On October 25th, a few months after his retirement, Joop Kroes received his PhD degree on his research into upward groundwater flows, an important factor in calculating the yield of food crops. Joop was one of the most important founders of the worldwide used SWAP-WOFOST model instruments. This is one of the reasons Wageningen is now leading internationally in agro-hydrological research.
"In my thesis I show that a physical basis for simulations of groundwater flow to the root zone and crops contributes to improved estimates of crop yields and groundwater replenishment" says Joop Kroes. "For good soil and water management it often appears necessary to apply complex modeling. This provides valuable insights into the processes that underlie the interaction between soil hydrology and crop growth".
In his dissertation, Kroes distinguishes two forms of vertical upward groundwater flow: 1) capillary rise from the groundwater and 2) recirculating percolation water that is created by pressure differences, even in the absence of groundwater. Joop Kroes: "A neglect of these upward groundwater flows can give deviations in the order of 3 to 26 percent in the calculated yields of the most common crops in the Netherlands. The upward groundwater flow is also important for the replenishment of the groundwater".
For his model calculations Kroes used a combination of the hydrological model SWAP and the crop growth model WOFOST, Wageningen models with a history of more than 40 years. "Joop recognized the importance of integrating soil hydrology, nutrient management and crop growth," says Jos van Dam, who has worked closely with him. "Thanks to Joop, the models are available as open source for researchers, students and practitioners".
The dissertation also provides an analysis of the changes in groundwater, climate and land use in the Pampas of Argentina. In recent years there have been frequent flooding caused by sharply rising groundwater levels as a result of monotonous land use with low evaporation demand. A greater differentiation of land use can offer a solution here. "An additional added value is that Joop also paid specific attention in his dissertation to the effects of drought and salinisation on crop yields, problems that are increasingly occurring in many parts of the world" according to supervisor Coen Ritsema.
Another nice detail is that Joop also participated in the WaterWijzer for six years, which was coincidentally launched in the week of his promotion. He can now safely say goodbye to the ESG.