Annemieke I. Gärdenäs

Review and comparison of models for describing non-equilibrium and preferential flow and transport in the vadose zone

Abstract In this paper, we review various approaches for modeling preferential and non-equilibrium flow and transport in the vadose zone. Existing approaches differ in terms of their underlying assumptions and complexity. They range from relatively simplistic models to more complex physically based dual-porosity, dual-permeability, and multi-region type models. A relatively simple dual-porosity flow model results when the Richards equation is combined with composite (double-hump type) equations for the hydraulic properties to account for both soil textural (matrix) and soil structural (fractures, macropores, peds) effects on flow. The simplest non-equilibrium flow model, a single-porosity model which distinguishes between actual and equilibrium water contents, is based on a formulation by Ross and Smettem [Soil Sci. Soc. Am. J. 64 (2000) 1926] that requires only one additional parameter to account for non-equilibrium. A more complex dual-porosity, mobile–immobile water flow model results when the Richards or kinematic wave equations are used for flow in the fractures, and immobile water is assumed to exist in the matrix. We also discuss various dual-permeability models, including the formulation of Gerke and van Genuchten [Water Resour. Res. 29 (1993a) 305] and the kinematic wave approach as used in the MACRO model of Jarvis [Technical Description and Sample Simulations, Department of Soil Science, Swedish University of Agricultural Science, Uppsala, Sweden (1994) 51]. Both of these models invoke terms accounting for the exchange of water and solutes between the matrix and the fractures. Advantages and disadvantages of the different models are discussed, and the need for inter-code comparison is stressed, especially against field data that are sufficiently comprehensive to allow calibration/validation of the more complex models and to distinguish between alternative modeling concepts. Several examples and comparisons of equilibrium and various non-equilibrium flow and transport models are also provided.

Modelling seasonal nitrogen, carbon, water and heat dynamics of the Solling spruce stand

Abstract Daily flows of water, heat, carbon and nitrogen from 1976 until 1991 were simulated with the one-dimensional models soil —soiln for a 105-year-old spruce stand on an acid brown forest soil at Solling (central Germany). Nitrogen deposition in the area exceeded 40 kg N ha −1 yr −1 . Comparisons were made with measured soil water tensions, chloride and mineral nitrogen concentrations in soil water, and tree biomass components. Down to 60-cm depth, the simulated soil water tensions agreed well with measured data when a summer surface resistance of 125 s m −1 was used to calculate potential transpiration. Modelling efficiency (an indicator of agreement between simulations and measurements) for pF-values of soil water tensions ranged from 0.55 to 0.87. At 100-cm depth, the modelling efficiency was only 0.15. The mean value of the chloride concentrations were, at the same time, underestimated. The average deep percolation was 38% of the precipitation. A lower summer surface resistance of 80 s m −1 was necessary to increase the agreement between simulated and measured chloride concentrations, which decreased the deep percolation to 31% of the precipitation. On average, simulation resulted in an accumulation of N in trees (21 kg N ha −1 yr −1 but a decrease of N in soil (7 kg N ha −1 yr −1 ). Simulated leaching was high (27 kg N ha −1 yr −1 ) and probably overestimated due to overestimated soil nitrate concentrations below the root zone. To evaluate the carbon and nitrogen model, too few detailed measurements were available on, especially, plant N uptake and amounts of soil mineral N.

Soil respiration fluxes measured along a hydrological gradient in a Norway spruce stand in south Sweden (Skogaby)

Soil respiration fluxes were measured continuously in order to assess the degree to which they were influenced by spatial and temporal variation in soil moisture. The synergistic effects of the variation in soil moisture with the one in soil temperature, soil organic matter and global radiation on respiration fluxes were also analysed. The measurements were performed using an open chamber system along a hydrological gradient in a Norway spruce forest in south Sweden (Skogaby) for 3 weeks in June 1995. The measured soil respiration fluxes were quite stable and somewhat larger compared with those reported in literature. The experiment took place during the shoot elongation period with intensive nutrient uptake, and it might be that soil respiration was dominated by mycorrhizal activity. Variation in the moisture content of the litter layer accounted for most of the spatial variation in respiration fluxes.

Simulated water balance of Scots pine stands in Sweden for different climate change scenarios

Abstract The effects of climate change on the water balance of Scots pine were studied with a coupled water and heat flow model called ‘SOIL’. Two forest soil types (a silty-sand and a sand) at five locations in Sweden were chosen to represent sites with different air temperature, growing season length and precipitation excess. The simulated water balance for the period 1961–1987 was compared with those simulated with two climate change scenario schemes: one is based on increased temperature and the other on both increased temperature and increased precipitation. Different assumptions regarding the effects of changes in leaf area index and minimum canopy resistance on transpiration were studied. Water stress increased substantially in the scenarios based on increased temperature only, which prevented transpiration from increasing. But with scenarios based on simultaneous changes in temperature and precipitation, water stress increased mainly during spring and transpiration increased by 50 mm and 100 mm in northern and southern Sweden, respectively, i.e. by 30–50%. When simultaneous changes in climate and stand characteristics were assumed, transpiration increased by 30–70% with the relative change being greatest for northern Sweden. Differences in water balance between the locations and soil types were less pronounced in the climate change scenarios than in the present climate scenario.

Open ventilated chamber system for measurements of H2O and CO2 fluxes from the soil surface

This paper describes the design of a ventilated ‘open’ chamber system for continuous measurement of fluxes of water and carbon dioxide from the soil surface. The system consists of four units: a chamber, an air switching unit, an infrared gas analyzer and a data logger. The chamber has the shape of a rectangular ‘box’ lying on the soil surface covering a ground area of 0.68 m−2. The wind speed through the chamber can be regulated with a fan from zero up to a maximum of about 0.9 m s−1. The soil evaporation measured by the chamber showed good agreement with evaporation measured by a balance. In situ measurement of evaporation by the chamber agreed well with evaporation estimated by a physically-based soil model. The soil water content under the chamber showed good agreement with the simulated values as well as with the water content measured outside the chamber. The magnitude of soil CO2 efflux was in good accordance with other data reported in literature. The main error with this technique was found in the measurement of the air flow through the chamber.

SOIL ORGANIC MATTER IN EUROPEAN FOREST FLOORS IN RELATION TO STAND CHARACTERISTICS AND ENVIRONMENTAL FACTORS

Published data on the quantities of organic matter in the forest floor (SOMfl) of European forest stands were collated. Studies of SOMfl were included if the total sampled surface was at least 0.15 m2, the surface of a single sample was at least 100 cm2, live material was sorted out, and ash or carbon content was determined. Data from 59 forest stands were compared with regard to stand characteristics and environment. Using a single variable, tree genus was most important for the amount of SOMfl (R 2 adj = 0.34). The amounts in spruce stands (41 × 103 kg ha−1) were significantly higher than those in larch, Douglas fir, oak and birch stands (4–11 × 103 kg ha−1). The best significant multiple model was genus combined with stand age and basal area (R 2 adj. = 0.72). The importance of litter quantity and quality for differentiating the tree genera is discussed.

Summer Rains and Dry Seasons in the Upper Blue Nile Basin: The Predictability of Half a Century of Past and Future Spatiotemporal Patterns

During the last 100 years the Ethiopian upper Blue Nile Basin (BNB) has undergone major changes in land use, and is now potentially facing changes in climate. Rainfall over BNB supplies over two-thirds of the water to the Nile and supports a large local population living mainly on subsistence agriculture. Regional food security is sensitive to both the amount and timing of rain and is already an important political challenge that will be further complicated if scenarios of climate change are realized. In this study a simple spatial model of the timing and duration of summer rains (Kiremt) and dry season (Bega), and annual rain over the upper BNB was established from observed data between 1952 and 2004. The model was used to explore potential impacts of climate change on these rains, using a down-scaled ECHAM5/MP1-OM scenario between 2050 and 2100. Over the observed period the amount, onset and duration of Kiremt rains and rain-free Bega days have exhibited a consistent spatial pattern. The spatially averaged annual rainfall was 1490 mm of which 93% was Kiremt rain. The average Kiremt rain and number of rainy days was higher in the southwest (322 days) and decreased towards the north (136 days). Under the 2050–2100 scenario, the annual mean rainfall is predicted to increase by 6% and maintain the same spatial pattern as in the past. A larger change in annual rainfall is expected in the southwest (ca. +130 mm) with a gradually smaller change towards the north (ca. +70 mm). Results highlight the need to account for the characteristic spatiotemporal zonation when planning water management and climate adaptation within the upper BNB. The presented simple spatial resolved models of the presence of Kiremt and annual total rainfall could be used as a baseline for such long-term planning.

Accumulation of wet-deposited radiocaesium and radiostrontium by spring oilseed rape (Brássica napus L.) and spring wheat (Tríticum aestívum L.)

The accumulation of (134)Cs and (85)Sr within different parts of spring oilseed rape and spring wheat plants was investigated, with a particular focus on transfer to seeds after artificial wet deposition at different growth stages during a two-year field trial. In general, the accumulation of radionuclides in plant parts increased when deposition was closer to harvest. The seed of spring oilseed rape had lower concentrations of (85)Sr than spring wheat grain. The plants accumulated more (134)Cs than (85)Sr. We conclude that radionuclides can be transferred into human food chain at all growing stages, especially at the later stages. The variation in transfer factors during the investigation, and in comparison to previous results, implies the estimation of the risk for possible transfer of radionuclides to seeds in the event of future fallout during a growing season is still subject to considerable uncertainty.

Interception and retention of wet-deposited radiocaesium and radiostrontium on a ley mixture of grass and clover.

The aims of this study were to assess the potential radioactive contamination of fodder in the case of accidental radionuclide fallout, and to analyse the relationship between interception and retention of radionuclides as a function of biomass and Leaf Area Index (LAI). The interception and the retention of wet deposited (134)Cs and (85)Sr in ley (a mixture of grass and clover) were measured after artificial wet deposition in a field trial in Uppsala (eastern central Sweden). The field trial had a randomised block design with three replicates. (134)Cs and (85)Sr were deposited at six different growth stages during two growing seasons (2010 and 2011) using a rainfall simulator. The biomass was sampled in the center of each parcel 2 to 3h after deposition and at later growth stages (1 to 5) during the growing season. The above ground biomass and LAI were measured as well. The interception of radionuclides by the ley was largest at the late growth stages; the spike and tassel/flowering (code 5:6) in the 1(st) year, and at flowering/initial flowering (code 6:5) in the 2(nd) year. There was a correlation between radionuclide interception and above ground plant biomass, as well as with LAI, for both radionuclides in both years. The highest activity concentrations of both radionuclides were measured after deposition at the late growth stages and were found to be higher in the 2(nd) year. The weathering half-lives were shorter at the earlier growth stages than at the later growth stages for both radionuclides. For the magnitude of deposition chosen in our experiment, it can be concluded that the above ground biomass is a good predictor and the LAI a more uncertain predictor of the interception of radiocaesium and radiostrontium by ley grass and clover.

Indications that site preparation increases forest ecosystem carbon stocks in the long term

ABSTRACT Mechanical site preparation (MSP) causes a mixing disturbance of the soil, which may increase decomposition of soil organic matter and subsequent carbon (C) dioxide emissions to the atmosphere. MSP also promotes the establishment and growth of tree seedlings, and hence ecosystem C fixation. However, there are uncertainties regarding the net effects of MSP on C stocks at the ecosystem scale. To assess decennial effects of MSP on ecosystem C stocks, C stocks in soil, ground vegetation and trees at three experimental forest sites with Pinus contorta, Pinus sylvestris and Picea abies in Sweden were sampled and measured for ca. 25 years in a control and after three MSP treatments: disc trenching, mounding and ploughing. After 25 years, all of the MSP treatments resulted in larger ecosystem C stocks than the control treatment due to positive effects on the tree biomass C stock. The tree C stock was highest after ploughing, intermediate after mounding or disc trenching and lowest in untreated control plots at all experimental sites. The MSP treatments did not affect the soil C stocks down to 30 cm. We recommend mounding or disc trenching to promote C sequestration as they disturb sites’ ecological, aesthetic and recreational values less than ploughing.