Predicting the effects of landuse on water quality – Stage I
Appendix 1: SPASMO Model (HortResearch)
A general description of the SPASMO model
The SPASMO model considers a 1-dimensional soil profile of 7 m depth, divided into 0.25 m intervals (slabs). Water transport through the soil profile is modelled using a water capacity approach (Hutson and Wagenet, 1993) that considers the soil to have both mobile and immobile pathways for water and solute transport. The mobile domain is used to represent the soil’s macropores (e.g. old root channels, worm holes and cracks) and the immobile domain represents the soil matrix. After rainfall or irrigation any dissolved solute is allowed to percolate rapidly through the soil in the mobile domain only. Subsequently, on days when there is no significant rainfall, there is a slow approach to equilibrium between the mobile and immobile phases, driven by a difference in water content between the two domains.
Crop water use
The model calculations run on a daily time step and are based on a simple water balance of the vineyard. A standard crop-factor approach is used to relate the water use of the grapevines to the prevailing weather and time of year (Allen et al., 1999). Plant water use depends on both the ambient meteorological conditions and the physiological stage of plant development. A two-step procedure is used to calculate plant water use, based on guidelines given by the Food and Agriculture Administration (FAO) of the United Nations (Doorenbos and Pruitt, 1977; Allen et al, 1999). Measured values of global radiation, air temperature, relative humidity and wind speed were used to calculate a reference evaporation rate, ETo [mm d-1]. From the modified Penman-Monteith equation, we obtain
where Rn [mm d-1] is the net radiation is expressed in units of an equivalent evaporation rate, Da [kPa] is the difference between the saturation vapour pressure at mean air temperature and the mean actual vapour pressure of the air, s [Pa oC-1] is the slope of the saturation vapour-pressure versus temperature curve, γ [66.1 Pa] is the psychrometric constant, and f(U) is a wind-related function given by
f(U) = 2.7 (1+U/100)
Here U [km d-1] is the 24-hr wind run at 2-m height. This reference value, ETo, defines the rate of evaporation expected from an extensive surface of green grass cover of short, uniform height, actively growing, completely shading the ground, and not short of water.
To account for the effect of plant physiological characteristics, a crop coefficient, Kc, is used to relate the reference evaporation rate, ETo, to the actual crop water use, ET. For routine calculations of crop evapotranspiration, the following equation is used:
ET = KC . ETO
where Kc is a dimensionless number that normally varies between about 0.2 and 1.1. The particular value of the crop coefficient, Kc, determines the evapotranspiration of a disease-free crop grown in a large field under optimum soil water and fertility conditions and achieving full production potential under a given growing environment. In other words it defines the maximum rate of water use expected from a particular crop. Various factors affect the value of Kc, including crop characteristics, crop planting or sowing dates, rate of crop development, length of growing season and climatic conditions. Here we have used standard values for Kc, set to a maximum of 0.70 during mid season, but reducing when the plants are under water or nutrient stress (Allen et al., 1999).
Crop water balance
Water uptake by the vines is assumed to be in proportion to the density of fine roots. For grapes, we have assumed the roots will ramify the soil profile to a depth of 1.5 m, with an exponential profile of root-length that places ¾ of the roots in the top ¼ of the root zone.
Irrigation is applied automatically to the grapes, whenever ½ of the available soil water has been consumed from within the root zone.
Because of the stony nature of the soils we have assumed them to be free draining so that no run off component has been included in the calculations. Instead, all the rain that falls is added to the soil profile.
Drainage through the soil profile is assumed to occur whenever the soil water content exceeds ‘field capacity’. The soil’s physical, hydraulic, and chemical transport properties are prescribed within each soil slab, and these data are obtained from the New Zealand Soils Database (Landcare, 1999).
Nitrogen transport through the soil
As water percolates through the soil profile it carries with it any dissolved solutes, such as fertilizer or pesticide. The nitrogen transport component of SPASMO is based on a simple nitrogen balance that accounts for plant uptake, the application of mineral fertilizer, exchange and transformation processes in the soil, losses of gaseous nitrogen to the atmosphere, and the leaching of nitrogen below the root zone. The SPASMO model considers both the organic nitrogen (i.e. in soil biomass) and the mineral nitrogen (i.e. ammonium and nitrate in solution) contained in the soil and the plant biomass. Dissolved nitrate is considered to be fully-mobile and to percolate freely through the profile, being carried along with the invading water. In contrast, the movement of dissolved ammonium is retarded as it to binds to mineral clay particles of the soil. The soil can receive inputs of organic carbon and nitrogen from plant residues, which is added to the litter layer of the top 0.25 m of soil, and inputs of mineral fertilizer which is applied to the soil surface during in spring time.
Crop growth
Plants play a key role in the nitrogen dynamics of the root zone, and so first it is necessary to ‘grow’ plants in the model. We assume that the amount of soil nitrogen removed by the grape vines will be determined by vine growth, and we estimate the nitrogen uptake from the growth of the various plant organs multiplied by their respective nitrogen concentrations. For the purpose of modelling the vine growth, the daily biomass is given a potential production rate per unit ground area, G (kg/m2/d) that is related, via a conversion efficiency, ε (kg/MJ), to the amount of solar radiant energy, Φ (MJ/m2/d), intercepted by the plant foliage,
The value of ε is related to the water and nitrogen status of the soil, while Φ depends on the daily sunshine, air temperature and the leaf area of the vines (King, 1993). We use an allometric relationship to partition the daily biomass production into the growth of foliage, shoots, roots and berry components. We also assume that plant growth will achieve a maximum only if soil water and soil nitrogen are non-limiting.
Net production of above and below ground biomass
Plant biomass is expressed in terms of the growth and senescence of the plant organs. For each plant organ we use a simple mass balance equation that considers:
• inputs of dry matter (DM) due to carbon allocation
• losses of DM as the plants senescence, and
• removal of DM at harvest (or thinning)
Nitrogen uptake by the crop
The model assumes that plant growth will achieve the maximum potential only if soil water and soil nitrogen (NO3- and NH4+) are non-limiting. The net uptake of nitrogen from the soil is set equal to the amount of nitrogen incorporated into the new biomass, minus the fraction of nitrogen that has been retranslocated, λ, from the old or senescing tissues. Uptake of nitrogen from the soil is assumed to be in proportional to the depthwise distribution of the fine roots.
Figure 14-1: Annual cycle of dry matter in a grape vineyard. The lines reflect the seasonal change in the biomass of the leaves (red), shoots (dark blue), fine roots (green) and berries (light blue).
Figure 14-1 shows the modelled seasonal pattern of vine growth which has been parameterized using data for sauvignon grapes reported by Coombe et al., (1988) and Gladstones (1992). We have assumed the grape vines are trimmed during summer, to control their vigour. Thus, in the model, as soon as the total above-ground dry matter (shoot plus foliage) reaches 2 Mg/ha, the vines are thinned by removing 20% of this DM. This results in 1 or 2 thinnings each year, which is in accord with normal management practice. Leaf fall was assumed to occur in mid May and the shoots were pruned in mid June by removing 75% of the shoot dry matter. Any vegetation removed during thinning and leaf fall was deposited back onto the soil surface. ‘Fresh organic matter’ slowly decomposes, returning the stored carbon and nitrogen to the litter and humus components of the soil biomass (see below).
The daily values of plant growth can be integrated over the whole season to provide an estimate of the annual nitrogen balance of the grape vines. From our model, we estimate the annual N-uptake to be about 60 kg-N/ha/yr. Approximately 2/3 of this nitrogen is returned to the soil biomass as leaf litter and shoot prunings. The remaining 1/3 of the nitrogen taken up by the plants is either removed in the grapes at harvest time, or recycled internally in the stem and roots of the grape vine. This ‘stored’ nitrogen is eventually remobilized during bud burst of the following spring. Thus, the harvest of grapes represents a small loss of nitrogen from the system, which we estimate to be just 16 kg-N/ha/yr. It is normal practice to add a dressing of mineral fertilizer in the spring time, if petiole analysis reveals a decline in leaf-N.
Table 14-1: Model output of the nitrogen balance of the grape vines.
| Component of Nitrogen Balance 1 | Mean (kg-N/ha/yr) |
Sth. Dev. (kg/ha/yr) |
|---|---|---|
| Plant Uptake | 59.8 | 8.1 |
| Removal in grapes | 16.1 | 2.3 |
| Stored in wood | 3.5 | 0.1 |
| Plant Returns to soil | 40.2 | 5.7 |
1 Plant Uptake = Removal in grapes + Stored in wood + Plant Returns to soil
Carbon and nitrogen dynamics of the soil organic matter
The decomposition of soil biomass adds to the amount of mineral nitrogen in the soil. This process is known as mineralization. Mineralization is modelled by dividing the soil organic matter into two pools – a fast cycling litter pool and an almost stable humus pool following Johnsson et al. (1987). This two-pool model then considers the amount of soil carbon and soil nitrogen that cycle within soil organic material. The relative amounts of these two components change daily to reflect inputs of new biomass and losses of older biomass as it decomposes. The nitrogen demand for the internal cycling of soil-C and soil-N is regulated by the C/N ratio of the soil biomass, rO, which is one of the model inputs.
Decomposition of soil litter carbon (CL) is a function of a specific rate constant (KL) which is influenced by temperature and soil moisture. The products of decomposition are CO2, stabilized organic material (humus) and, conceptually, microbial biomass and metabolites. The relative amounts of these products are determined by a synthesis efficiency constant (fE) and a humification fraction (fH). During harvest and leaf fall we assume 10% of the fresh organic matter goes into the litter pool while the remaining 90% is added to the stable humus pool.
A similar set of mass balance equations are used to describe the turn-over of carbon and nitrogen in the humus pool. Decomposition of soil humus (CH) is assumed to follow first-order kinetics with a specific rate constant (KH) which depends on temperature and soil moisture.
Mineralization of soil organic matter
All carbon and nitrogen turn-over reactions can result in a net production (mineralization) or a net consumption (immobilization) of ammonium, depending on the C/N ratio of the biomass, rO, in the two pools. From a consideration of mass balances, any increase in NH4+-N, due to mineralization, must be equal the decrease in organic-N from the two organic matter pools. The model also recognises that, if no ammonium is available for immobilization, then nitrate can be used.
During all simulations reported here we chose typical values for most of the parameters: the rate constants were KL=0.015 d-1 and KH=0.00005 d-1; constant values were used for the efficiency of carbon turn-over, fE=0.4, the humification fraction, fH=0.2, and the C/N ratio of the soil biomass, rO=10.0, as suggested by Johnnson et al. (1987).
Mass-balance equations for fertilizer
The Fertilizer transport model allows for an input of mineral nitrogen in the form of either Urea, Ammonium or Nitrate. This option allows us to simulate different forms of mineral fertilizer that are broadcast onto the soil surface. Here we are considering just the application of CaNH4(NO3)2 and so the amount of urea added is set equal to zero.
Once the ammonium is applied to the soil surface, its fate is determined by six competing processes:
• inputs from the mineralization of soil biomass
• retardation due to the adsorption of ammonium to the soil particles
• losses due to the volatilization of ammonia gas
• losses due to the nitrification of ammonium into nitrate
• losses due to the drainage of ammonium below the root zone
• losses due to plant uptake
Similarly, once the nitrate is applied to the soil surface its fate is determined by the two inputs and the following five processes:
• Inputs of Nitrate from fertilizer application,
• Inputs from the nitrification of ammonium
• Retardation due to the adsorption of nitrate (= 0 in Renwick stoney silt loam)
• Losses due to immobilization
• Losses from denitrification,
• Losses due to plant uptake
• Losses due to the drainage of nitrogen beyond the root zone.
We consider denitrification to be a microbial process that is rate-limited by the amount of soil organic carbon (the energy source) and mineral nitrogen (the nutrient source) available to the microbes.
Mass-balance equations for pesticide
The pesticide transport model allows for a wide range of pesticides to be applied onto the soil. Here, we are concerned only with simazine and we characterize its transport properties via a KOC and a T1/2 value. The KOC value reflects the mobility of a pesticide, and it is a measure of the affinity of the pesticide to bind to the organic matter in the soil. The half-life, T1/2, reflects the persistence of a given pesticide, and it is a measure of the time it takes for half of the pesticide to be degraded by the soil microbes.
Once the pesticide is applied to the soil surface, its fate is determined by five competing processes
• Retardation due to pesticide binding to the soil organic matter
• Degradation due to microbial activity
• Losses due to volatilization to the atmosphere
• Losses due to uptake by plants (assumed =0)
• Losses due to deep drainage
Model calculation procedure
The above mass-balance equations are solved numerically, to generate the depthwise distribution of the average concentration of dissolved ammonium, nitrate and pesticide within the soil profile. Each of the rate constants that describe the various transformation processes are based on laboratory-measured reference values using standard functions FW and FT to account for the effects of soil moisture and temperature (Johnsson et al., 1987). We have used a standard modelling approach that we consider to be appropriate for this report because (1) it has a sound theoretical basis, (2) it has proven successful in other simulations, (3) it uses local weather and soil data as input, and (4) the results are expressed in term of a risk analysis for a given input of fertilizer and/or pesticide.
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