Slide 7: Water balance modelling
Water is key to savanna ecosystem modelling, because they are water limited rather than temperature or light limited. Even when they are nutrient limited, the nutrient availability is controlled by when the soil is moist. Here is a very simple way of modelling water balance, using only the information easily available from climate change simulations: monthly rainfall, maximum temperature and minimum temperature. It ends up as an index of the number of days in the year for which plant growth is possible ('G days')
Basically, the water balance is the sum for each month of the average daily amount of precipitation, divided by the daily evaporation, multiplied by the number of days in the month. This assumes that the rainfall is greater than the evaporation; if this is not the case, then the rain over evaporation term is given to be 1. The E0, or open water evaporation is calculated using the second equation. We needn't go into the details of this term, but it is worth noting that it incorporates air temperature (the monthly mean, minimum and maximum) as well as the altitude and latitude of the cell being modelled. This equation was developed by Linacre.