A Closer Look at Drought: Why it matters how we measure

How we measure a phenomena, in this case drought, is as important as what we choose to measure. Sensible methods of measurement form the foundation of any critical analysis.

A recent paper by Sheffield et al. (2012) focuses on this topic, in particular with applications to the Palmer Drought Severity Index (PDSI), a common index used to monitor the onset and progression of meteorological drought conditions. Sheffield et al assert that because the index uses a rudimentary method for estimating evaporation it has an overly pessimistic view of what drought has been in recent decades. Sheffield argues that globally drought has not increased in recent decades when a more physically based method of estimating evaporation is used.

Criticism of PDSI for its simplistic method of measuring evaporation is not new. In fact, Dai and others have researched this same issue but came to the conclusion that using a more physically based method yields results comparable to current estimates. There have been numerous potential explanations for the difference in conclusions, but that is a conversation for another post. For a critique of the Sheffield paper see here, with reactions from Kevin Trenberth and Aiguo Dai. A short summary of the paper may also be found here.

Regardless of the Sheffield paper, it is critical that we continue to evaluate how we measure drought, particularly as dynamics driving multi-season drought evolve. What was once an accurate index (or indicator) of drought for any given region will not necessarily always be so as the climate changes. Although the reasons for this are many, one is that indexes are pared down to be understandable and operationally useful. In the case of the PDSI, what some see as an extraneous level of complexity, others see as a necessity to accurately represent drought. Whether or not this is actually the case undoubtedly warrants further study, both in how the physical phenomena responsible for drought evolve in a changing climate and how those relationships are reflected (or absent) in various drought indices.

Of particular interest to me is how faithfully the PDSI reflects changes in the hydrological cycle as the phenomena driving observed drying trends change and as the accepted definition of normal is forced to evolve. Altered seasonal snow melt, shifts in large scale atmospheric circulation and increased atmospheric moisture carrying capacity will have undeniable consequences for the hydroclimate in many regions. How these changes are reflected in the PDSI and how sensitive it is to the complexity of its mathematical underpinnings has real world implications for agriculture and water managers, particularly those across the US who use the index as a means of triggering drought relief.