Invited Review Paper A review of the effects of climate change on riverine flooding in subtropical and tropical regions

Tropical and subtropical regions can be particularly severely affected by flooding. Climate change is expected to lead to more intense precipitation in many regions of the world, increasing the frequency and magnitude of flood events. This paper presents a review of studies assessing the impacts of climate change on riverine flooding in the world’s tropical and subtropical regions. A systematic quantitative approach was used to evaluate the literature. The majority of studies reported increases in flooding under climate change, with the most consistent increases predicted for South Asia, South East Asia, and the western Amazon. Results were more varied for Latin America and Africa where there was a notable paucity of studies. Our review points to the need for further studies in these regions as well as in Australia, in small to mid-sized catchments, and in rapidly urbanising catchments in the developing world. Adoption of non-stationary flood analysis techniques and improved site-specific socio-economic and environmental model scenarios were identified as important future directions for research. Data accessibility and mitigation of model uncertainty were recognised as the principal issues faced by researchers investigating the impacts of climate change on tropical and subtropical rivers. This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/). doi: 10.2166/wcc.2019.175 s://iwaponline.com/jwcc/article-pdf/10/4/687/640385/jwc0100687.pdf Rohan Eccles (corresponding author) Hong Zhang School of Engineering and Built Environment, Griffith University, Gold Coast Campus, Gold Coast, Queensland 4215, Australia E-mail: rohan.eccles@gmail.com David Hamilton Australian Rivers Institute, Griffith University, Nathan Campus, Brisbane, Queensland 4111, Australia

downscaling and bias correction methodologies, and hydrological model applications. Selection of the most appropriate models and techniques for a given catchment can be challenging, as catchment size, topography, location, and climatic conditions must all be taken into account. Changes to sea levels and anthropogenic activities (land-use changes, urbanisation, water demand, and flood mitigation/control structures) may also be considered, adding further complexity to models.
Each step in the modelling process involves assumptions, which inevitably cause some degree of error, and is compounded with each successive modelling step (Praskievicz & Chang ). The mitigation and quantification of this model uncertainty is a major consideration in impact studies.
Review articles assessing the impacts of climate change on riverine flooding can be found on a global (Hunt & Watkiss ; Kundzewicz et al. ), European (Kundzewicz et al. ; Madsen et al. ), and country-wide scale (Miller & Hutchins ) but are lacking elsewhere. This paper aims to critically evaluate the current literature in the tropical and subtropical regions of the world and examine the factors that are unique to these climates. For this purpose, a systematic quantitative literature approach has been adopted (Pickering & Byrne ), whereby articles have been coded into a customised database to quantitatively assess the literature. This is the first review of its kind conducted in tropical and subtropical regions and the first to adopt a quantitative approach.
To maintain a comparable standard of literature, 'grey' literature, including reports, conference papers, unpublished articles, theses, and book sections have been excluded from this review, rather only English language peer-reviewed journal articles have been considered. Scopus and Web of Science were searched using a defined set of search terms. The inclusion criteria specified that articles must consider climate change scenarios, the use of hydrological modelling, and analysis of extreme river flows in either tropical or subtropical regions. A total of 134 peer-reviewed journal articles were included in this review from an initial evaluation of 4,711 ( Figure 1). The multitude of papers included in this review allows for a more comprehensive analysis of the literature than that found in other similar reviews. A critical evaluation of the following aspects of the literature has been included, (i) the geographic distribution of studies, (ii) the chosen methodologies in terms of downscaling, bias correction, hydrological model choice, and analysis, (iii) the key findings of the literature and, (iv) the implications of these findings and future directions for research.

DISTRIBUTION OF THE LITERATURE
An updated version of the Köppen Climate Classification (Peel et al. ) was used as the basis to delineate subtropical and tropical catchments ( Figure 2). When river basins covered numerous climate zones, only when the majority of the catchment was within tropical or subtropical climates was it considered in this review. As such, studies on the lower Mississippi River were excluded, while those on the lower Yangtze were included. The literature was approximately evenly split between tropical and subtropical catchments, covering 5 continents and 37 countries. Most of the research (59%) was published on Asian river basins, with the remainder published in Africa (13%), the Americas (12%), and on a global scale (16%). Figure 3 presents the number of studies included in this review on a per-country basis, excluding those studies conducted on a global scale.     . For the catchment size plot, <10,000 is the range between 1,000 and 10,000 km 2 , <100,000 is the range between 10,000 and 100,000 km 2 , and <1,000,000 is the range between 100,000 and 1,000,000 km 2 . There is a clear need for further tropical and subtropical river basin research to be conducted throughout Australia, Latin America, and Africa.

RESEARCH METHODOLOGIES ADOPTED IN THE LITERATURE Climate models
The selection of GCMs, Regional Climate Models (RCMs), Approximately 71% of the studies adopted an ensemble approach in which two or more climate models were employed to provide multiple projections as has been widely rec-  reported in their global study that the global hydrological models contributed more to uncertainty in streamflow changes than the GCMs. They therefore recommended that future studies adopt an ensemble of hydrological models in addition to an ensemble of GCMs.

Consideration of dams
Many studies in this review were carried out on sizeable river basins regulated by many large dams and reservoirs. The Mekong Basin has seen major dam developments over recent years for hydropower and irrigation purposes, with many more in the planning phase. When all planned dams are complete, the active storage capacity is expected to increase to 100 km 3 from the 5 km 3 initially available in

Global studies
In addition to the studies completed on catchment, regional, and national scales, several global studies have been con-

IMPLICATIONS AND FUTURE DIRECTIONS Implications
In order for research undertaken by the scientific community to be relevant to engineers and decision makers, it is essential that uncertainty is estimated and mitigated (Andersson et al. The additional effects of sea-level rise and anthropogenic activities (e.g., hydraulic structures construction, land-use changes, and urbanisation) further complicate the issue. In some instances, these effects may be more pronounced than those of climate change (e.g., Budiyono et al. ; Zhao et al. ). The combination of these changes may be especially devastating in some developing nations of the tropics and subtropics. Bangladesh, for instance, may be jointly affected by more intense cyclones (storm surges), increased extreme river flows, and sea-level rises, all of which may exacerbate flooding. Adaptation strategies and emergency action plans are required to mitigate economic damage and fatalities from such events. These plans must be flexible and robust to account for the range of plausible scenarios and allow future adjustments to be made with advances in modelling (Mathison et al. ). Such plans may be more difficult to implement in the developing nations of the tropics/subtropics, as governments understandably may not prioritise them over more immediate issues.

Future directions
Continual improvements must be made to climate models, particularly in the modelling of land-surface processes if projections are to become more reliable (Okazaki et al. ). Increasing the availability of sub-daily climate model outputs would be advantageous especially for studies con-