Appendix 2.6.2. Description of the HillPlan model
HillPlan is a modelling tool that has been developed to forecast, amongst other things, the changes in vegetation in upland United Kingdom under a range of livestock management strategies. The structure of HillPlan is shown in Figure 1.
Figure A2.6.2.1. The structure of HillPlan
The tool includes a model with a number of linked sub-models (Figure 1) that represent the relevant processes. The seasonal pattern of vegetation production is modelled for grass, based on Hutchings and Gordon (2001), using input data on weather, soils and fertiliser levels to predict grass production with a daily time step. An empirical model of heather ( Calluna vulgaris) production is based on Palmer (1997) and Read et al. (2002) using weather, soil and defoliation data as inputs. Thirteen different vegetation types are represented. Animal foraging behaviour is based on a modification of the ideal free distribution (following Armstrong et al.1997), in which instantaneous rates of intake are estimated, for each vegetation type, from bite mechanics and the nutritive value of the vegetation selected and then animals are distributed in the landscape as a function of the relative estimated intake rates. Long-term changes in the proportion of plant species are predicted based on Birch (1999) and Birch et al. (2000) using relative growth rates of different species.
These sub-models are implemented in a framework within which the user specifies the areas of different land management units (or fields). Each land management unit comprises a series of vegetation patches containing different vegetation types. The areas of patches and types must be specified. The numbers and species of domestic livestock (sheep and cattle) and their basic characteristics (large, medium or small body size) and the dates during which they occupy each land management unit are also required as input data. For this project each region was regarded as one land management unit.
The main output is the change in area of different vegetation types over time, although a large number of intermediate and explanatory variables can be output, of which some of the more important are animal intake, utilisation rates of different vegetation types and grass height.
The model can be run for a number of years. Normally the model is run for a longer period of time than that which is of immediate interest to the user. This is because the outputs should not be regarded as precise predictions of the exact timescale of changes, but rather as indicative of the likely trends. By selecting a longer run that that which is actually required this ensures that the user does not miss any changes occurring immediately beyond the period of interest. HillPlan Version 1.4.0.435 was used for this study.