The water productivity model Global-WP calculates crop water productivity. Global-WP is based on remote sensing derived input data sets and can be applied at local to global scales. The model is a combination of Monteith's theoretical framework for dry matter production in plants and an energy balance model to assess actual evapotranspiration. PhD research of Sander Zwart showed that combining both approaches results in omission of two parameters which are usually difficult to estimate: the evaporative fraction and the atmospheric transmissivity. The result is that water productivity can be assessed from four spatial variables: the albedo, the vegetation index, the extraterrestrial radiation, and air temperature.
For more information, please read the PhD thesis of Sander Zwart. Sander Zwart successfully defended his PhD thesis on Benchmarking water productivity in agriculture and the scope for improvement. Sander Zwart was a WaterWatch colleague for eight years, during which he combined his activities at WaterWatch in water productivity with his PhD research. Sander used the Global-WP (WatPro) model to assess water productivity of wheat at a global scale.
GLOBAL-WP has undergone some significant adjustments and improvements, as compared to its predecessor WatPro. Plant growth and water use are driven by environmental conditions that can or cannot be controlled by farmers. In order to analyze the effects of climate on water productivity, an important new feature is introduced in GLOBAL-WP: The model has the ability to correct for climatic conditons, by means of normalizing the water productivity results for accumulated reference evapotranspiration during the growing season.
Country average statistics derived from GLOBAL-WP for rice producing countries (not corrected and corrected for climatic conditions)
Other GLOBAL-WP improvements include taking into account the stress on crop production yielded by temperature (both high and low), as well as better reflecting the crop stomatal responses to ambient conditions by introducing a stress factor related to vapour pressure deficit occuring during the growing season. The expression for longwave radiation in the GLOBAL-WP formulation has also been altered and is made temperature dependent. The yield of a crop can be estimated from the accumulated biomass production during the growing season multiplied with a harvest index. GLOBAL-WP incorporates a spatially variable, crop specific harvest index.
The adjustments and improvements in the GLOBAL-WP formulation allow the model to be used for a variety of staple crops, delivering an innovative insight in the variability of water productivity at a very high spatial resolution, on a global scale.