## 2 Methodology
### Model Parameterization and Calibration  
The SAVANNA model was parameterized to describe the response of changes of different plant and livestock groups. Weather data from 1980 to 2010 were used as the base scenario (Department of Meteorological Services, Zimbabwe) The model parameters used for this study are based on previous SAVANNA research [18; 20; 21; 22; 30] a goat production report by [4] and initial population data collected from Veterinary department in Nkayi [31]. Plants were placed into six functional groups based on previous modeling research [14; 18; 22; 23] to simplify parameterization and to provide computational efficiency when executing simulations. The plant functional groups were based on functional groups described by [32] and classified as follows: Grass (C\({}_{4}\) photosynthetic pathway), shrubs (C\({}_{3}\) photosynthetic pathway), fine-leaved palatable trees, broad-leaved palatable trees, broad-leaved unpalatable trees and _Colophosphamidem propane._ In addition, the grass layer was parameterized with the C\({}_{4}\) photosynthetic pathway while all woody shrubs and woody trees were parameterized as C\({}_{3}\) photosynthetic pathway species [33]. Livestock groups were also placed into functional groups for similar parameterization and computational efficiency. Herbivore functional groups consisted of cattle sheep, goats, and donkeys. Parameters for herbivore groups were provided by [18] and [34]. As input maps/grids, SAVANNA uses geographic layers (ESRI/ArcMap grids) describing elevation (United States Geological Survey [35] slope, aspect, vegetation, and land use [36], soil class [37], and distance to water to simulate the growth of plants and distribution of livestock. Each grid cell was divided horizontally into one of three vegetation types and vertically into layers of vegetation facets (grass-dominated, shrub-dominated, and tree-dominated) and soils (deep Kalahari sands, Brown loamy sands and Grayish Brown sands). As GIS grids are used as input into SAVANNA, the model outputs both time series files and GIS grids as well. In this study, a 2 km \(\times\) 2 km grid cell size was used to model the rates of plant production, response of animals to forage availability and ecosystem function under varying climatic conditions. The 2 km grid resolution was selected to provide computational efficiency yet enough detail to simulate the varying vegetation and livestock areas over the entire communal areas of the Nkayi district. The soil input grid was creates using the three soil classes described by [37] (Figure 3) with soil-layer moisture and carbon characteristics derived from [38] and [39]. Figure 2: Schematic representation of the SAVANNA model [17]. The vegetation and land use maps were derived from previous land degradation studies [36] and converted to 2 km grid resolution (Figure 4) along with estimates of woody cover and shrub/grass biomass for three primary vegetation types (forest, tilled/fallow field, and degraded). Initial grass biomass amounts were set low to reflect low forage production [36] with 25 g/m\({}^{2}\) for fields, 35 g/m\({}^{2}\) for conservation forest areas and 10 g/m\({}^{2}\) for degraded areas. Woody tree cover in forest vegetation areas was initialized at 70% (20% fine-leafed palatable, 20% broad-leafed palatable, 20% fine-leafed unpalatable and 10% propane). Woody tree cover in degraded vegetation areas was initialized at 40% (10% fine-leafed palatable, 10% broad-leafed palatable, 10% fine-leafed unpalatable and 10% propane). Woody tree cover in field areas was initialized at minimal levels (<5%) to represent some presence around human dwellings. Woody shrub biomass levels were initialized at 30 g/m\({}^{2}\) and 0.9 m in height for forest and degraded vegetation types and 5 g/m\({}^{2}\) and 0.1m for field areas to represent fallow zones and field borders. Figure 4: Nkayi District land use map converted to 2 km resolution grid used as inputs to the SAVANNA model.