Farmer aging and succession

MtM ABM description

Farmer aging

Farmers age by one year per model tick. Hence the model time scale is 1 year per tick, but this is subject to change. A 5 year model time step may be considered more appropriate.

After aging, a farmer is tested for exiting the industry (or retiring or dying, or however we want to think about it). They exit with a probability given by p_{\mathrm{exit}} = \left\{ \begin{array}{cl} \frac{1}{91-\mathrm{age}} & \text{if age} < 91 \\ 1 & \text{if age} \geq 91 \end{array} \right. This means that farmers are not guaranteed to exit until they are 90, reflecting the rather surprising number of old farmers in survey data.

Age of farmers at model initialisation

The age of farmers at model initialisation is drawn from a :beta-binomial distribution, with n=70, \alpha=5, and \beta=4, and ages then offset by adding 20 (so that 20 is the youngest possible age). An example distribution for 250 farmers is shown below, with the red line showing the associated probability density function.

This distribution has been roughly matched to survey evidence of farmer ages (it has not been fitted, just matched approximately by eye).

Age of new farmers

Incoming farmer ages are also drawn from a beta-binomial distribution with n=45, \alpha=4, \alpha=2.

This distribution might appear to skew older than the model age initialisation, but note that the range of values tops out at 65 years as shown in the direct comparison of the probability density functions below.

In any case, this paramterisation appears to keep the farmer age distribution fairly stable over time.

More detailed fitting and analysis of these (hard-coded) parameter settings around farmer ages is probably desirable.

Entrant farmers and landuse change

A new farmer will consider whole-farm landuse change (see Farmer decisions about land use for details) if any of the farm’s holdings is losing money. This pathway to land use is more likely to lead to land use change at the whole farm level than the more piecemeal changes that tend to be made by incumbent farmers.

Update History

Date	Changes
2025-02-19	Initial post.