The Monte Carlo simulation was conducted for 300 iterations with a randomly imputed DSM created for each realisation. The simulation took 16 h on a desktop with an Intel i7-3930 CPU for the 3657 by 7138 cell DSM. The mean grid layer for the contributing upstream area of each grid cell was calculated and the natural logarithm applied to effectively visualise the differences between high and low values. Fig. 4 shows a map of the contributing upstream area weighted by the snow area to simulate the effect of snowmelt. The map also shows the moss quadrats coloured by moss health values. The spatial distribution of the contributing upstream area seems to be a good proxy for water availability from snowmelt. There appears to be a strong relationship between moss health as measured at the quadrat locations and the contributing upstream area as evidenced by the quadrats dominated by healthy moss growing in or near melt streams. The western side of the study area is much drier than the eastern side due to the lack of snow cover in the higher parts of the catchment. It is therefore important to use the snow layer to increase the weight of the upstream area on the eastern side to simulate the increased water availability from snowmelt.