There are four different environmental processes that are modelled within the fields of fuel game. These are: soil fertility, beneficial bugs (biodiversity), carbon emissions, and water quality. Each of these processes are modeled for each individual field within the game, and some are also aggregated at the farm or global level. A description of each of the calculation is explained below.

 

Soil Fertility

Soil fertility is a measure of the total amount of organic nitrogen present (in kg per acre). The nitrogen cycle model describes this process along with the calculation of this value in detail. The value reported on the individual fields ("Soil Fert (N)") reflects the total amount of organic nitrogen present in both dead and living tissues. This is distinguished from the farm level measure because it includes living tissues (particularly roots) which persist from one year to the next in perennial plants such as switchgrass. The root nitrogen will be pooled with the rest of the organic nitrogen at the next change of cropping choice on the field. This is intended to reflect the total buildup of organic nitrogen over time and provide a measure which is more indicative of the long-term health of the field.

The value reported at the farm level ("soil health" in the right hand graph) only reflects the amount of organic nitrogen present on the fields that is ready for mineralization (ie., not that bound up in living roots). This is meant to reflect the pool of potential nutrients that are immediately available in the calculation of inorganic nitrogen (and subsequently, the yield) of the next crop. On the farm level, this value is reported as a value between zero and one, to reflect how close the player is to the maximum reasonable fertility levels possible (2850 kg/acre).

 

Beneficial Bugs

An index describing the relative suitability of the landscape for insects that provide natural pest suppression can be calculated for each location by examining the habitat value of the surrounding landscape within 1.5km. To start, each of the eight surrounding neighbors are assigned a habitat score based on their crop cover: 1.0 for grass, 0.5 for alfalfa, or 0.0 for corn. The average habitat score of these eight neighbors (H_N) is calculated. Finally, they are combined with the habitat score of the current location (H_L to calculate the biocontrol index:

BCI_L = 0.25 + 0.19 H_L + 0.62 H_N

This model is adapted from Meehan et al. (2012). Note: this BCI calculation as described in the source is bounded at (0.25, 1.06); as a result, the resulting BCI value should be adjusted (as not to exceed unity) as:

BCI_score_L = (0.25 + 0.19 H_L + 0.62 H_N) / 1.06

At the farm level, this value is averaged across all of the players fields and normalized using the same bounds as at the field level. This value is reported as "beneficial bugs" in the "Overall" tab of the environment graphs.

 

Carbon Emissions

Emissions produced directly by farming activities are calculated for each field, and are broken down into two categories—those produced during the agricultural production phase (ag phase), and those produced during the conversion of the feedstock to ethanol (refinery phase). This provides a better sense to players of where in the production line the emissions are coming from and how their decisions affect these sources.

 

Ag Phase

The ag phase emissions are comprised of emissions from two sources; farming practices, and the carbon cycling involved in plant growth. Emissions from farming practices are produced by five different actions: tilling, planting, fertilizing, pesticide application, and harvesting. These are only applied when appropriate (i.e. there are no tilling emissions if the player does not till). Planting and harvest phases are applied each year for corn, while they may not be necessary each year for switchgrass or alfalfa. It is assumed that pesticides are applied equally across all options.

Total emissions for the farming practices can then be approximated using:

CO₂ eq(kg/ac) = 16.4 ⋅ P_plant + 37.05 ⋅ P_harvest + 11.3 ⋅ (kg of fert) + 10.16 ⋅ P_Pesticide + 289.7 ⋅ P_tillage

where P is a binary value representing whether or not the particular phase is applied for that year.

Emission rates for fertilizers and pesticides are calculated on a per-unit basis using values provided by the GREET model for production, application, transportation, and breakdown. Emission values for planting and harvest were not available; instead emissions were calculated as proportional to the energy usage as approximated in the energy model for corn. The energy model suggests that results suggest that 4.25% of the energy goes to planting, 9.60% to harvesting, and 75.06% on tillage. Emissions were then by applying these proportions to the total ag phase emission levels (5316 MJ/ha based on 'average' yield approximations) reported in the EBAMM model (16.4 kg CO₂ eq /ac for planting, 37.05 kg for harvest, and 289.7 kg for tillage).

 

SEQUESTRATION

 

Refinery Phase

To calculate emissions produced during the refinery phase, we assume that there is a single flexible plant that can accommodate any feedstock. emissions during this phase are derived from the process of refining the crop into ethanol. These emissions can be calculated as

CO₂ eq(kg/Mg feedstock) = 0.047 ⋅ F_cellulose + 0.41957 ⋅ F_cornGrain

Emission estimates (in kg CO₂ / litre of fuel) are obtained from the EBAMM model, and converted assuming conversion rates of 0.38 l/kg for cellulose and 0.4 l/kg for corn grain.

At the farm level, emissions are presented in two ways. The average emission level per field is used to calculate a normalized score from zero to one, with zero being the minimum (-1912 kg ₂ per field where carbon is sequestered) and a maximum level of emissions (9320 kg CO₂ per field). This value is reported in the "Overall" tab of the environment graph and used to calculate the overall environmental score. Total farm emissions in kg of ₂ (summed across all the fields on the farm) are presented on the "Emissions" tab, along with the average emissions for all of the farms.

 

Water Quality

Water quality is calculated as a function of the amount of inorganic nitrogen that is not taken up by the crops but is instead leached into the ground water. The calculation for this is detailed on the nitrogen cycle model page. This value is normalized between zero and the maximum value of 110.075 kg N per acre, and presented as a percent on each of the fields. At the farm level, the values for all fields are averaged, normalized, and presented in the "Overall" tab as a percentage. Additionally, the total amount of nitrogen leached from the farm is presented on the "Water" tab, along with the average total runoff for all farms.