9 June 2016
The response of Californian earthflows to drought
A really interesting paper has just been published in Geophysical Research Letters examining the response of Californian earthflows to the ongoing, epic drought. This paper was led by Georgie Bennett, currently at the US Forest Service and Colorado State University, but who will be joining us here at the University of East Anglia in January 2017 as a full faculty member. California has been suffering a serious drought for many years now. In the paper, Bennett et al (2016) mapped 98 active earthflows in a 140 square kilometre area of the Eel River catchment in California. This is an area with multiple large, often deep earthflows that are an important part of the geomorphic system. I have taken one of the larger earthflows from Google Earth below:
The mapping was very detailed, undertaken using aerial photographs. The mapping focused not just on the boundaries of the landslides, but also on the displacement of trees located on the landslides, which allowed a calculation of the velocity of the landslides with time. This data was then compared with an index of drought, the commonly-used and widely accepted Palmer Drought Severity Index (PDSI), which has been used previously to show that the 2012-2015 drought in California is unprecedented.
The results are fascinating – I highlight here two key aspects. First, over the last 70 years or so the velocity of the earthflows has markedly decreased. This image shows the mean velocity, with error bars and the PDSI. It is notable that in recent years the PDSI has become exceptionally low, and the earthflows have responded by slowing down:
Bennett et al. (2016) also looked at the differential response of the Californian earthflows of different depths to the drought conditions. They found that the shallower (typically 5 – 15 m thick) landslides showed quite variable response to the drought conditions, with the majority slowing down but a few actually accelerating. However, all of those deeper than 15 metres decelerated. In some ways I find this a quite surprising result – I would have expected that the deep landslides would have been less susceptible to the climate than the shallow ones. The result is robust, but the cause is not clear and is intriguing. Bennett et al (2016) suggest that it could be that the shallow landslides respond to short term and local effects (vegetation change, a large storm) that might occur within a drought, whilst the deeper Californian earthflows are just responding to the climate forcing.
This is a really interesting study, showing very elegantly the ways in which the landslide system is responding to climate change. With greater changes to come in the years ahead as the global climate warms further, we will see many more responses of this type.
Bennett, G. L., Roering, J. J., Mackey, B. H., Handwerger, A. L., Schmidt, D. A. and Guillod, B. P. 2016. Historic drought puts the brakes on earthflows in Northern California. Geophysical Research Letters. Doi: http://dx.doi.org/10.1002/2016GL068378