65% for the system with bare soil during winter. Discrepancies between simulated and measured discharge could have been due to spatial variability in soil and plant properties in the field, as well as to errors in the estimated unsaturated hydraulic conductivities or root water uptake functions used in the simulations. In simulations based on either modified soil properties or root depths, similar changes in water dynamics were obtained, demonstrating the need for independent determinations of the soil hydraulic properties.

Predicting silage maize yield and quality in Sweden as influenced by climate change and variability

Abstract In recent decades European silage maize production has extended northwards, into Scandinavia, and the importance of maize in fodder production has increased substantially. For the northward expansion of maize production it is of interest to evaluate both the role of climate change that has occurred already, and scenarios for possible future climate change. The aim of this study was to assess for Swedish climatic conditions, the annual variation in silage maize yield and quality (dry weight and starch contents) of cultivars currently grown in Germany. The MAISPROQ simulation model currently used in German maize production was applied to evaluate the effects of (i) cultivar differences (four cultivars; four sites; 2003–2009), (ii) intra-regional variation among ten sites representing three regions (two cultivars; 2003–2009), and (iii) climatic variability among two historical periods during 1961–2009 and three future periods during 2011–2100 using A2-emission climate scenarios and the Delta-method (two cultivars; four sites). Forage quality assessments strongly influenced the assessments of harvest time and thereby the yield. Changes in simulated yield of the tested cultivars were high for the past climate, but relatively small under future climatic conditions due to earlier harvest caused by improved forage quality. By the end of the 21st century an appropriate fodder quality would be achieved every year in the south of Sweden, whereas in the middle of Sweden (60°N) about 30% of the years would not be successful, even for the earliest cultivar. In the east, increased water stress counteracted the positive effect of a prolonged growing season. It was concluded that adaptation of field experiments to model calibration requirements remains to be done, in order to enable extrapolation of observations from Swedish field trials to a changing future climate.

Implications of Spatial Variability of Soil Physical Properties for Simulation of Evaporation at the Field Scale

Soil water retention curves in eight soil profiles on a sandy soil were determined by field measurements of soil water content and soil water matric potential made during 19 days of continuous drying using time-domain-reflectometry and tensiometers. Evaporation from the different profiles was simulated using the retention curves and estimated unsaturated conductivity functions, meteorological data as driving variables, and measured soil water matric potentials at 40 cm depths as the lower boundary condition. Good agreement was obtained between simulated and measured evaporation from different locations in the field. Areal mean evaporation obtained as the arithmetic mean of individual simulations was 2% smaller than the evaporation obtained using mean physical properties in one simulation. This difference was small compared to the variation in simulated evaporation between individual locations (62–142% of the meanevaporation).

Boreal Forest Surface Parameterization in the ECMWF Model—1D Test with NOPEX Long-Term Data

The objective of the present study was to assess the performance and recent improvements of the land surface scheme used operationally in the European Centre for Medium-Range Weather Forecasts (ECMWF) in a Scandinavian boreal forest climate/ecosystem. The previous (the 1999 scheme of P. Viterbo and A. K. Betts) and the new (Tiled ECMWF Surface Scheme for Exchange Processes over Land, TESSEL) surface schemes were validated by single-column runs against data from NOPEX (Northern Hemisphere Climate-Processes Land-Surface Experiment). Driving and validation datasets were prepared for a 3-yr period (1994-96). The new surface scheme, with separate surface energy balances for subgrid fractions (tiling), improved predictions of seasonal as well as diurnal variation in surface energy fluxes in comparison with the old scheme. Simulated wintertime evaporation improved significantly as a consequence of the introduced additional aerodynamic resistance for evaporation from snow lying under high vegetation. Simulated springtime evaporation also improved because the limitation of transpiration in frozen soils was now accounted for. However, downward sensible heat flux was still underestimated during winter, especially at nighttime, whereas soil temperatures were underestimated in winter and overestimated in summer. The new scheme also underestimated evaporation during dry periods in summer, whereas soil moisture was overestimated. Sensitivity tests showed that further improvements of simulated surface heat fluxes and soil temperatures could be obtained by calibration of parameters governing the coupling between the surface and the atmosphere and the ground heat flux, and parameters governing the water uptake by the vegetation. Model performance also improved when the seasonal variation in vegetation properties was included.

Influence of shelterbelt type on potential evaporation in an arid environment

The results from three studies in central Tunisia are presented, the first and second in November‐December of two consecutive years and the third in April‐May. The spatial variation in reductions of potential evaporation behind the shelterbelts, measured with the Andersson evaporimeter, is reported. The types of shelterbelt systems were a mechanical shelter of porous nylon netting on the open plain, a cactus Opuntia ficus indica shelter with very low porosity, a nursery with an Acacia cyanophylla shelter within a system of rows of tall Eucalyptus camaldulensis situated 40 to >170 m from the shelter, and an orchard of dispersed young fruit trees sheltered by young plantations of Acacia cyanophylla and Casuarina equisetifolia shelterbelts. The mechanical shelter reduced potential evaporation similarly in the three studies. The cactus shelter was inefficient owing to its low porosity. In the nursery the tall eucalyptus trees reduced potential evaporation at distances >20H (H is distance in terms of height of...