Analyzing the informative value of alternative hazard indicators for monitoring drought risk for human water supply and river ecosystems at the global scale
Abstract. Streamflow drought hazard indicators (SDHI) are mostly lacking in large-scale drought early warning systems (DEWS). This paper presents a new systematic approach for selecting and computing SDHI for monitoring drought risk for human water supply from surface water and for river ecosystems that is also relevant for meteorological or soil moisture drought. We rec-ommend considering the habituation of people and ecosystems to the streamflow regime (e.g., a certain interannual variability or relative reduction of streamflow) when selecting indicators. Distinguishing four indicator types, we classify indicators of drought magnitude (water deficit during a pre-defined period) and severity (cumulated magnitude since onset of the drought event). We quantify eight existing and three new SDHI globally using the global hydrological model WaterGAP2.2d. We recommend streamflow hazard indicators that should be included in large-scale DEWS as they are suitable for risk systems that are differently adapted to low water availability and characterized by either perennial or intermittent streamflow regime and the existence or not of large reservoirs. Drought magnitude is best quantified by return period or relative deviation from mean, and severity by return period or water volume below a threshold relative to mean annual streamflow. Both anomaly and deficit indicators should be provided.
Claudia Herbert and Petra Döll
Claudia Herbert and Petra Döll
Streamflow drought hazard indicators for monitoring drought risk for human water supply and river ecosystems at the global scale https://doi.org/10.5281/zenodo.6647609
The global water resources and use model WaterGAP v2.2d: model description and evaluation https://doi.org/10.1594/PANGAEA.918447
Claudia Herbert and Petra Döll
Viewed (geographical distribution)
Review on “Analyzing the informative value of alternative hazard indicators for monitoring drought risk for human water supply and river ecosystems at the global scale”
In this paper the authors present an approach for the selection and calculation of streamfloe drought hazard indicators for monitoring human surface water supply and for river ecosystems. In doing so, the authors discuss and propose to consider the habituation of people and ecosystems to the streamflow regime. For this purpose, eight existing drought indicatoss are compared and quantified and three new ones are proposed based on the global hydrological model WaterGAP2.2d.
This article has positive aspects, in particular the effort in modelling that the authors have made is remarkable, however it needs to be reviewed thoroughly as there are several points that need to be clarified and addressed by the authors. In short, the article has potential, but the authors need to make an effort to focus the analysis and make a readable manuscript. I hope that the comments below will help in this direction.
Structure of the article
First, the structure of the paper could be improved. It is very long, repetitive in some aspects, lacking clarity on the objectives and results, that should be better highlighted both in the description and conclusions sections.
Similarly, the order of the sections does not follow a story, some sections could be shortened, removed, or moved to increase the readability. This structure surely makes it very difficult to read. For example, the introduction should be significantly shortened to focus on the description relevant to the objectives of this work. Section 3.4 should be moved into the methodologies section. In addition, there is an annex that includes only one figure that is quite relevant. I suggest including it in the main text, if the number of figures is an editorial requirement I suggest removing some other figures (e.g. Figure 9).
The methodology can also be reduced, focusing on the description of the proposed new indicators and model validation. In addition, some aspects should be clarified, e.g., eq(1) for SPI, X is noted as a generic variable, if this variable is precipitation this representation could be misleading.
Focus of the manuscript and sectoral risk representation
The authors indicate that the focus of the article is on analysing indicators for monitoring drought risk in very specific sectors (human water supply and river ecosystems). I agree that the selection of the hazard indicator is key to determine this dimension of drought risk and the discussion on that direction is more than welcome and needed. However, apart from referring to these sectors as the focus of the article and several speculative and unsupported assertions, there is no information in this article on how drought specific indicators affect these sectors. The dynamics of how these sectors or systems are affected is surely complex, depending on various factors that determine their exposure and vulnerability beyond whether they rely on upstream reservoirs or the systems are seasonally dependent. However, this is not enough to characterize the vulnerability of these sectors. Further discussion and analysis in this regard is needed.
On the one hand, considering the way the article is structured, orienting the analysis in the description and comparison between indicators, derived metrics, etc. in a concise and targeted manner can improve the focus and structure of the article. On the other hand, to strictly evaluate whether the proposed indicators are valid to represent risk, a more detailed analysis of the proposed sectors is needed, with a description of their vulnerabilities and how their impacts are produced as a consequence of the combination of the different dimensions of risk.
Grid cells – case study selection and description
The selection of the two grid cells seems to be motivated by characteristics derived from some of the modelled variables. However, no description of these sites exists in the manuscript. A quick search turns up that one gridcell is in central eastern Paraguay (perhaps including a portion of the Paraguay river) and the second near Firenze (Arno). Both points with very different realities regarding how they might be exposed and how they are vulnerable to droughts and surely each will have a very specific risk profile. Here I see a missed opportunity, as one of the objectives of this paper is to find out how the exposed systems can be used. Surely, a discussion along these lines would greatly improve the discussion on the usefulness of the various indicators. Indeed, the comparison between indicators is merely informative, which does not enable identification or validation between them (Please refer to my final comment)
Similarly, March 2002 has been used to describe Figures 3 and 4. But it is not clear why this period was chosen or how this comparison can be extrapolated to the whole period.
Comparison not validation
As proposed in this article, the fundamental purpose of any drought indicator is to represent the sectoral impacts in the best possible way. Indeed, the validation of the best indicator should be consistent in how it represent sectoral impacts. In this sense, it is perfectly legitimate to compare indicators, but it is not possible to validate or rank one over another without looking at independent variables that represent potential impacts. Please elaborate further on this issue