Model description - Data sources and management
A challenge faced by any LCA modelling framework is its data-intensive nature. The availability and quality of data varies considerably within and between key parameters. GLEAM data collection involves a combination of research, direct communication with experts and access to public and commercially available LCA inventory packages.
Data resolution and disaggregation
All data needed for GLEAM can be classified into basic input and intermediate input. Basic input data is defined as primary data such as animal numbers and distribution, herd parameters, mineral fertilizer application rates, crop yields, etc. and are taken from literature, databases, surveys and expert consultation. Intermediate data are an output of GLEAM calculations which, in turn, are the basis for further calculations.
Intermediate data include animal growth rates, feed rations, animal energy requirements, etc. Due to its GIS nature, GLEAM requires many of the input data to be as geographically accurate as possible. The model uses a variety of spatially explicit data, of which none has a resolution coarser than 5 arc minutes (ca. 10 km x 10 km at the equator). This is the same resolution at which GLEAM generates the outputs.
Livestock distribution maps
One of the major inputs of GLEAM is the geographic distribution of animal populations. Total animal numbers at national level are reported in FAOSTAT. The spatial distribution of animal numbers and the production systems used in GLEAM are based on the Gridded Livestock of the World. They rely on disaggregation of (sub) national numbers using a number of natural and socio-economic predictors, such as land cover, length of growing period, human population or irrigation. Additional disaggregation is calculated in GLEAM for beef feedlots and industrial chickens into broilers and layers.
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Herd and flock parameters
The key biological parameters needed for herd and flock parameters incorporate data on fertility, growth rate and replacement rate. Specific values for the different cohorts, production systems and dairy and non-dairy herds, in the case of ruminants, were established through extensive literature research, expert consultation and the conduction of surveys.
Feed rations
Feed rations differ between animals in different cohorts, production systems and regions. Diet composition is the result of the combination of intermediate calculations in GLEAM, literature search, surveys and expert knowledge. The key parameters identified for all feed materials are the dry-matter yield per hectare, net energy content and nitrogen content. While the dry-matter content determines the potential feed availability for a given region, the energy and nitrogen content define the feed conversion efficiency, eventual GHG emissions and nutrient use.
For GLEAM 3.0, emission factors from feed production, processing and transportation are elaborated from the LEAP database of GHG emissions related to feed crops or based on several literature reports and largely depend on the quantity and type of fuel used. Specific regional emission factors are used in all regions for emissions from nitrogenous, phosphorus and potassium fertilizers used for feed production. Similarly, an emission factor of 25.53 kg CO2-eq is used per kg of pesticide applied.
Manure management
There are significant differences on environmental impacts depending on manure management systems (MMS). Data is required on the presence and extension of each particular MMS within a geographic region. Climatic information is also relevant, as they shape to a great extent the nature and magnitude of manure management related impacts. GLEAM relies on the adaptation of existing national inventories reports of MMS, expert knowledge and literature reviews. The types of MMS used in GLEAM are the ones defined by the IPCC (2006) guidelines.