A Drought By Any Other Name

What is a drought? I know I don’t know — I live in the temperate northeastern United States and my field site is frequently wrapped in fog — but I get the feeling that I am not alone. According to a paper born from a Colorado State University graduate student seminar on ecology and drought, we should all be asking ourselves this question. Drought seems to have lost its meaning for ecologists, and not in the semantic satiation way, where if you say a word over and over again it become aural nonsense.

Ingrid J. Slette and her co-authors published ‘How ecologists define drought, and why we should do better’ in Global Change Biology this summer. As Slette tells it, “This project grew out of discussions during a grad student seminar course about ecology and drought. Everyone in the class approached drought from a different perspective, and when we looked to the literature to find a definition of drought that we could all agree on as a starting point for the class, we couldn’t find one.” The class decided they needed to take a step back, and they shifted from synthesizing the impacts of drought to simply defining it. This might seem like a trivial point of semantics, but as they write in their paper, “Failure to define or characterize drought conditions in the published literature challenges out ability to advance ecological understanding.” You can’t compare studies, or compile a meta-analysis without understanding the idiosyncratic environmental conditions hidden under the catch-all term ‘drought.’

Perhaps we should not be surprised that ecologists can’t agree on drought; as I discovered while reading Slette’s paper, meteorologists and climatologists also struggle to define drought. But, the sticking point is not that we don’t have a clear definition of drought, it’s that ecologists use the term ‘drought’ in the literature as if we do. When Slette and her team surveyed 564 publications from the last fifty years of drought research, less than a third of the papers explicitly defined drought or cited a definition of drought. In addition, they report: “ecologists most often use the term drought as a synonym for generally dry conditions (~30% of papers). In other words, authors state they are studying drought without quantifying and/or contextualizing how dry conditions are relative to normal (e.g., by reporting stardardized index values, or some measure of deviation from average conditions).” But wait, it gets even juicer — it turns out that hand-waving about drought may be distracting ecologists from noticing the actual climatic conditions at their study sites.

Slette and her coauthors selected a subset of studies from their review that were (a) bad at defining drought, but (b) good at providing details about their geographic location. They pulled the location coordinates and timeframes of these studies to calculate Standardized Precipitation Evapotranspiration Index (SPEI) values using the Global SPEI database. Only half of the droughts in this subset were characterized by especially dry SPEI values, outside the range of normal climate variability for their ecosystem. They found that 87% of the drought studies took place during times that were drier than average for the study site, but 13% of these “drought” studies were from periods that were slightly wetter than average based on estimated SPEI values. And while there may have been extremely local conditions that were truly dry at some of these “wet-droughts”, we don’t know because the authors did not report on them or place them within the context of the local long-term climate records.

I asked Slette about the review process for this paper. I had seen on twitter that it was her first publication as a lead author, and I wondered if journal editors had recognized the importance of this topic. I assumed that Slette may have faced the same challenges as authors of ‘advice papers’ who struggled to find the right home for their work. Both this paper and Dyson et al’s advice for urban ecologists working on private property had origin stories in graduate students creating the resources that they were searching for early in their careers. Slette and her seminar wanted a straightforward ecological definition on drought and couldn’t find it. Slette wrote, “I anticipated that it would be quite difficult to get this paper published, but I was actually pleasantly surprised by how well the editors and reviewers received it. Choosing to submit this paper as an Opinion was an important decision in terms of finding a good home for it, I think that turned out to be a better fit for it than as a primary research article.” Then, I asked her about her own research, aside from writing sharp reviews of ecological literature. I wanted to know what definition of drought she used and how it had changed since writing her definition paper.

Slette is a PhD candidate at CSU, and she answered, “I study how changing precipitation amounts and variability affect plant production. Specifically, I have been studying how experimentally-imposed extreme droughts affect plant root production and aboveground vs. belowground resource allocation in Central U.S. grasslands. For these experiments, drought was defined as a reduction in precipitation similar to what this area experienced during the Dust Bowl, about a 2/3 reduction from average. After writing this review paper, I am much more cognizant of all drought definitions, including my own. In every paper that I write from now on, I am definitely going to include more detail about the conditions of the drought itself, not just about its impacts.”

Finally, I asked her if the process of mining hundreds of papers for definitions of drought has made her a tougher reviewer or raised her standards for precise language from other ecologists. “I will definitely become a tougher reviewer now! I’m going to evaluate for precise wording and ask for lots of information about study design and justification.” I think that anyone who reads Slette’s paper will walk away with similar raised standards. And those of use who work in wet ecosystems should think about this too — we need to evaluate how we define our own work and what assumptions are hidden in our terms and jargon. As Slette notes, “I hope that the positive feedback and acceptance of this paper signals increased interest in (re)evaluating how ecologists define their work.”

References:

Slette, I.J., Post, A.K., Awad, M., Even, T., Punzalan, A., Williams, S., Smith, M.D. and Knapp, A.K., 2019. How ecologists define drought, and why we should do better. Global change biology.

Banner image: Ingrid J. Slette, Konza edge drought experiment