Model description

APSIM is an agricultural production system simulator developed and used for improving risk management under variable climate (Keating et al., 2003; Holzworth et al., 2014). A configuration of APSIM model (version 7.5) was used on a daily time step, which included the Maize crop module (APSIM-Maize), and the APSIM soil water module (SoilWater), soil Nitrogen module (SOILN) and SurfaceOrganic Matter module (SurfaceOM). A description of all the APSIM modules can be found at www. apsim.info (including references and source code). A brief overview of the modules is provided herein.

APSIM-Maize

The MAIZE module simulates maize development, growth, yield, and N accumulation on a daily-time step in response to daily weather (temperature, rainfall and solar radiation), soil water, soil N, and crop and soil management. Phenology is simulated using a photo- thermal-time approach (Jones and Kiniry, 1986), which assumes that the development rate increases as a multi-linear function of thermal time for the 0 to 44o_{C temperature range} with an optimal temperature for development of 34oC. The phenology routine calculates 11 growth stages and nine phases (time between stages). Each day, the phenology routine calculates the accumulated thermal time accumulation (in degree days; o_{Cd) from eight 3-} hour estimates, third-order polynomial interpolations between the minimum and maximum daily temperatures (Kumudini et al., 2014), except the duration between sowing and germination, which is influenced only by plant available soil moisture. Accumulated thermal time is used to determine the duration of each phase. Between emergence and silking, daily thermal time accumulation is reduced by water and/or N stresses to result in delayed phenology when the plant is under stress. Maize is assumed to be insensitive to photoperiod until the end of the juvenile stage. Between the end of the juvenile phase and floral initiation, the development rate can be sensitive to photoperiod depending on the

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cultivar. This is followed by an inductive phase (photoperiod sensitive), which is terminated by tassel initiation. Because maize is a short-day plant, tassel initiation is delayed if day length exceeds 12.5 hours (Jones and Kiniry, 1986).

Biomass accumulation is simulated as the minimum daily growth limited by either radiation (potential growth) or by crop water supply (water-limited growth). The calculation of the balance between demand for and supply of soil water is used to

determine whether the environment is energy-limited (defined by radiation intercepted and radiation use efficiency) or water-limited (defined by transpiration and transpiration efficiency adjusted for vapour pressure deficit) (Monteith, 1986). The partitioning of dry matter to different plant organs depends on the growth stage. From emergence to silking, dry matter is allocated to leaves and the stem. From silking to physiological maturity, the growing grain is the largest sink for dry matter. Dry matter allocation in the grain is calculated as the product of grain number and maximum grain growth. The grain number per plant is determined by the average daily growth rate per plant between floral initiation and the start of grain filling, while grain size is determined by the grain growth rate, the effective grain-filling period, and the redistribution of assimilates post-anthesis. Crop N demand is driven by growth-stage dependent critical N concentration limits for different organs, which the simulated crop attempts to maintain. N is re-translocated to the grain from other plant parts and demand is driven by the critical N content but this demand is lowered if the plant is under N stress. Soil N supply is via mass flow and if crop N demand cannot be satisfied by mass flow to the roots, it is supplied by diffusive flow.

SoilWater

The soil water dynamics are simulated in the SoilWater module, which uses a multi-layer, cascading water balance (Jones and Kiniry, 1986). Processes include runoff, evaporation, and both saturated and unsaturated flow between conceptual soil layers. Inputs to

SOILWAT include bulk density, drained upper limit (DUL), crop lower limit (CLL), and saturated (SAT) soil water contents. Saturated flow occurs as a fraction of the amount of water greater than DUL. The fraction that drains in one day is specified by the coefficient SWCON, which takes into account soil texture differences (Jones and Kiniry, 1986; Ritchie et al., 1986). SWCON values of less than 0.5 d-1 are typical for heavy, poorly draining clay soils, and values greater than 0.8 d-1 are typical for coarsely textured soils

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that have high water conductivity (Probert et al., 1998). When the soil water content drops below the DUL, water movement depends on the gradients between adjacent soil layers and diffusivity, which is a function of the average water content between the two layers and the diffusivity coefficients (diffus_const and diffuse_slope). The bare soil run off curve (cn2_bare) specifies the proportion of rainfall that infiltrates and the proportion that is lost through surface runoff. Runoff from rainfall is computed using the USDA curve number approach (USDA, 1972). Potential evapotranspiration (Priestley and Taylor, 1972) is calculated using an equilibrium evaporation concept. Soil evaporation is calculated via two parameters U and CONA, which determine the first and second stages of soil evaporation (Ritchie, 1972). For the first stage, the soil is sufficiently wet and the soil evaporation is energy-limited and occurs at a rate equal to the potential evaporation rate. The second stage starts when the cumulative soil evaporation exceeds the upper limit of the first stage, where the soil starts to dry and water from within the soil starts to evaporate. Crop specific parameters (crop-soil factors) determine the rate of root extension (parameter XF, 0–1 multiplier on the rate of root growth) and the maximum rate at which a crop can extract water from a particular soil layer (KL, day-1). The user can specify XF for each soil layer (0: no root growth; 1: root growth at potential) to simulate barriers that can impede root growth through a particular layer (e.g., low pH and soil compaction).

SoilN and surfaceOM

The SoilN module describes the dynamics of carbon (C) and N for a layered soil profile. These layers are defined by the model user, and are typically the same as for the soil water simulations. Processes include soil organic matter decomposition, N immobilisation and mineralisation, and nitrification and denitrification. The input parameters for SoilN include pH, organic carbon (OC), finert (inert C fraction) and fbiom (microbial biomass fraction). SoilN treats soil organic matter as three pools, a fast decomposing microbial biomass (BIOM), intermediate (HUM), and a recalcitrant pool (INERT). The fresh organic matter (FOM) consists of the roots from the previous crop and any crop residue. The surfaceOM module describes the fate of surface residues and considers the above-ground crop residues that can be removed from the system, incorporated into the soil by tillage, and/or left on the surface. Residues incorporated into the soil and decomposing roots first enter the FOM pool, where they are transformed into either the rapid turnover microbial biomass (BIOM) pool or the slower turnover, less available humic (HUM) pool.

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