The KNMI has updated the calculation of the precipitation deficit. This measure of drought shows how much more water evaporates than falls as precipitation. Until now, the deficit was calculated between April 1 and September 30, but from 2026 onwards, the KNMI will also monitor the precipitation deficit year-round: from January 1 to December 31. This makes drought more visible earlier in the year and aligns monitoring better with the changing climate.
Year-Round Precipitation Deficit Monitoring
KNMI monitors potential drought, among other things, using the precipitation deficit. Until now, this precipitation deficit in the Netherlands was calculated for the growing season, the period from April 1 to September 30, averaged over 13 stations. However, KNMI research shows that meteorological drought can also occur before April 1 and that the chance of this is increasing due to climate change. For this reason, monitoring is now expanded: the precipitation deficit will also be tracked year-round, from January 1 to December 31. This change better accounts for drought early in the year and aligns with broader recommendations for year-round drought monitoring.
What Does This Mean for Users?
The current monitoring from April 1 is now extended with the calculation from January 1. This keeps the basis for users the same for now, allowing a smooth transition. The interpretation remains the same: a higher precipitation deficit indicates more severe drought. The new drought monitor provides a more complete picture of drought formation throughout the year, enabling earlier warnings. In many years, the change will make no difference when the precipitation deficit after April 1 is around zero. For years with a dry start to spring, such as the dry March in 2025 (see figure), this extension leads to higher precipitation deficit values in summer.
In the background, KNMI is working on the KNMI Data Explorer. Here, daily measurements, long-term averages, and monitoring of various variables will be combined. The KNMI drought monitoring, including the precipitation deficit and via SPI and SPEI indicators, will eventually be integrated here as well.
Expansion of National Drought Monitoring
The adjustment to the precipitation deficit aligns with advice from the Expertise Network Freshwater and Drought, titled Beyond the Seasons: Year-Round Prepared for Drought. This advice advocates broadening and improving drought indicator monitoring and supports KNMIs choice to monitor the precipitation deficit year-round. Continuous drought monitoring is relevant because, from a hydrological perspective, drought results from conditions over the year or multiple years. Weather conditions outside the growing season are also relevant. For example, the winter of 2018-2019: after the very dry summer of 2018, groundwater levels in many parts of the Netherlands were not sufficiently replenished in winter. As a result, the 2019 growing season started with lower than normal groundwater, which amplified drought effects that year.
KNMI collaborates with various partners for broad and accurate drought monitoring. Besides the KNMI observation network, where precipitation, temperature, and solar radiation are measured, they also monitor soil moisture, groundwater, and river discharges. All in all, drought monitoring helps water managers and users to quickly and effectively take measures in case of impending drought.