Flood Risk Management in Arid and Semi-Arid Areas: A Comprehensive Review of Challenges, Needs, and Opportunities

Abstract

Despite water shortages and infrequent rainfall in arid and semi-arid areas, their recent floods show that flooding tends to be more severe and life-threatening. However, flooding is a poorly understood natural hazard in arid and semi-arid areas and its management needs to address a set of challenges, some of which are unique to these zones. Hence, it is of great demand to develop a comprehensive understanding of Flood Risk Management (FRM) difficulties and needs that are significant in these regions. This review paper aimed to understand the current state of FRM in arid and semi-arid areas and to explore challenges existing in FRM of these zones, based on the authors’ experiences from practice-based research. The study identified four main classes of challenges: (i) regional features and flood characteristics, (ii) operational challenges, (iii) technical challenges, and (iv) management/organizational challenges. It was then concluded that these FRM challenges are multidimensional and coupled together. After discussing the challenges, the study addressed requirements and provided an outlook on solutions, further improvements, and possibilities for FRM in these regions.

1. Introduction

Despite continuous prevalent droughts in arid and semi-arid areas, devastating floods also hit these regions and cause major impacts on the economy, human activity, and environment [1]. Even in recent decades, the floods that occurred in arid and semi-arid regions have been responsible for a high number of fatalities and considerable economic damage in these regions. For example, catastrophic floods hit Riyadh in Saudi Arabia during 2009–2016, resulting in 168 deaths and millions of USD of economic damage to homes, businesses, and transportation infrastructures [2]. Floods in Jordan in the past five decades have claimed 345 human lives and affected 24,321 people [3]. A flood that occurred in Afghanistan in 2014 led to about 400 fatalities [3]. In Iran, records indicate an increase in flood losses from 17.2 to 132 million USD in the period between 1950 and 2000 [4]. In 2019 alone, flooding accounted for the death of 78 people around the country and caused over 1.1 billion USD in damage in Iran [5]. Similarly, floods in Egypt were responsible for massive economic damage and loss of life in 2015 [3]. In 2017, 10 members of a family were drowned by flash flooding in Arizona [6]. The deadly flooding in 2015 in southern Utah caused about 20 fatalities [7]. Flooding in Mexico in 2020 caused around 1530 million USD in damages [8].

Studies show that the number of floods is increasing in recent years in arid and semi-arid areas (Figure 1). In this regard, climate change worsens the intensity and frequency of extreme weather conditions in arid and semi-arid regions as it leads to heavy rainfall, high temperatures, and longer periods of drought [9], potentially exposing these areas to greater flood damage in the future. The problem is further amplified by population growth and human activities such as urbanization, inappropriate land use, economic development, and missing awareness.

In addition to climate change and human activity that cause widespread impacts on flooding [10], there are several unique conditions that expose arid and semi-arid areas to severe flooding and unexpected flood damage. On one hand, the high diversity in climate, geography, geology, vegetation, and hydrology of arid and semi-arid areas poses difficulties for local flood managers. On the other hand, human- and system-related challenges such as drought-based management and institutional frameworks have adverse effects on Flood Risk Management (FRM).

These facts all emphasize the need for a comprehensive understanding of FRM challenges in arid and semi-arid areas in order to mitigate flood negative impacts on the environment, economy, and society—now and in the future.

Figure 1. Number of floods from 2013 to 2022 in Iran, Afghanistan, Egypt, and Sudan [11].

Figure 1. Number of floods from 2013 to 2022 in Iran, Afghanistan, Egypt, and Sudan [11].

Therefore, this review article first presents and classifies challenges of flood management in arid and semi-arid regions based on the authors’ experiences from practice-based research, and secondly provides needs as well as opportunities for FRM improvement in these areas. By addressing the challenges and opportunities, it aimed to help local policymakers further in developing feasible and practical FRM plans in these areas. To achieve the main goals, this review article specifies the following objectives: (i) identifying major difficulties and challenges of FRM in arid and semi-arid regions, (ii) recognizing needs and providing solutions to address the challenges, and (iii) discussing opportunities for further improvements in FRM in these areas.

The structure of this review paper is as follows: Section 2 presents the explored challenges of FRM in arid and semi-arid regions considering flood characteristics and regional features, operational challenges, technical challenges, and organizational challenges. The needs are then introduced in Section 3, followed by potential solutions and opportunities.

2. Challenges of FRM in Arid and Semi-Arid Areas

In the literature, there are several definitions for arid and semi-arid areas, all of which shed light on water scarcity and draw attention to regular drought as the main natural hazard in these regions. According to Grove et al. [12], arid lands correspond with those receiving mean annual rainfall of around 25–200 mm while semi-arid regions have a total mean annual rainfall of 200–500 mm. Another common definition is the aridity index based on the ratio of mean annual precipitation to mean annual potential evaporation. In this context, the aridity index of arid zones and semi-arid zones is defined as 0.03–0.2 and 0.2–0.5, respectively [1]. Although one-third of the Earth’s surface is classified as arid and semi-arid regions [13], little attention is paid to floods in these lands, and flood management faces specific difficulties and challenges in arid and semi-arid areas due to technical and operational precautions as well as the nature of floods and geographic characteristics of these areas. In addition, organizational precautions such as capacity building of human resources, risk awareness, and society engagement play pivotal roles in flood management with regard to economic, social, and political aspects, which have been not included as a part of the existing FRM practices in these regions.

2.1. Regional Features and Flood Characteristics

Flooding in arid and semi-arid areas has unique characteristics that distinguish it from flooding in humid regions, and hence causes particular challenges to FRM in these regions. These difficulties result from the temporal and spatial combination of physical processes/characteristics in these regions including geographical features, climatic conditions, precipitation characteristics, catchment hydrological response, and debris flow.

(a)

Climatic and geographical characteristics

Climatic conditions of arid and semi-arid areas vary from place to place (mountains to riverplains) and from time to time (spring to winter) [14], which make them difficult to predict. Nevertheless, these lands are well recognized for low amounts of annual rainfall of 25–500 mm [12], high temperature in summer reaching peaks up to 50 °C [14], high solar radiation, low air humidity of around 14% in some regions [13], long-lasting drought even for a decade, and water scarcity [15].

These weather conditions have negative impacts on vegetation and soil in arid and semi-arid regions. On one hand, they cause sparse vegetation cover [16] which subsequently results in a low rate of interception and transpiration. On the other hand, low air humidity, high solar radiation, and high temperature lead to high levels of evaporation and hence to low soil moisture, which in turn results in the development of shallow soil with weak profile diversity [13]. Poor vegetation combined with extensive dry conditions also causes relatively unstable dry lands and loosely structured soil with low organic matter content and high vulnerability to water erosion [17]. Furthermore, the surface soil layer in arid and semi-arid regions is rarely saturated compared to that in humid zones. Consequently, infiltration-excess runoff is the dominant process in runoff generation during rainfalls in contrast to humid regions, where saturation-excess is the runoff generation mechanism [18].

(b)

Precipitation characteristics

In arid and semi-arid regions, rainfall is the most common type of precipitation especially in summer. Rainstorms tend to be more variable across space and time in these regions than in humid zones. Unstable atmospheric conditions in arid and semi-arid lands lead to short high-intensity rainfall over small area extent [12]. Such rainfalls are sudden, irregular, and infrequent, sometimes even with long periods of no rain in between [12]. These precipitation characteristics make them difficult to forecast. Furthermore, extreme variability in rainfall and geographical characteristics of these areas cause flood response to be scale-dependent [19]. Such dependency poses challenges in both the examination of flood characteristics and their simulation.

More challenges arise in mountain catchments of arid and semi-arid areas where precipitation is mostly dominated by snowfall. Unexpected increases in temperature or rain-on-snow events cause earlier spring snowmelt [20] in these regions generating a large amount of runoff after rainfall. This can lead to more tremendous spring floods on the one hand and to more severe erosion and debris flows on the other hand.

(c)

Flood response characteristics

Prolonged droughts combined with shallow soil and scarce vegetation cover have a significant influence on the hydrological response of arid and semi-arid regions. The spatio-temporal variability of rainfall contributes to the spatio-temporal variability of flood responses throughout the catchment [16]. Flooding happens infrequently, suddenly, and quickly over a short period of time in these regions. The response of the catchments to intense rainfall is rapid, reaching its peak in a very short time often only a few hours [1,14]. Flood hydrographs in arid and semi-arid regions are characterized particularly by sharp rising limbs, high peaks, and steep recession limbs [18]. Therefore, any modeling of flood response should consider such characteristics and variabilities, which is a challenging task.

(d)

High debris flow and sediment transport

Loose soil, poor vegetation, and sharp increases in flood velocity of arid and semi-arid regions cause excessive erosion, extensive sediment transport, and large debris flows when floods occur. The erosive force of debris flow amplifies the destructive power of flooding, which in turn leads to more severe damage to critical infrastructures, structures, and people. Massive debris flows in these regions also contribute to sedimentation [21,22,23,24] along hydraulic structures such as dams and reservoirs, and diminish their storage capacities. Culverts can become blocked with debris flows, influencing the downstream flow regime. Water pollution is another common problem after flooding, as suspended particles are transported along and deposited in flood plains, contaminating both soil and water [25].

It should be noted that the acquisition of required region- and flood-related data with the accepted quality poses additional barriers to achieving a good process understanding of regional features and flood characteristics in arid and semi-arid regions. This increases the flood vulnerability of these areas compared to humid regions.

As a phenomenon of high complexity, flooding itself presents unique challenges in these regions and could be less devastating in the absence or independence of other drivers as observed in humid regions. However, the compound nature of flooding in arid and semi-arid areas not only increases its occurrence likelihood in these regions but also intensifies its adverse impacts compared to humid regions. Ignoring this causal chain in flood generation in such areas leads to underestimation of the event and its consequences, thus resulting in the poor assessment and management of associated risk.

Table 1. Main regional features and flood characteristics playing a role in the compound nature of floods in arid and semi-arid regions.

Climatic Characteristics	Geographical Characteristics	Precipitation Characteristics	Flood Response Characteristics	Debris Flow and Sediment Transport
Spatial and temporal distribution	Sparse vegetation	Spatio-temporal variability of rainfall	Scale-dependent flood response	excessive erosion and large debris flow
High temperatures in summer	Low soil moisture	Short, high-intensity rainfall over small area extends	Spatio-temporal variability of floods	water and soil pollution
High level of evaporation	Shallow soil and loosely structured soil	Sudden, irregular, and infrequent rainfall with a low amount	Infrequent, sudden, and quick flooding	amplification of destructive flow power
High solar radiation	Weak profile diversity	Snowfall in mountains	Short concentration time	-
Low air humidity	Low organic matter in soil	-	Sharp rising limbs, high peaks, and steep recession limbs of the flood hydrograph	-
Long-lasting drought and water scarcity	Rarely saturated soil	-	-	-

summarizes the main features playing a role in the compound nature of floods in arid and semi-arid regions. Figure 2 shows a cause-and-effect diagram among regional features and flood characteristics in arid and semi-arid areas. A schematic representation of the relationship among these factors is illustrated in Figure 3.

2.2. Operational Challenges

Post-flood investigations in arid and semi-arid regions reveal insights into operational difficulties additional key challenges that flood managers face in these lands. These challenges are mainly tied to prolonged drought as well as regional features and flood characteristics, and so are not generally the concerns in humid regions.

(a)

Drought-based reservoir management

One major operational challenge to FRM in arid and semi-arid regions results from the water management of dam reservoirs in these areas. In comparison to humid regions, arid and semi-arid areas face low mean annual rainfall, prolonged droughts, and infrequent floods, causing drought to be the main natural hazard paid attention to [12,26] by administrative organizations and the public authorities. In dry seasons, water resources management in these zones commonly relies on dam reservoirs [27,28,29,30] designed to operate as multi-purpose structures. Consequently, the operational aim is to ensure a sufficient water supply by storing water during wet periods and then releasing it in times of scarcity [31,32]. Protracted droughts and rare flooding of arid and semi-arid regions shift the main focus of these multi-purpose structures toward water-saving policies. Hence, drought management plays a main role in risk-based decision processes of reservoir operations [28,29,30,33], and operational rules of most existing dam reservoirs are only conditioned for long regular drought periods. It can be shown that in humid areas, volumes of reservoirs are reserved for flood protection while each single drop of water is saved in arid and semi-arid areas.

Problems do arise when sudden and heavy rainfalls occur after long periods of no rain, causing a rapid rise in and flow of water. On one hand, the available capacity of constructed dam reservoirs might not be sufficient for such flooding. On the other hand, the rapid progress of floods leaves a very short time in reservoir operation management by administrations to release the water downstream in due time. Additionally, long drought experiences and expected periodic droughts increase concerns among authorities about releasing the water that was stored previously [28,29,30,34]. Such operational conflicts between the key purposes of these hydraulic structures force public authorities and dam operators to make hard decisions as to whether they need to release the water to mitigate the damage of current flooding or keep it to ensure water security for future dry periods. While the prioritization between drought and flood management is a major concern for authorities [27,32,35,36], no clear operational policies exist on the amount and time of water release for such a sudden flood potential. The lack of emergency operation rules exacerbates the problem further. Delays in flood control operation can even lead to much more devastating flood damage in comparison to the originally occurring flooding itself [37].

(b)

Drought-based water infrastructures

Not only is the management of reservoirs and dams in arid and semi-arid regions mainly based on drought conditions but also local critical water infrastructures have been designed for drought conditions [38]. Particularly, urban and rural areas of arid and semi-arid regions in developing countries suffer from poor or inappropriate infrastructures such as drainage systems [15] in coping with large amounts of flood waters entering the system [39]. Although these systems are widely applied for flood mitigation in humid regions, there is still a lack of knowledge concerning the design suitable for dealing with mid-drought flooding in arid and semi-arid areas. Another emerging issue is related to the lack of well-defined flood structural measures. The absence of flood protection measures makes not only these regions highly vulnerable to flooding [40] but also contributes to low flood awareness among both authorities and the public.

(c)

Sediment deposition in hydraulic structures

Hydraulic structures and infrastructures in arid and semi-arid regions are subjected to severe sedimentation [15,21,22,23,41,42,43,44], posing enormous challenges to flood management. During short and heavy rainfall, these structures deal with sedimentation, debris flows, and high concentrations of sediments that disrupt drainage regulations, culverts, and bridges. Sediment trapping poses serious problems both to hydraulic structures’ sustainability and downstream channels. Hydraulic structures are filled with sediment within years, which causes a loss in their capacities. Such a disruption influences the downstream flow regime, even increasing the water level at bridges or surrounding inundated areas. Another main issue is the importance of regular maintenance of these systems to remove debris and sands where sediment dredging activities are expensive.

Table 2. Operational challenges in Flood Risk Management (FRM) of arid and semi-arid areas: their causes and consequences.

Operational Challenges	Components
Drought-based reservoir management	Drought as the main natural hazard to be paid attention
	Shifting the main focus of multi-purpose dam to water saving policies
	Drought management as the main concern
	Operational rules conditioned for long regular drought periods (water storage in wet periods, water release in dry seasons)
	Insufficient available capacity after sudden and heavy rainfall
	Operational conflicts after heavy rainfall
	Lack of clear operational policies for a sudden flood potential
	Lack of emergency operational rules
	High uncertainty about releasing water or storing that after sudden rainfall
Drought-based water infrastructures	Design of critical infrastructure for drought conditions
	Lack of knowledge about suitable design for dealing with sudden flood
	Lack of well-defined flood structural measures
Sediment deposition in hydraulic structures	Sediment trapping
	Loss of capacities
	Influencing downstream flow regime
	Increasing water levels at bridges
	Increasing water levels in inundated areas
	Lack of regular maintenance
	Lack of regular sediment dredging activities

provides the discussed operational challenges in FRM of arid and semi-arid areas.

2.3. Technical Challenges

Another class of key challenges for flood management of arid and semi-arid areas arises from technical difficulties including data issues, modeling problems, and warning challenges.

(a)

Data challenges

In arid and semi-arid regions, a major category of technical challenges results from a lack of appropriate data. One main problem lies in the lack of field measurements and data availability in arid and semi-arid zones of developing countries, as these areas are mostly poorly gauged catchments with insufficient hydro-meteorological data [3,45,46,47,48,49]. Generally, there is a limited number of rain gauges in these parts [49,50] and they are widely distributed, and so the recorded data cannot appropriately represent the spatio-temporal variability of rainfall. Not only in developing lands but also in developed countries, many existing gauges are deteriorated [19,39,50,51,52] due to flooding, debris flow, and high sedimentation.

The lack of a central database for data storage and analysis is another issue in developing countries due to insufficient financial budgets. Weak documentation of past flood events in these lands represents also a significant obstruction to further flood management and assessment. Very few surveys and information are available about prior floods [50,53,54] especially concerning smaller catchments. Inconsistent and missing values in the available data are other data-related issues.

(b)

Flood modeling challenges

The lack of observed rainfall and streamflow data in arid and semi-arid areas causes further challenges to the modeling process as it increases the difficulties for calibration and validation of hydrological modeling as well as prediction of future flood characteristics. The lack of appropriate historical as well as real data constrains the ability to use modeling tools to improve flood forecasting and create early flood warnings. Even near-real-time rainfall information that can be provided by radars is particularly subject to errors and uncertainties in arid and semi-arid areas as rainstorms occur suddenly with high intensity and over small areas.

Nevertheless, rainfall forecasting is a challenging task in arid and semi-arid regions due to their rainfall characteristics [49,55,56,57]. Furthermore, sudden occurrence of rainstorms and flooding demands real-time high-resolution simulation of present climatic conditions that feeds them to forecasting models as the initial conditions. Such real-time forecasting models are still state-of-the-art not only for arid and semi-arid regions but also for humid zones where frequent flooding occurs [52].

Another modeling challenge that has been less studied is related to the rainfall-runoff models used in these areas. Many of these models are characterized for humid areas [18,58] and do not present arid and semi-arid conditions well. In some regions, the high diversity in characteristics of the area needs different parameter values in hydrological modeling [59] and possibly different approaches, thereby introducing new modeling challenges. Both geographical features and hydrological characteristics demand rich observed and monitored data which is a challenging task in flood events, particularly in arid and semi-arid areas, as discussed before. The compound nature of floods in arid and semi-arid regions poses additional modeling challenges due to the complexities and different characteristics of processes.

(c)

Flood warning challenges

The nature of floods and the rapid response of catchments, particularly in mountainous areas of arid and semi-arid lands, results in an extremely short lead time for warning and evacuation in these regions. Due to the lack of high-resolution flood forecasting models in arid and semi-arid regions, low-resolution models are mostly used to forecast flooding, and are imprecise and subject to errors and uncertainty for further warning [49,52,56,57]. Even in some developing countries, flood alerts are frequently issued based on meteorological information [3,53,56,60,61], and warning messages are not well understandable by the public. Furthermore, multi-dimensional meanings and different interpretations of flood alerts provide explicit challenges to the effectiveness of warnings. Another main challenge is the difficulty in reaching the public in arid and semi-arid zones of some developing countries. On one hand, in many of these regions, the population is poor [62,63] without adequate access to communication technologies to be informed about flooding. On the other hand, dissemination of warnings to at-risk organizations and people faces problems, particularly in rural areas, due to the failure of telecommunication systems.

Table 3. Technical challenges in FRM of arid and semi-arid areas: their causes and consequences.

Technical Challenges	Components
Data challenges	Lack of appropriate data
	Poorly gauged catchments
	insufficient hydro-meteorological data
	Limited number of rain gauges and their widespread nature
	Deteriorated existing gauges
	Lack of a central database
	Insufficient financial budget
	Weak documentation of past flood events
	Insufficient surveys about past floods
	Inconsistent and missing values in the available data
Flood modeling challenges	Difficulties in the prediction of flood characteristics
	Demanding rich observed and monitored data
	Difficulties in the calibration and validation of models
	Uncertainties and errors in radar-related data
	Challenges in rainfall forecasting, flood modeling, and early warning
	Demanding real-time high-resolution flood models
	Lack of characterized rainfall-runoff models for arid and semi-arid conditions
	Compound nature of floods in arid and semi-arid areas
Flood warning challenges	Short lead time for warning and evacuation
	Imprecise flood forecasting models
	Errors and uncertainties for further warnings
	Lack of easily understandable warnings
	Multi-dimensional meanings of flood alerts and different interpretations
	Difficulties in reaching the public
	Lack of adequate access for the public to communication technologies
	Problems in the dissemination of warnings due to the failure of telecommunication systems

provides an overview of the technical challenges and their components in flood management of arid and semi-arid regions.

2.4. Management/Organizational Challenges

The arid and semi-arid areas of developing countries also face another category of challenges related to management and organizational aspects. This category includes weak institutional FRM frameworks, poor flood awareness, lack of public training and risk communication, uncontrolled urbanization, and crisis-based management.

(a)

Weak institutional frameworks for flood management

Governance and institutional settings can increasingly challenge flood management in arid and semi-arid regions. Developing countries mostly suffer from a lack of a proper institutional framework [53,54,61,64,65] which identifies responsible actors and their roles in an effective FRM. Poor institutional arrangement leads to overlapping responsibilities among actors and organizations as well as unclear involvement, particularly for flood emergency responses. Another emerging challenge is cross-collaboration within different levels of governmental agencies with diverse sectors. A lack of coordination between various actors with different interests provokes conflicts, thereby posing difficulties in managing flooding in these countries.

Such a weak institutional framework causes further challenges for flood management across various provinces and districts. There is generally limited collaboration and data exchange across borders [53,61]. The problem is exacerbated for basins located in different provinces where conflicts arise in cooperation among organizations as well as the implementation of flood management strategies.

(b)

Poor awareness of flooding

Flood awareness is an essential element for adopting flood risk reduction behaviors. Flooding in arid and semi-arid areas, itself, seems to be a contradiction to the public. Rare flood occurrence and its infrequent climatic signals associated with regular signals of drought cause inhabitants and public authorities to overlook flood occurrence [2,49,54,66,67]. In developing countries, the low level of flood risk awareness is even evident in their historical development where no considerations have been made in modernization, internal migration, and urbanization, neither by public sectors nor by residents.

There are various factors contributing to poor flood awareness in arid and semi-arid regions. Apart from personal and demographic characteristics discussed in the literature [68], there also exist other determinants that result from unique features of arid and semi-arid regions. One main causal factor of poor flooding awareness is serious droughts that occur regularly in arid and semi-arid regions for a long period with considerable damage and lasting social, cultural, and emotional impacts on people [69,70]. Long-lasting effects of drought combined with incomplete ecosystem recovery after drought leave a lasting imprint on a society’s collective memory [71]. Periodic drought signals enhance the local awareness of drought and strengthen the memory of a society in terms of recognizing drought as the only threatening water-related hazard in the region, producing a false sense of security against flooding.

(c)

Lack of public training

Overlooking flood occurrence by governments causes them not to have any risk reduction actions and defined programs for educating people about flooding. The absence of flood-informing symbols and warning signs is one example [64,72,73], which contributes to poor flood awareness among residents and tourists in arid and semi-arid regions. A lack of public training leads to low resident knowledge about the environment they are living in and its potential hazards. The local communities in arid and semi-arid regions neither know [54,67] nor are trained on how to deal with nature and flooding [53,65].

Poor public knowledge of flooding causes flood-prone areas of arid and semi-arid regions to be densely populated zones, particularly in developing countries [63,74]. Rapid population growth in these areas close to water where lands are cheaper and access to water is easier prompts urban expansions into regions that are highly exposed to flooding risk. Such a sudden and unplanned increase in population density leads to considerable informal urbanization, deforestation, and change in urban hydrology exposing these areas to a higher risk of flooding. Nevertheless, the economic status and income level of societies play a crucial role in this regard as low-income households live in areas with poor infrastructures. These regions are highly exposed to flooding, where the low quality of materials used in buildings amplifies the problem.

Furthermore, poor flood awareness in the population leads to ignoring any flood warnings and in turn emerging high-risk behaviors such as taking photos or watching the incident during the time of flooding [75]. Local inhabitants even center their long-term behaviors and coping strategies based on drought events. For instance, smallholder farmers relocate close to water [76] or migrate to urban areas [53,64,77] to cope with future drought and water scarcity. Such a reliance of residents on prior drought experiences for their decision-making shows the crucial roles of “prison of drought experience” [78] and “path dependency” of responses to drought, even in wet seasons. Obviously, these path-dependent strategies in arid and semi-arid regions increase the exposure and vulnerability to flooding.

(d)

Lack of risk communication, social engagement, and social preparedness

Communicating the risk of flooding to the public and stakeholders is another main challenge that flood managers face in arid and semi-arid areas. In some mentioned regions the governance arrangement is a “top–down” approach [65,79] which limits opportunities for knowledge sharing and risk communication among residents and local governments. Another underlying factor is historically inaccurate risk communications, which destroyed societal trust as well as societal sensitivity to future flood risks.

Poor public awareness of flooding in arid and semi-arid regions is a barrier to the active involvement of the society in taking private loss reduction measures [80,81]. Furthermore, low flood awareness acts as a disincentive to effective community engagement in the implementation of FRM plans established by governments [80,82].

Poor social preparedness of the society is another issue that matters in FRM in arid and semi-arid regions. This is strongly connected to a weak institutional framework, lack of flood management plans, and poor flood risk communication between governments and the public. Citizens of arid and semi-arid areas are shocked by flooding and are unsure how to respond to it. A lack of disaster education and local community engagement pose serious threats to society when it is subjected to sudden and rare flooding.

(e)

Uncontrolled urbanization

Rapid uncontrolled urbanization poses further challenges to flood management in arid and semi-arid areas. On one hand, the shift in residence of the population to urban areas raises flood exposure, particularly when areas with high flood potential are chosen as settlement options. On the other hand, urbanization exposes people to extreme flooding due to the increase in flood peaks and volumes [83] without having adequate infrastructures for collecting excess runoff and discharge. In addition to infrastructures, the design, typology, and construction materials of buildings and houses in arid and semi-arid regions have been traditionally adapted to dry and arid climates without further concerns about extreme rainfall and potential flooding. Nevertheless, the challenge of urbanization is strongly connected to the lack of appropriate flood management plans which presents issues for sustainable city development in arid and semi-arid regions. The lack of laws and regulations [53,54,61,65] that limits informal urbanization and enforces sustainable urban development in these regions is another issue that matters here.

(f)

Crisis-based management and lack of flood national action plans

Unlike European countries, in which the Floods Directive (FD) has been defined as a general legalization with determined goals and timeframe [84], most Asian and African countries do not have a defined framework. Despite being a serious disaster in arid and semi-arid regions, flooding has received little attention among public authorities. Rare flood occurrence makes it difficult for local experts to broaden their knowledge about flooding and its specific characteristics in these regions.

Such low flood knowledge and experience cause flooding to be neglected [85,86] by authorities and FRM is not seen as a priority in their disaster management. Furthermore, the infrequent occurrence of flooding in arid and semi-arid regions leads to a lack of specific flood management strategies in a cycle of preparedness, response, and recovery [1,3,5,54,61] which significantly increases the local community’s vulnerability to flooding. No flood hazard and risk maps have been prepared [53,54,61,65] before being shared with the public and stakeholders, which also complicates the determination of flooded zones for emergency responses. A lack of preparedness measures exacerbates the problem. No maps of evacuation routes and safe locations are available in the time of sudden flooding and previous unflooded places are even incorrectly assumed to be safe among the public.

Instead of a predefined systematic flood management plan, local public administrations often adopt crisis management protocols [53,54,65] and undertake immediate measures from the moment of issuing warnings via meteorological organizations. Crisis-based flood management can only limit the adverse impacts of that individual flooding. Moreover, emergency responses are short-term and prone to failure in these regions because an important precondition for their performance is timely flood pre-warnings [45,50,52] which is particularly a challenging task in arid and semi-arid regions. The reliance of governments on emergency responses without public training presents also considerable challenges.

Most of the addressed management/organizational challenges in arid and semi-arid regions are related to the lack of national flood action plans; therefore, the majority of them will be met if proper national flood action plans are adopted in these regions.

Table 4. Management/organizational challenges in FRM of arid and semi-arid areas: their causes and consequences.

Management and Organizational Challenges	Components
Weak institutional framework for flood management	Lack of proper institutional framework
	Overlapping responsibilities
	Unclear level of involvement in flood emergency responses
	Cross-collaboration
	Lack of coordination
	Conflict of interests
	Limited collaboration and data exchange across borders
	Conflicts in the implementation of flood management strategies for basins located in different provinces
Poor awareness of flooding	Considering drought as the only threatening water-related hazard
	Flooding as a contradiction to the public
	Overlooking flood occurrence
	Infrequent flood signals
	Periodic drought signals
	Historical development of catchments based on regular drought
	Lasting imprint of drought on the society’s collective memory
	Low flood experience among the public
	False sense of security against flooding
Lack of public training	Lack of defined programs for educating people about flooding and risk reduction actions
	Absence of flood-informing symbols
	Absence of flood warning symbols
	Emerging high-risk behaviors
	Centering coping strategies on the basis of drought events
	Presence of drought experience
	Path dependency of the responses to drought
Lack of risk communication, social engagement, and social preparedness	Lack of appropriate communication due to the “top-down” governance arrangement in some regions
	Inaccurate risk communications that destroy trust in warnings of future flood risks
	Lack of active involvement of the society in taking private loss reduction measures
	poor/lack of knowledge of the public about private flood risk reduction measures
	Lack of training programs to empower people with the knowledge of loss reduction measures
	Lack of promoting mechanisms to encourage people to be actively involved in flood management programs
Uncontrolled urbanization	rapid uncontrolled urbanization
	Shift in residence of the population to urban areas
	Increase in flood peaks and volumes
	Traditional adaptation of buildings (design, typology, and construction materials) to dry climates
	lack of laws and regulations
	Lack of sustainable urban development
Crisis-based management and a lack of flood national action plans	Difficulties for public authorities in broadening flood knowledge due to rare flood occurrences
	Low flood experience among public authorities
	Lack of specific flood management strategies
	Lack of/inaccurate flood hazard and risk maps
	Lack of/insufficient preparedness measures
	Lack of evacuation routes and safe locations
	Reliance of governments on crisis-based management

summarizes the discussed management/organizational challenges and their components.

3. Needs and Opportunities of FRM in Arid and Semi-Arid Areas

In order to tackle the challenges identified in the previous sections, it is essential to promote initiatives and adopt appropriate policies. Therefore, in the following sections, we discuss the requirements of FRM in these regions and provide several practical solutions and possibilities to meet the needs.

(1)

Shifting flood modeling from a single-driver event to a compound event

The challenges discussed are coupled together in practice and often experienced together rather than independently. They will become more challenging due to human activities, changes in lifestyles, and population growth. Climate change is expected to continue in the future, causing more extreme climate conditions. There will be more dry seasons but also more extreme rainfall events in arid and semi-arid regions.

The expected extreme condition for the future of arid and semi-arid areas increases the significance of improving our understanding of the compound events behind floods in these areas. Proper modeling approaches regarding flooding are required to facilitate the shift from a single-driver event to a compound event in these regions. Furthermore, it is important to understand the governed casual chain and the discussed physical processes in flood generation and to identify the underlying combined mechanisms that amplify a flood’s negative impacts on arid and semi-arid regions. Figuring out the compound nature of flooding, one can then deal with the existing technical, operational, and organizational issues of FRM in these regions.

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Establishing rainfall-runoff models for arid and semi-arid areas

It is important to establish rainfall-runoff models that are designed for arid and semi-arid regions to present arid conditions in flood generation. High diversity in these regions’ characteristics demands continuous collections of related data. To present the characteristics of the whole region different interconnected hydrological models are required. In mountainous catchments, early snowmelt should be paid attention to in the hydrological modeling of arid and semi-arid areas for both reliable flood prediction and drought management.

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Coupling debris assessment and hydraulic modeling

Concerning the high potential of sedimentation and debris flow in these regions, there is a significant need for more accurate hydraulic models to investigate culvert and bridge blockages and assess the amount of sediments deposited at those hydraulic structures. The effect of bridge blockage on the water velocity needs to be investigated through coupling debris assessment with hydraulic modeling.

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Addressing reservoir operational conflicts in reservoir management strategies

When basins with dam reservoirs in arid and semi-arid regions are subjected to heavy rainfalls or flooding, it is required to see these structures as multi-purpose measures that operate in both flood and drought conditions. For this aim, it is vital to establish clear operational policies on the amount and time of water release due to heavy upstream rainfall to mitigate damage, now and in the future. Emergency operational rules are another key strategy to address operational conflicts occurring for a sudden unexpected flood event in those basins versus water shortage in drought conditions. In order to perform a reliable flood assessment and to predict downstream inundated areas, hydrologic information on upstream reservoirs is required in addition to reservoirs’ outflows. Nevertheless, such reservoir management strategies should be a part of a more comprehensive FRM in arid and semi-arid regions.

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Setting up an institutional framework for flood management

Short available time for flood forecasting in arid and semi-arid regions demands coordinated actions among organizations, especially in African and Asian countries, where there is a lack of a Floods Directive (FD) for orderly actions. It is of high importance to establish an appropriate institutional framework with central parts to set instructions, coordinate actions between governmental agencies, specify their roles and level of involvement, and share responsibilities. Regular communication among actors with different interests is particularly crucial in arid and semi-arid areas to avoid cross-collaboration and reduce conflicts.

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Developing flood hazard and risk maps

The development of flood hazard and risk maps is a key to assisting authorities in informed decisions concerning controlled urbanization, adequate infrastructures, and drainage systems as well as emergency responses and evacuation routes. More importantly, flood hazard and risk maps should be shared with residents to make them aware of their exposure to flooding. The public should be also informed in advance about safe locations and evacuation routes.

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Communicating with the public

There is a need to communicate with the public effectively inform them about the potential flooding and train them on how to behave to cope with flooding. For this aim, public authorities should provide flood-related knowledge on a regular and continuous basis at various levels. Continuous improvement of flood risk perception is of great importance and should be ensured. Joint training and education tools, which allow the exchange of knowledge and experience as well as providing better involvement of target groups, are in demand. Installation of flood-informing symbols and warning signs is a key to raising the attention of residents and tourists to flooding in these regions. These help to inspire local people to become involved in every step of the flood management process. Another important factor is to make the public familiar with flood alerts to understand them well and interpret the warnings correctly in a disaster.

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Building capacity through participation

To achieve the goals, participatory capacity building at both macro and micro levels is required in arid and semi-arid regions. More specifically, provincial and federal organizations should be informed clearly about flooding through meetings and workshops. Here, research institutions and universities play a very crucial role in shedding more light on the scientific aspects of FRM in close collaboration with public authorities through sharing their projects’ findings as well as state-of-the-art of FRM in similar regions around the world.

Finally, joint capacity building of residents through face-to-face training as well as indirect method training such as inundation maps, newspapers, and media should be performed. Joint public training through self-warning and self-protection activities such as building mobile flood protection walls and using social media can significantly enhance the involvement of the public in FRM.

Materials and brochures should be provided to tourists who travel to a region for social and recreational activities.

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Strengthening community capacity through digital platforms

To further engage local inhabitants and enhance their flood awareness in an informative and interactive way, digital capacity-building platforms can be established to not only provide the population with information about flood risk but also encourage them to share their experiences and knowledge. The platform can provide training materials such as videos, pictures, texts, and instructions about types of floods, the hazards people are exposed to, and flood protection possibilities. Surveys can be included in the platform which allows collecting the required data for further analyses. Through establishing a forum, people can exchange their knowledge, discuss the situation, and share their photos and videos with others about flooding. Such digital platforms not only provide an opportunity for risk communication but also provide researchers with inhabitants’ insights about flood-related issues.

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Involving the community in FRM activities

After communicating with the public and building their capacity, it is of high importance to engage them in the cycle of FRM activities. Such social preparedness and proactive community involvement can then contribute to the success of action plans and acceptance of decisions made at governmental levels. Furthermore, it is the key to increasing resilience, without which society will be still in danger. Local inhabitants can be involved in FRM activities by taking private flood adaptive behaviors such as water barriers, wet floodproofing, and dry floodproofing measures.

To improve the active involvement of the public in FRM through private flood adaptive strategies, incentive mechanisms such as insurance and governmental financial support play crucial roles. It emphasizes joint responsibility and preparation at all levels of society and public authorities.

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Including private flood adaptive behaviors in flood modeling

The contribution of local inhabitants to FRM through private flood adaptive behaviors leads to communication, human interactions, and adaptation. It is, therefore, essential to include private flood adaptation into flood modeling, assess the interplay among adaptation options, and explore the role of promoting mechanisms. Such social interactions and their interplay are complex and result in multiple feedbacks over time horizons.

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Changing the flood modeling perspective from macro- to micro-level

Current flood models are not capable of including complexities caused by community involvement and human behaviors. Hence, there is a need to change the flood modeling perspective from macro- to micro-level and apply an alternative framework that can address such aspects. To fill this knowledge gap, Agent-Based Modeling (ABM) is an innovative approach that allows the modeling of social processes in combination with engineering practices. ABM enables modelers to integrate dynamic adaptive individual behaviors into flood risk assessment. This new approach helps also policymakers of arid and semi-arid regions to make informed decisions on the implementation of risk-transfer mechanisms and external incentives for enhancing private flood adaptive decisions in society.

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Establishing an integrated early warning system

In arid and semi-arid areas where floods appear suddenly, time is a determining factor to implement emergency responses successfully. Hence, it is of great importance to forecast incoming floods through an Integrated Early Warning System (IEWS). An IEWS needs to include several interconnected numerical sub-models for monitoring real-time hydro-meteorological data, predicting weather on a wide range of space and time scale, simulating the hydrologic response of the catchment, predicting inundated areas, assessing the potential flood risk, evaluating the level of dangers, and finally issuing a warning to decision-makers and society.

While such an IEWS requires making use of various models interconnected together, it gets very complex and needs rich data. Another technical requirement concerns warnings [3] which should be broadcasted timely, in an understandable way, and through appropriate communication tools even in mountainous catchments of arid and semi-arid regions. Furthermore, flood forecasting in arid and semi-arid regions is associated with various sources of uncertainty [33] for which probabilistic forecasting can be a useful approach to present a range of possible scenarios.

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Monitoring and observing flood events regularly, using earth observation

Given the importance of data in the flood modeling process, it is significant in arid and semi-arid regions to collect field-based geographical and hydrological data on both national and local levels. The collected data need to be stored, analyzed, and accessible by experts and decision-makers.

Data exchange and sharing of information should be enhanced among organizations and institutions. To improve the documentation, field studies and survey after the flood occurrence is very important to collect information on the flood characteristics and associate damage as in-situ data. Missing data need to be collected after the event by interviewing local inhabitants who experienced those flood events [16]. Such documentation will be a useful source of historical data in the future for improving flood forecasting and hydrological modeling. If it is possible, it is recommended to increase the density of rainfall and river monitoring networks for enriching gauged data for effective flood monitoring in arid and semi-arid areas.

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Using remote sensing technologies for data acquisition

There is an essential need for remote sensing data acquisition to overcome the difficulties existing for observation and monitoring of regional and flood characteristics in arid and semi-arid areas. Radar technology can help improve our ability in rainfall measurement in parts where there is insufficient rainfall data due to limited numbers of gauges. Weather radar is also a useful tool for obtaining rainfall estimates in areas exposed to deterioration due to sedimentation and debris flows. The radar rainfall data can be combined with observational rainfall gauges for a more reliable data assimilation. Nevertheless, uncertainties and challenges are involved with using radar technology which should be considered within the context of flood modeling.

New technical development and remote sensing provide satellite images and create better observation methods for flood prediction. It is very useful in arid and semi-arid regions where lack of data plays a critical role in poor understanding and management of flooding. The interconnection of radar, satellite, and GIS techniques can contribute to further improvement. Remote sensing can assist in collecting data about the catchment including near-surface soil moisture, land surface temperature, snow cover, topographical characteristics, land use, and vegetation cover. Furthermore, it is useful for flood inundation mapping of past floods by post-processing of related satellite data [5]. Remote sensing is also an applicable tool to find the best locations of new dams to diminish flash flood losses [45]. Nevertheless, any traditional methods of data collection such as land monitoring and field measurement are of high importance to both enrich the databases and to assist modelers in the verification of the acquiesced data through advanced techniques.

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Adopting sustainable land use practices

Sustainable land use practices can play a vital role in reducing the risk of flooding in arid and semi-arid areas. Practices such as soil erosion control measures [12], reforestation, and watershed management [40] can help improve soil structure and restore vegetation cover in these regions. Furthermore, floodplain management should be paid attention due to flood damage resulting from dry riverbeds in arid and semi-arid areas during periods of heavy rainfall.

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Changing to integrated water resource management

Integrating Water Resource Management (IWRM) can help balance competing demands for water in these areas and diminish the risk of flooding. Flood water can be harvested in arid and semi-arid regions [45] to provide sources of water through groundwater recharge or water storage, especially for dry periods when the area suffers from water shortage. Flood harvesting is not only a good practice for water resources management but also plays a vital role in flood management through activities such as storing water in artificial ponds to diminish the flood risk at the outlet. Such adaptation solutions can improve the sustainability of society and impact the recovery speed [5].

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Establishing a systematic flood national action plan

Last but not least, it is crucial to establish a systematic national action plan that includes practical flood management and the above-discussed strategies in a cycle of preparedness, response, and recovery to deal with flooding before, during, and after flooding. In this regard, well-defined structural measures play also central roles which should be combined with non-structural measures appropriate for arid and semi-arid regions. It should be noted that the compound nature of flooding in arid and semi-arid areas needs to be also considered in the establishment of action plans as well as the generation of multi-hazard and multi-risk maps. Only through that, a comprehensive flood risk evaluation and management is possible in arid and semi-arid areas to mitigate the simultaneous and amplified impacts of various geophysical and hydro-climatic drivers. The challenge of urbanization can be better overcome through integrating urban planning and FRM in these regions.

4. Conclusions

Arid and semi-arid areas are typically characterized by water scarcity and regular long-lasting drought and it is often assumed that they face insignificant flooding problems due to low annual rainfall. However, flood events in recent decades show that these regions are extremely exposed to flooding and hence, they are dealing with two opposite ends of the hydro-climatic spectrum. Despite being a serious natural hazard, flooding remains a poorly understood natural phenomenon in these regions and their FRM is a challenging task.

This review study aimed to recognize these main challenges and to inspect the unknown but important components resulting in these challenges. By identifying core challenges, this review research outlined that FRM challenges are diverse and multidimensional and four main classes rolled out in arid and semi-arid zones. One key challenge is related to unique regional features and flood characteristics presenting particular difficulties to FRM in these areas. Another critical challenge arises from operational difficulties where water and infrastructure management are mainly based on prolonged drought. Technical difficulties including data issues, modeling problems, and warning challenges cause significant challenges to FRM in these areas. Last but not least, management and organizational challenges play crucial roles in the failure and ineffectiveness of FRM strategies in arid and semi-arid regions.

After addressing the challenges in detail, we discussed the requirements imposed and provided practical solutions to overcome these FRM challenges and meet the necessities of a successful FRM. In this regard, we recommend focusing on both social and political aspects for an effective FRM in arid and semi-arid areas and addressing pivotal gaps considering cultural determinants.