WATCH has made the first steps towards quantifying the effects of these changes at global and regional scales. Key to this is our understanding of evaporation.
WATCH has produced a new global data set of evaporation from land. This is on a 0.25 degree x 0.25 degree (~ 25km x 25km) grid, and has produced daily estimates for the period 1984 – 2007. This provides functionality beyond existing data sets in that it breaks down evaporation into separate components of transpiration through plant leaves, direct evaporation from the soil surface, forest rainfall interception, and snow sublimation.
This breakthrough has been made possible by the availability of high-quality satellite data, coupled with novel and innovative approaches taken by WATCH researchers.
The data set can be used itself for validating climate models, and it will help hydrological modellers to produce improved river discharge estimates at regional scales. And, data-sparse areas such as Africa will benefit greatly.
Analysis of the data set reveals how different variables affect evaporation at different times and in different places. It is, therefore, highly tempting to seek out trends, but this must be done with caution. Although the data go back to 1984 (when the input data from satellites first became available), we need at least another ten years of data before trend analysis becomes valid. However, the data do appear to support the suggestion that total global land evaporation has reduced over the last ten years.
Climate change is only one of the drivers that will affect the water cycle in the future. Human activities – particularly deforestation – are changing large areas of the globe. This change – generally to shorter vegetation of crops and pasture – affects evaporation. This, in turn, has the potential to change not only river flows, but also large-scale weather patterns. Engineering projects such as hydro-electric and irrigation schemes cannot be ignored, and the scale of these will only increase in response to a growing and an increasingly wealthy world population.