Exploring ERA5 reanalysis potentialities for supporting landslide investigations: a test case from Campania Region (Southern Italy)

Abstract

Recently, ECMWF has released a new generation of reanalysis, acknowledged as ERA5, able to deliver a comprehensive, free, and operative picture of the past weather, exploiting the data assimilation of historical observations from different sources (satellite, in situ, multiple variables) for both atmospheric and soil variables. Experiences concerning flooding issues suggest that ERA5 could support also landslide investigations. In this sense, a number of questions may be raised: (i) can ERA5 data be reliable in reproducing rainfall histories leading to a landslide event? (ii) can ERA5 soil moisture estimations be reliable proxies for antecedent slope wetness condition? (iii) can ERA5 be implemented in a landslide early warning system to improve the performances? This study tries addressing these questions referring to some historical Campanian events (Southern Italy) with a special focus on the fully investigated event occurred in Nocera Inferiore on March 4, 2005. The study shows the following: (i) ERA5 precipitation can appropriately reproduce actual rainfall histories leading to the events occurred in the last years; (ii) ERA5 soil moisture may act as proxy of slope wetness conditions; (iii) ERA5 data could be easily implemented in LEWSs in areas poorly covered by field monitoring networks.

Appendix. HTESSEL soil model

HTESSEL soil model schematizes the vertical movement of liquid water in the soil unsaturated zone relying on the Richards (1931) equation:

$$ \frac{\partial \theta (s)}{\partial t}=\frac{\partial }{\partial z}\left[k\left(\theta (s)\right)\frac{\partial }{\partial z}\left(\frac{s}{\gamma_w}+z\right)\right]+{S}_{\theta}\left(\theta (s),z\right) $$

(1)

where t and z are time spatial coordinate, θ and s are the volumetric water content and soil suction, θ(s) and k(θ(s)) represent the soil water characteristic curve (SWCC), and the hydraulic conductivity function (HCF), S_θ(θ(s), z) is a volumetric sink term corresponding to root extraction, and γ_w represent the unit weight of water.

Using such an equation requires soil hydraulic parameters stated through the SWCC and HCF. The HTESSEL model accounts for SWCC using the acknowledged equation proposed by van Genuchten (1980):

$$ {S}_e=\frac{\theta -{\theta}_r}{\theta_s-{\theta}_r}={\left[1+{\left(\alpha s\right)}^n\right]}^{-\left(1-\frac{1}{n}\right)} $$

(2)

where S_e is the so-called effective saturation degree, θ_s and θ_r are the saturated and the residual volumetric water content, n and α [kPa-1] are two empirical parameters.

As for HCF, the HTESSEL model combines the van Genuchten SWCC with the capillary model by Mualem (1976) relying on the saturated hydraulic conductivity k_sat (m/s), and an empirical parameter, λ, that accounts for pore tortuosity and connectivity:

$$ k={k}_s{S_e}^{\lambda }{\left(1-{\left(1-{S_e}^{\raisebox{1ex}{$n$}\!\left/ \!\raisebox{-1ex}{$n-1$}\right.}\right)}^{\left(1-\frac{1}{n}\right)}\right)}^2 $$

(3)

Regarding boundary conditions, such a model implements:

• at the uppermost boundary, P − E − R (where P, E, R are precipitation, evaporation and surface runoff, respectively); specifically, the evaporation results, as well as transpiration, from the surface energy budget computed by using a resistance-based approach while the surface runoff relies on a Hortonian mechanism;

• at the lowermost boundary, a “free drainage” condition corresponding to the “unit gradient” or infinite “layer assumption” as detailed in Reder et al. (2017).

The outputs are yielded at hourly scale as volumetric soil water considering a four-layer representation of soil (layer 1 = 0–7 cm, layer 2 = 7–28 cm, layer 3 = 28–100 cm, layer 4 = 100–289 cm).