The period in which we monitor drought starts today. The KNMI keeps track of the precipitation deficit every year from April 1 to September 30. In recent months, significantly less precipitation has fallen than average, with March being record dry.
The precipitation deficit is the difference between the amount of moisture that evaporates and the amount of precipitation that falls. During the meteorological dry season (April to September), evaporation is usually greater than precipitation. This leads to soil drying out and a precipitation deficit. This is normal during this time of year. There are exceptions, such as last year, when the amount of precipitation was greater than evaporation.
Dry Start
This year, it is noticeable that in the months of February and March, significantly less precipitation has fallen than the long-term average. In March, an average of about 6 millimeters of precipitation fell across the country, compared to the normal 53 millimeters, a new record for the month of March. If we had started tracking the precipitation deficit on January 1 of this year, it would now (up to March 30) stand at 46 millimeters.
Evaporation Increases Due to More Sun and Higher Temperatures
In recent decades, evaporation has increased. This is due to more solar radiation and higher temperatures. However, the amount of precipitation has remained approximately the same. As a result, the difference between evaporation and precipitation is becoming increasingly larger, and the soil is drying out more during the growing season. Since the beginning of this century, both evaporation and the difference between evaporation and precipitation have increased by about 60 millimeters. The relative increases are 13 percent and 130 percent, respectively. Proportionally, a small increase in evaporation leads to a large increase in drought.
Precipitation Forecast for the Coming Weeks
For the next two weeks, about 6 millimeters of precipitation is expected. On average over 1991-2020, more than 20 millimeters falls in the first half of April, so the month starts much drier than average.
Monitoring Drought Throughout the Year
There are many ways to monitor drought. Each index has its pros and cons. One disadvantage of the precipitation deficit is that it is only meaningful in the summer half-year; in winter, almost no moisture evaporates. Therefore, we use, like many other countries worldwide, the Standardized Precipitation Index (SPI) and the Standardized Precipitation-Evapotranspiration Index (SPEI). The SPI and SPEI provide information about both dry and wet conditions and are valid throughout the year. The SPI compares the amount of rain that has fallen with what is normally expected according to climatology. The SPEI compares in the same way the climatology of the difference between rain and potential evaporation. For drought monitoring, there is a disadvantage: the SPI does not take evaporation into account. The SPI thus serves as a complement to the precipitation deficit.