To explore the links between climatic warming and rainfall in drylands, scientists from the Universities of Cardiff and Bristol analysed more than 50 years of detailed rainfall data (measured every minute) from a semi-arid drainage basin in south east Arizona exhibiting an upward trend in temperatures during that period. The analysis demonstrated a decline in rainfall intensity, despite an increase in total rainfall over the years. Interestingly, the study shows that there is a long-term decline in heavy rainfall events (greater than 25 mm/h) and an associated increase in the number of smaller storms each delivering less rainfall. This result is contrary to commonly held assumptions about rainfall trends under climate change.

Lead author, Dr Michael Singer from School of Earth and Ocean Sciences at Cardiff University, said: “In drylands, convective (or short, intense) rainfall controls water supply, flood risk and soil moisture but we have had little information on how atmospheric warming will affect the characteristics of such rainstorms, given the limited moisture in these areas.” Co-author, Dr Katerina Michaelides, from the School of Geographical Sciences and Cabot Institute at the University of Bristol, said: “Our findings are consistent with previous research in the Colorado Basin which has revealed a decline in runoff in the upper part of the Basin. “Our work demonstrates that there is a more regional decline in water resources in this dryland region, which may be found in other dryland regions of the world.”

Since trends in convective rainfall are not easily detected in daily rainfall records, or well-simulated by global or regional climate models, the researchers created a new tool to assess the effects of climate change on rainfall patterns and trends in dryland areas. Their new model, STORM, simulates individual rainstorms and their expression over a river basin, and it can represent different classes of climate change over many decades. Drs Singer and Michaelides employ STORM to show that the historical rainfall trends likely resulted in less runoff from this dryland basin, an effect they expect to have occurred at many similar basins in the region. Dr Singer added: “We see this model as a useful tool to simulate climate change in regions and cases where traditional models and methods don’t capture the trends.”

Paper: ‘Deciphering the expression of climate change within the Lower Colorado River basin by stochastic simulation of convective rainfall’ by M. Bliss Singer and K. Michaelides in Environmental Research Letters.”

In May 2015 extensive flooding occurred in Texas and Oklahoma due to heavy rainfall. This was not caused by a hurricane, but rather by an El Nino, which indeed leads to extreme rainfall events, according to a study by Wang et al. 2015.

In another study by Wang et al. 2014, researchers found a flooding pattern in the region of the Missouri River which happened to follow a Pacific Ocean cycle. Knowledge of this link now allows better forecasts to be made and preventive efforts to be taken. Washington University in St. Louis reminded that not everything can be attributed in knee-jerk fashion to climate change. Extreme rain at the end of 2015 in Missouri led to terrible flooding. Part of the blame here was assigned to the changes in building in flood-prone areas of the river which led to an obstruction of the outflow.

In another study the University of Colorado in Denver was able to show that also the state of Colorado is poorly prepared for flooding. Important bridges and infrastructure urgently need to be upgraded. Damage that occurred in a flood in 2013 would have been much less had the structures been strengthened and better taken care of.