New evaporation model for water management

“Formulas from 1948 were being used.”

Monthly evaporation models, important for water management, can be improved by studying the dispersion of rain for each month. This is one of the conclusions in the research project of Marieke de Groen. She will defend her thesis on Monday the 29th of April. De Groen: “The subject was neglected for a long time. The monthly models contained formulas from 1948.”

Our main source of food – the agriculture sector – is very dependant on the weather. To determine whether it is wiser to build a dam, irrigate, or make better use of rainwater, monthly models are made. Evaporation plays an important part in these models. De Groen: “If you have 100 mm of rainfall in one month, it could al have fallen on one day, or maybe during 10 days. This is quite important for plants; after all, you don’t water your own plants only once a month.”

In her research, De Groen made a distinction between transpiration and interception. Transpiration is when water is absorbed by the plant through the roots and then evaporates through the leaves. Interception is when part of the rain-water doesn’t reach the ground, but lands on plant leaves and evaporates from there. “Transpiration causes plants to grow, interception doesn’t. That is why the distinction is important,” says De Groen. “This is also important in climate models. The classical models fall short on two points: they don’t take the dispersion of the rainfall over the month into account and they don’t deal with the difference between transpiration and interception.”

De Groen worked at the international hydraulics institute IHE in Delft, which does research in Zimbabwe. During her research period in Zimbabwe, she realised that the occurrence of ‘rain days’ takes place along the lines of the so-called Markov-theory. This means that the probability for rain on a certain day is only dependant on the amount of rain the day before. The theory helped De Groen to create simple monthly models for transpiration and interception. “The classical models simplistically assume that all the rain fell at the beginning of the month and they drastically simplify the relationship between the amount of water in the ground and the amount used by plants,” says De Groen, “I will show that you can make better models by using different assumptions, and they apply world-wide.”

The method developed by De Groen is especially valuable for water managers and hydrologists that depend on a limited amount of data in a limited time frame to be able to make strategic management decisions.