Flood and drought risk assessment for agricultural areas (Tagus Estuary, Portugal)

. Estuaries are coastal systems particularly vulnerable to climate change effects and within these systems, agriculture is one of the most potentially affected sectors. This paper proposes a risk assessment approach for helping the decision-making process at a local level, addressing two risks that affect agricultural areas located in estuarine margins: the unavailability of fresh water for irrigation resulting from the upstream propagation of estuarine brackish water during 10 droughts, and land inundation by high water levels associated with high tides and storm surges. For each risk, quantitative consequence descriptors are presented to support risk level determination and evaluation through a continuous consequence/probability diagram. The approach applicability is discussed through its application to the Lezíria Grande de Vila Franca de Xira, located in the Tagus Estuary (Portugal). Results indicate that the approach is appropriate to support risk owners in taking actions to mitigate the risk. The flexibility of the approach to be adapted to local conditions and updated 15 through time, and the ease of its application by the risk owner can be pointed out as the main strengths. level estimation and evaluation, as it integrates the uncertainty in the process. In 425 addition, this tool is suitable for communicating the risk in a simple way to the risk owner. The applicability of the developed approach was explored through the application in the Lezíria Grande agricultural area, known to be affected by those two hazards. Results show that concerning fresh water scarcity, the risk increases with the duration of the droughts and when low river flows occur for several consecutive weeks, even using the Risco River as an alternative source of water for irrigation is not sufficient to meet the water needs. The total dependence of irrigation on the Tagus and Sorraia fluvial 430 discharges, with other users upstream, suggests that previous knowledge of the water availability reserved in Spain and Portugal and the consumption expected for the different sectors upstream is essential in assessing the risk of fresh water unavailability for irrigation. Real-time knowledge of the upstream discharges, existing consumptions and possible runoff of the margins under extreme events. To simulate salinity intrusion the system of models SCHISM was also used, but was implemented in 3D baroclinic mode. The numerical model is forced by tides at the oceanic boundary, river flows at the riverine boundaries (Tagus and Sorraia) and atmospheric data at the surface. The model was previously calibrated and extensively validated in the Tagus estuary against field data (Rodrigues and 680 Fortunato, 2017; Rodrigues et al., 2019). Results showed its ability to represent the circulation and salinity patterns. At Vila Franca de Xira (the station located farther upstream and nearest to Conchoso), in particular, salinity errors were about 2 psu (Rodrigues and Fortunato, 2017). At Conchoso, the Root Mean Square Error and the Mean Absolute Error were 0.4 psu and 0.3 psu, respectively; the model tends to overestimate the data. The maximum difference between the data and the model results at the peak salinity was about 2 psu (Rodrigues et al., 2019). A detailed description of the model implementation and 685 validation can be found in Rodrigues and Fortunato (2017) and Rodrigues et al. (2019).


Risk assessment approach 60
A risk assessment approach is developed to address two natural hazards that often affect agricultural areas located near estuaries, particularly those dependent on surface water for crop irrigation and presenting low topography. These hazards are: i) water salinity increase due to droughts, and ii) estuarine high water levels that can promote inland inundation. In order to support stakeholders and decision-makers in the definition of mitigation and adaptation strategies, the approach should be easy to perceive by the stakeholders and suitable to be updated according to local conditions. The approach is summarized in 65

70
The risk assessment has to be preceded by the establishment of the risk context, which defines the risk management objectives, the consequence descriptors and the criteria to grade consequences, likelihood and risk (ISO, 2009b). The risk context depends on the site-specific characteristics and must be supported by historical information, and stakeholders and the risk owner judgement. The risk owner is the person or entity responsible for the risk management (ISO, 2009a). 75 As discussed above, several approaches are available to operationalize risk analysis and evaluation. Risk matrices, combining qualitative or semi-quantitative information on consequence and probability, are used in several risk management standards and guidelines to rank and prioritize risk (ISO, 2009b). Despite several disadvantages pointed out in the literature (e.g. Cox, 2008;Duijm, 2015), risk matrices are widely used in risk acceptance discussion and risk communication to broader audiences, supporting decision-making, as they present complex concepts in a simple way (Woodruff, 2005;Ale et 80 al., 2015). As an adequate tool to deal with risk level uncertainty, in both consequence and expected frequency, a continuous consequence/probability diagram is chosen in the present study as the suitable technique to assess risk.
The consequence is defined as an event outcome that affects the risk management objectives (ISO, 2009a). The proposed approach defines quantitative consequence descriptors of the two hazards through indicators of the potential economic impact for the risk owner. 85 https://doi.org/10.5194/nhess-2020-422 Preprint. Discussion started: 5 February 2021 c Author(s) 2021. CC BY 4.0 License.
For the water salinity increase during droughts, the consequence descriptor was defined as the water unavailability for irrigation during the most critical period for the crops to be watered. The water unavailability for irrigation (Wu) is given by Eq. (1): Concerning estuarine high water level, several elements are exposed to hazards such as the land, people or infrastructures 90 including dykes that prevent lowland inundation during high spring tides. When dykes are present, inundation normally occurs when the water level is above the dyke crest or when the dyke is breached. This exposed element can provide a direct quantification of the hazard economic impact for the risk owner. Thus, the high water level consequence is based on the dyke overflowing and the chosen descriptor is the relative cost of dyke damage repair, considering that the risk owner is the organization responsible for repairing the dykes. The relative cost of dyke damage (RCDD) given by Eq. (2): 95 Criteria to grade the consequence severity should rely on past events information from the area where this approach is applied, with the stakeholder's involvement. The same must be followed when selecting classes of likelihood, defined as the chance of something happening and can be presented as a probability of an event.
In the present approach, the definition of risk levels considers the ISO (2009a) criteria and the tolerable risk concept that is 100 normally used to assist decision-makers (Marszal, 2001). Tolerable risk is defined by ICOLD (International Commission on Large Dams) in 2002 as "a risk within a range that society can live with so as to secure certain net benefits". It is a range of risk that cannot be neglected or ignored and should rather be kept under surveillance and reduced if possible (Bowles, 2003).
Below tolerable risk, the risk is acceptable, i.e. risk is considered insignificant or adequately controlled, and above risk is unacceptable (HSE, 2001). For the hazards considered, risk is divided in three levels in the consequence/probability diagram 105 corresponding to different bands: a) high risk (red band), where the level of risk is considered intolerable and risk treatment is essential whatever its cost; b) medium risk (yellow band), where the risk is considered tolerable; c) low risk (green band), where the level of risk is considered negligible, so no risk treatment measures are needed. Risk tolerance limits depend on the study area characteristics and should be defined based on information from past events and risk owner judgement.
After establishing the risk context for the area where the approach is applied, hazard scenarios based on historical data and 110 stakeholder's information have to be defined to support risk assessment. Consequence descriptors are evaluated for the defined scenarios and risk levels are determined, compared and evaluated against risk criteria and tolerance limits previously defined. Results provide scientifically-supported information to help stakeholders and risk owners to discuss the acceptability of the risk magnitude. The consequence descriptors can be evaluated through the analysis of model results, and historical and monitoring data. 115 https://doi.org/10.5194/nhess-2020-422 Preprint. Discussion started: 5 February 2021 c Author(s) 2021. CC BY 4.0 License.

The Tagus estuary
The Tagus estuary, located at the mouth of the Tagus River basin (Fig. 2), is framed by the largest metropolitan area of Portugal, hosting along its margins 1.6 million inhabitants (Tavares et al., 2015). With a surface area of about 32,000 ha, the estuary presents a marked contrast of occupation between both margins: extensive artificial areas are present along the 120 northern margin and agricultural and semi-natural areas including a Natural Reserve (one of the most important sanctuaries for birds in Europe with about 14,000 ha) in the eastern area. Agriculture is the most relevant economic activity in the Tagus estuary upper region, in particular irrigated agriculture. Two different types of water resources management are present: the collective management existing in the irrigation perimeters of state / public initiative, either through distribution from reservoirs (Vale do Sorraia) or through direct extraction from the Tagus River (Lezíria Grande de Vila Franca de Xira); and 125 individual management carried out by farmers outside these perimeters.
The main source of freshwater discharging into the estuary is the Tagus River, with an average, maximum and minimum annual flows of 336 m 3 s -1 , 828 m 3 s -1 and 102 m 3 s -1 , respectively (APA, 2012). The Sorraia and the Trancão rivers also contribute to the freshwater inflow into the estuary. The Tagus is the longest river of the Iberian Peninsula with a watershed of 80,100 km 2 distributed between Portugal (30%) and Spain (70%). The hydrological regime is highly modified by several 130 reservoirs constructed since the 1950's in both countries, along the Tagus River and its tributaries. Although a convention was signed between the two countries in 2001 to agree on annual water releases in the Tagus River at the international border, particularly during droughts these releases are irregular and difficult to account for (Henriques, 2018). Therefore, the water availability downstream strongly depends on the water resources management practices in the basin.
The hydrodynamics of the Tagus estuary is primarily driven by tides. The tidal range varies between 0.55 m and 3.86 m at 135 the coast (Guerreiro et al., 2015), and increases inside the estuary due to resonance (Fortunato et al., 1999). During extreme conditions, other forcings may also be important. High river flows can increase water levels in the riverine stretch of the estuary (Vargas et al., 2008) and stratify the water column . During storms, wind, atmospheric pressure and surface waves can also increase the water levels significantly .
The upper part of the estuary is affected by natural hazards with different meteorological and oceanographic origins, often 140 with relevant socio-economic impacts. Droughts can result from extremely dry periods aggravated by the impact of the water management practices in the Tagus river basin. These water scarcity events significantly reduce the river flow reaching the estuary and consequently increase the saltwater intrusion, as observed in 2005 and 2012 (Rodrigues et al., 2019). The vulnerability of the water for human consumption, in terms of quantity and quality, was assessed for the EPAL (Public Water Supply Company) water intake located in the Tagus estuary upper sector (Valada do Tejo) for different climatic 145 scenarios (Rodrigues et al., 2012). Both the results of that study and those of Rodrigues et al. (2019) suggest that only very low river flows would lead to a significant increase of the salinity in the area. Historical data show that the Tagus estuarine margins are also vulnerable to floods from two different origins that can widely affect the agricultural lands due their low https://doi.org/10.5194/nhess-2020-422 Preprint. Discussion started: 5 February 2021 c Author(s) 2021. CC BY 4.0 License. elevation (Freire et al., 2016;Rilo et al., 2017): extreme water discharges in the Tagus and Sorraia rivers (riverine flood), and strong winds and low atmospheric pressure conditions combined with high spring tides . 150

Lezíria Grande Public Irritation Perimeter
The Lezíria Grande de Vila Franca de Xira Public Irrigation Perimeter (Lezíria Grande) is an important economic agricultural area, located on the Tagus estuarine eastern margin, about 40 km from the estuarine mouth (Fig. 2). This very productive area with 13,420 ha of alluvial soils of both fluvial and estuarine origins belongs to the Metropolitan Area of Lisbon and is part of the municipalities of Vila Franca de Xira and Azambuja. The Lezíria Grande occupies low elevation 160 terrains, between mean sea level (MSL) and 2 m above MSL, reclaimed from the estuarine bed and protected from flooding by a 62 km long system of dykes, along the margins of the Tagus, Sorraia and Risco rivers. The dykes are made of soil covered by vegetation and in some places their outer flanks are protected with riprap. Available topographic data indicate that the dykes crest reach heights between 2.4 and 7.2 m above MSL. The southern area of the Lezíria is part of the Tagus Estuary Natural Reserve. The Lezíria Grande has a relevant impact on the local and regional economies, with an annual 165 The Lezíria Grande presents a complex irrigation and drainage system network of channels 720 km long that are connected to the adjacent rivers (Tagus, Sorraia and Risco) by water intakes and drainage gates. The main water intake that supplies the freshwater for the farmland irrigation is located in the Tagus River, at Conchoso, and includes a pumping station (Fig. 2).
The total irrigated area is about 10,000 ha, 60 % of which are irrigated by surface irrigation and 40 % under pressure.
As the Conchoso water intake is located close to the upstream limit of the salinity propagation, the availability of water with 175 quality for irrigation strongly depends on the freshwater input from the Tagus River into the estuary. Because the effect of droughts in the freshwater input usually starts in July, the critical month for irrigation, crops can be lost, with relevant economical losses. During the most recent severe droughts, in 2005 and 2012, several emergency measures were undertaken in the Lezíria Grande to minimize the negative impacts, such as the water supply exclusively from the Risco river water intake and the construction of a temporary weir at the Sorraia river. The installation of the pumping system at the Conchoso 180 water intake, allowing the extraction of water from the Tagus River during low tide, and the construction of a removable weir in the Risco River are recent improvements to increase the resilience to droughts.
Due to its low elevation terrains, the Lezíria Grande is vulnerable to flooding episodes of both riverine and estuarine origins.
High water discharges of the Tagus and Sorraia rivers can promote dyke breaching and extensive agricultural lands inundation as occurred in February 1979 (Rebelo et al., 2018). During this event, the Lezíria Grande dyke was ruptured both 185 in the north and south sides, originating either displaced or evacuated people and relevant economic losses. About 2,000 people were reported to have been affected due to dyke failures in the surrounding area (Loureiro, 2009).
Estuarine high water levels caused by spring tides and severe storm surges can also overflow and damage the Lezíria Grande dykes, promoting extensive inland inundation. The most dramatic estuarine flood event that affected this area, destroying all the channels and dykes, occurred on February 15, 1941 (Madaleno, 2006). This event affected several locations along the 190 Portuguese coast with devastating human and physical impacts (Muir-Wood, 2011;Freitas and Dias, 2013) and is considered the major calamity that affected the Iberian Peninsula in the last 200 years. Besides the severe damages in infrastructures, the impacts in the upper estuary include human losses and drowned cattle (Muir-Wood, 2011). The most recent estuarine flood event that affected the Lezíria Grande occurred on February 27, 2010 resulting from the passage of the storm Xynthia in the Portuguese territory (André et al., 2013;Fortunato et al., 2017). The dykes in the southern area of the Lezíria Grande were 195 overflown and damaged and the farmland flooded. As the event occurred out of the active farm season, witnesses report that only up to 5 families and some cattle had to be evacuated. After the event the dykes were repaired and elevated in some places. and Rural Development (DGADR), acted as risk owner as they are the most representative stakeholder. The overall risk management objective of the Lezíria Grande is the management and exploitation of a public irrigation infrastructure during extreme weather conditions. These conditions can be aggravated by climate change effects, namely more extreme droughts 205 and floods. For the present application and considering the saltwater intrusion hazard, the risk management objective is to ensure water with good quality at the Conchoso intake, i.e. water with salinity below 1 psu, during the agricultural irrigation campaigns. Despite the natural conditions that contribute to the droughts, the water resources management practices at regional and local levels affect this objective. At the regional level, the volumes discharged from Spain during exceptional meteorological conditions and the EDP hydropower production regime are the main conditioning factors for the water 210 availability downstream. The adaptive capacity of the farmers, such as improving the adequacy and efficiency of the irrigation practices, changing the type of cultures, and increasing emergency planning and response capability are examples of local water resources management factors. Concerning the high water level hazard, during estuarine floods, the specific risk management objective considered is to avoid the dykes overflow and damage, preventing inland inundation. Due to the Lezíria Grande low topography, the dyke integrity is crucial to protect the farmland, support facilities and infrastructures 215 from being inundated and damaged, not only during extreme events but also daily during high tide. This objective can be reached by flood adaptation measures, including raising the dykes height, as decided after the 2010 flood event, increasing the area of salt tolerant crops, and increasing emergency planning and response capability.

Consequence and likelihood criteria
For the water salinity at Conchoso during droughts, which results from the upstream saltwater propagation, the consequence 220 is evaluated using the consequence descriptor presented in Eq. (1), considering the water unavailability for irrigation during the month of July as it is the most critical for irrigation. Water unavailability is computed on a weekly basis and the minimum weekly water need is considered as 1,029x10 3 m 3 , which corresponds to the worst case scenario based on historical needs (Aqualogus/Campo d'Água, 2016). The usable volume of water is estimated by multiplying the time during which water with salinity below 1 psu is available at the Conchoso intake per week by the maximum pumping capacity at the 225 Conchoso station. The Conchoso pumping rate capacity considered was 4.5 m 3 s -1 , which corresponds to the pumping rate capacity with low water level (Aqualogus/Campo d'Água, 2016a). This criterion is justified by the absence of reservoirs in the Lezíria and it is assumed that the water is used as soon as it is abstracted. The severity grade criteria of this consequence were defined based on the past occurrences and their consequences (Table 1).
During the most recent droughts, the consequences were more severe in 2005 than in 2012 as less water was available. In 230 2005, fresh water was unavailable at the Conchoso water intake from mid-July onwards. The water was therefore supplied to the Lezíria exclusively from the Risco River water intake and the consequences were very severe with significant losses of crops (Rodrigues et al., 2016). Thus, severity is considered low when less than 1 % of the water needed is unavailable for irrigation, leading to negligible losses of crops. The severity is considered medium when 1 %-25 % of water is unavailable, https://doi.org/10.5194/nhess-2020-422 Preprint. Discussion started: 5 February 2021 c Author(s) 2021. CC BY 4.0 License.
while high severity corresponds to 25 %-50 % of water unavailable for irrigation. Very high severity corresponds to over 235 50 % of the water unavailable, which can lead to very significant losses of crops and, consequently, economical losses.
For the high water level, the consequence is evaluated based on the descriptor presented in Eq. (2) affecting the risk management objective is to avoid the dykes being overflown and damaged. This criterion was defined based on the impact of the dyke repair cost on the risk owner annual income (Table 1). Severity is considered low when the dyke repair cost is less than 1 % of the annual income, which corresponds to twice the dyke annual maintenance cost.
Considering that the impact of the February 2010 storm event, which was about 4 % of the annual income, has a mediumlow severity, the upper limit of this class is defined as 10 %. Very high severity consequence is considered when the dyke 245 repair cost exceeds 30 % of the annual income. The likelihood criteria for both hazards are presented in Table 2.   High -H 0.5-1 1-2

Risk criteria
For the water salinity, risk tolerance limits are defined based on the water availability and the possibility to fulfil the needs from alternative water sources (Table 3). Risk is considered low when the water available at the Conchoso water intake is sufficient to meet the irrigation needs. Thus, the criterion followed to define the upper limit of the low risk is: the water 255 unavailable for irrigation is less than 1 % for high likelihood events (i.e., events with a return period RP=1 year). Medium https://doi.org/10.5194/nhess-2020-422 Preprint. Discussion started: 5 February 2021 c Author(s) 2021. CC BY 4.0 License. risk level, which corresponds to the tolerable risk, corresponds to events during which the water available at Conchoso cannot meet the irrigation demands, but the Risco River can be used as an alternative source to fulfil the needs. The upper and lower limits of the tolerable risk band were defined based on estimates of the minimum and maximum volumes of water available in the Risco River. The minimum volume of water available in the Risco River was defined based on Rodrigues et 260 al. (2019), which estimates that the volume of water available in the Risco River ranges from 1-4x10 6 m 3 . Considering that the Risco River should provide an alternative water source during one month, the minimum weekly volume available in the Risco River is 0.25x10 6 m 3 , which corresponds to 24% of the total water needs for irrigation per week. The maximum water volume available is determined by the water abstraction capacity. The average abstraction capacity was taken as 0.97 m 3 s -1 (Aqualogus/Campo d'Água, 2016), which corresponds to a weekly volume of 584,558 m 3 (57% of the irrigation needs). 265 Thus, the risk is considered tolerable if the water unavailable in Conchoso is less than 24% (for high likelihood events, i.e., RP=1 year) or 57% (for low likelihood events, i.e., RP=100 years). The risk is considered high when the water available from both Conchoso and the Risco river is insufficient to fulfil the irrigation needs and risk treatment is required. For the high water level, risk tolerance limits are defined based on the potential impact of the dyke damage cost on the risk owner annual profit, measured by the ratio between the repair cost and the risk owner annual income. Risk is considered low when the damage repair cost is negligible relative to the annual income. Thus, the upper limit of the low risk band is defined as: the cost of the dyke repair does not exceed the annual dyke maintenance cost, that represents about 0.5 % of the annual 275 income, for high likelihood events with RP=1 year, and the double for events with low/medium low likelihood (RP=10 years) (Table 4). Medium risk level corresponds to the tolerable risk, i.e., the impact of the dyke damage repair cost on the annual income is tolerable for the risk owner. The lower limit of the tolerable risk band corresponds to the upper limit of the low risk. The upper limit of the tolerable risk is defined by considering that the dyke repair cost should not endanger the financial viability of the risk owner. The impact of the February 2010 event, already presented, is used to help defining this 280 limit: for high likelihood events (RP=1 year), the risk is considered tolerable if the repair cost does not exceed 4 % of the annual income, and 4 times more in case of events with low/very low likelihood (RP=100 years) (Table 4). Above the tolerable risk upper limit, risk is considered high and unacceptable, and in this case risk treatment is required whatever its cost to reduce the risk level.

Water salinity
As stated above, the salinity at the Conchoso intake depends mostly on the Tagus River discharge and on the water management practices in the Tagus watershed. The size and strong artificialization of the watershed, shared between two 290 countries, make the hydrologic modelling a complex and time-consuming task far beyond the scope of this work. The capacity of flow regularization in the Spanish part of the basin reduces the average flow at the Spanish-Portuguese border by 27% (Aus Der Beek et al., 2016). Therefore, hazard scenarios were constructed based on available data and on past event information. July is the critical month for crop irrigation and the upper salinity limit for irrigation is 1 psu. Considering these conditions, and to provide a wide range of events for the risk assessment, five scenarios of Tagus river discharge were 295 established (Table 5). A scenario combining the worst recent drought (SD2) with the possible sea level rise of 0.5 m was also considered. This value is representative of the prediction for the end of the 21st century considering the Representative Concentration Pathway (RCP) scenarios RCP2.6 and RCP8.5 (Rodrigues et al., 2019). For all scenarios the likelihood was estimated based on relevant historical events and on probability estimates, and follows the criteria already presented in Table 2. Further discussions about the scenarios can be found in Rodrigues et al. (2019). The quantitative consequence 300 descriptor defined previously is assessed for the different scenarios through numerical modelling. Numerical models implemented and validated for the study area are described in Appendix 1. Figure 3 presents the different scenarios projected in the consequence/probability diagram for the water unavailability; the horizontal and vertical bars represent the expected uncertainty for consequence and likelihood, respectively. Consequence is low for all the scenarios in the first week, since the water available fulfils all the needs for irrigation. As time progresses the consequence increases for all the scenarios with exception of scenario SD1 (climatological, mean river flow of 136 m 3 s -1 ), in which water is always available for irrigation. The consequences are also more severe when the river flow is lower, as expected, although very low river flow scenarios (SD4, SD5) have low likelihoods. The estimated consequences for the scenarios agree with the observed occurrences during recent droughts (2005,2012), as described by the risk owner. During July and August of both 2012 and 2005, droughts represented by scenarios SD2 and SD3 respectively, salinity reached 310 concentrations at the Conchoso water intake that were inadequate for irrigation. In 2012, in particular, water with salinity of about 1.1-1.2 was used for irrigation, which reduced the production. However, the adverse impacts of the 2005 drought were more severe for the farmers in the Lezíria, since the drought itself was more severe and the ABLGVFX had fewer resources and was less prepared to deal with these events. More severe consequences are also estimated for scenario SD3 comparatively to scenario SD2 (Fig. 3). Since the consequence of all the scenarios is estimated based on numerical 315 simulations there is an associated uncertainty. To estimate the uncertainty of the consequence, the maximum difference between the data and the model results at the peak salinity (2 psu) was used and the estimations described previously were performed considering the water salinity <3 psu. Results suggest that its influence on the consequence severity is higher for low river flow scenarios, and in some cases, consequences can range from "Very high" to "Low". However, it should be noted that the criteria used to define the uncertainty corresponds to the maximum peak difference, which explains the larger 320 variability in the consequence. Regarding the risk diagram, results indicate that for all the scenarios with exception of the climatological scenario (SD1) the risk is intolerable in the last week (Fig. 4), showing that the risk increases with the duration of the droughts. When the low river flows occur for several consecutive weeks, even using the Risco River as an alternative source of water for irrigation is not sufficient to meet the water needs for irrigation. Thus, for events similar to these scenarios, risk treatment is mandatory to reduce risk level and may include the use of alternative water sources, the selection of alternative crops, the reduction of 330 the irrigated area and/or investments regarding water storage.

High water level
Estuarine high water levels are forced by spring tides and severe storm surges, which are associated with very low atmospheric pressure conditions. Based on the past extreme events of 1941 and 2010, described in Sect. 3.2, that caused overtopping of the Lezíria Grande dykes and inundation of agricultural lands, four scenarios of extreme water levels were 345 defined (Table 6). Extreme water level conditions of the scenarios result from the oceanographic and meteorological conditions of the events. The same sea level rise scenario for the end of the century used for the salinity was considered here (SF4), combined with the storm surge and tide conditions of the 1941 cyclone (SF2). The scenarios were assessed through numerical models implemented and validated for the study area and that are described in Appendix 1. The model estimates of the extent of dyke overflown entails uncertainties associated with several error sources. The tidal levels predicted by the 350 model have errors of the order of 15 cm in the upper estuary, while errors associated with the storm surge can reach about 10 cm . Topographic errors, in particular, in the dykes' crest height, were taken as 10 cm. Taking the overall error as the square root of the sum of the squares of the individual errors leads to a vertical uncertainty of 20 cm. To determine the uncertainty in the estimate of the overflown dyke length, we considered that a difference of 50 cm in water level between two simulations (scenarios S2 and S4 described below) leads to a discrepancy of 130 % in the overflown 355 extent of the dyke. Assuming a linear relationship between the horizontal and vertical dimensions, the uncertainty in the estimate of the length of the dyke overflown is 50 %. Table 6. Scenarios for maximum water levels considering different storm surge, tide and sea level rise (SLR) conditions. W m is the maximum water level at Cascais tide gauge and Q is the Tagus river flow. For all high water level scenarios, the area where the dyke is potentially affected is located in the southern half of the Lezíria (Fig. 5). In scenario S1, about 1 km of dyke near the Lombo do Tejo island is affected. In the scenario SF2, the dyke is affected in the same zone but the length doubles. When a spring tide is considered (scenario SF3) the length of the affected dyke increases up to 4 km, extending the affected area to north of the Alhandra island and to the southern extreme of the Lezíria. The length of the potentially affected dyke increases to 8 km if sea level rise is considered (SF4). 370 Figure 6 (a) presents the different scenarios projected in the consequence/probability diagram for the relative cost of dyke damage (RCDD). Again, the expected uncertainty for both consequence and likelihood is represented by horizontal and vertical bars. The consequence severity of the scenarios with low (SF1) and medium severity (SF2) is consistent with the known impacts of the 1941 and 2010 events, which were much higher in 1941, as described in Sect. 3.2. Medium severity, corresponding to a dyke repair cost of 1 to 10 % of the ABLGVFX annual income, can be reached for low likelihood 380 scenarios with RP between 10 and 100 years. The consequence severity is "high" (repair cost is up to 30% of the ABLGVFX annual income) for the very low likelihood scenario (scenario 3, RP>100 years). In this case, besides the 1941 storm surge conditions, an extreme tidal range is considered (equinoxial spring tide). Very high consequence severity, expressed by the dyke repair cost over 30 % of the ABLGVFX, is reached if sea level rise is considered (SF4). Limitations of the model can underpredict the severity level. The model was run with a fixed geometry, i.e., the bathymetry and topography were assumed 385 to remain unchanged during the simulations not considering dykes' erodibility. In reality, events of this type can erode and breach the dykes at several locations, as actually occurred in February 2010, increasing the potential dyke length affected.

Scenario
Because the 1941 scenario is more energetic (in terms of wind speed, water currents and waves), the breaching should also be more severe. Hence, the length affected during this scenario is probably more underpredicted by the model than for the 2010 scenario. None of the scenarios considered has an associated low risk ( Fig. 6 (b)), i.e., the dyke is not overflown. This 390 is explained as this risk is not associated with average oceanographic and meteorological conditions. In all scenarios without sea level rise, the risk conditions are moderate (tolerable level), indicating that risk has to be monitored regularly to decide if adaptation measures have to be taken to reduce the risk level. However, as sea level rises, risk will become unacceptable.
Hence, risk treatment will be required in the future to bring the risk down to an acceptable level. The risk assessment approach presented in this study intended to integrate the hazard dimensions that most affect agricultural areas located in estuarine margins. Highly dependent on water resources, agriculture is one of the economic sectors most vulnerable to climate change effects (Aleksandrova et al., 2016). Its vulnerability increases when agricultural areas are 405 located in estuaries where changes in hydrological regimes and sea level rise can impact both salt water landward intrusion and low-lying areas inundation (Kimmerer and Weaver, 2013). The main challenge of the approach developed herein was to find suitable consequence descriptors of the two hazards that incorporate scientific-based data but can easily be applicable by the risk owner and be updated in time. For this, the difference in elements at risk, coverage and temporal scale of impacts for the two hazards were considered in the definition of consequence descriptors for risk assessment. 410 For saltwater landward intrusion due to droughts, the water resources availability are the element at risk. The scarcity of suitable water for irrigation has an economic impact for the risk owner, mainly due to crop losses resulting from lack of water or/and salinization of land if salty water is used. Enabling risk owners with tools that anticipate the expected water availability is thus essential to support decision making both before and during the agricultural campaign. An adaptive risk assessment in a temporal scale of weeks during the most critical period for crop irrigation is suitable for helping manage 415 water unavailability.
For estuarine high water levels associated with tides and storm surges, the elements at risk are mainly the agricultural land itself, and infrastructures such as dykes, support facilities, roads and access infrastructures. Damages in those assets and ultimately the loss of agricultural land due to inundation have a direct economic impact for the risk owner. In agricultural lands located in low-lying estuarine areas, dykes or other protection structures are normally present to prevent frequent land 420 inundation during high spring tides. Using the relative cost of dyke damage as a consequence descriptor has the advantage to provide a direct quantification of the hazard economic impact for the risk owner, and to be easily estimated through information that the risk owner normally has access to.
Due to the uncertainty of the factors that control both risks, a continuous consequence/probability diagram was found to be the most adequate technique for risk level estimation and evaluation, as it integrates the uncertainty in the process. In 425 addition, this tool is suitable for communicating the risk in a simple way to the risk owner. The applicability of the developed approach was explored through the application in the Lezíria Grande agricultural area, known to be affected by those two hazards. Results show that concerning fresh water scarcity, the risk increases with the duration of the droughts and when low river flows occur for several consecutive weeks, even using the Risco River as an alternative source of water for irrigation is not sufficient to meet the water needs. The total dependence of irrigation on the Tagus and Sorraia fluvial 430 discharges, with other users upstream, suggests that previous knowledge of the water availability reserved in Spain and Portugal and the consumption expected for the different sectors upstream is essential in assessing the risk of fresh water unavailability for irrigation. Real-time knowledge of the upstream discharges, existing consumptions and possible runoff https://doi.org/10.5194/nhess-2020-422 Preprint. Discussion started: 5 February 2021 c Author(s) 2021. CC BY 4.0 License. from the rice crops, particularly those located along Sorraia River, will definitely contribute to decision-making regarding the best periods for estuarine water intake. 435 Considering the estuarine inundation, the results presented above show that presently the risk in Lezíria Grande is moderate.
The hazard can be significant, but only for very extreme events with a high return period. However, sea level rise will increase the risk. Hence, the risk owner should consider risk reduction measures, as they will become necessary in the future.
Furthermore, the sea level rise considered herein was based on the 5 th IPCC assessment report (IPCC, 2014). Since that report was published, several studies indicate that sea level may rise faster than anticipated (Shepherd et al., 2012;Khan et 440 al., 2014;Scambos and Shuman, 2016;Seo et al., 2015;Martín-Español et al., 2016;Kopp et al., 2017). Hence, the possibility that the 0.5 m rise in sea level used in scenario SF4 is reached long before the end of the century should be considered. Finally, the uncertainty in both the probability and the consequence are large. Further studies and data collection should therefore be conducted to reduce these uncertainties. Examples include considering dyke breaching and simulating the combined effect of river floods and storm surges (Zhang et al., 2020). 445 Differences in the temporal scales of both risks have an impact on the time horizon of risk assessment and consequently on the selection of possible actions to be taken to reduce risk. Results highlight the differences between the hazard consequences of the two risks for the risk owner, with different extent and impact level depending on the hazard severity.
The fresh water scarcity can have economic and even social consequences at other risk management levels, as farmers, agroindustry and local communities, particularly if production is severely affected in quality and quality having impact in related 450 trade and services. Besides the economic impact for the risk owner, inundation can have consequences for farmers if the agricultural land loss is high. Considering the context of the study area, a broader impact of the consequences in agroindustry and local communities can be considered negligible.
The risk assessment approach application in the study area raised some challenges. The definition of both consequence and risk criteria have to be based on in situ knowledge and historical information. Even if the risk owner has most of the 455 information required, other relevant data are often dispersed in different institutions requiring their aggregation and a prior informed-analysis. The definition of hazard scenarios is another important point to be considered when this approach is applied. As stated before, hazard scenarios have to be anchored on past events information. Valuable information about historical events can be found in a variety of sources, including databases where systematized data are suitable for supporting risk assessment . Several global and national disaster databases are available (e.g. EM-DAT, 460 2013; DISASTER database, Zêzere et al., 2014) but their resolution is inappropriate for local scale analyses. Regional and local databases are scarcer (e.g. Rilo et al., 2017) but should be used and their development encouraged. The choice of events for the scenarios definition should cover a wide range of consequences and probability, to provide a suitable risk spectrum. Whenever possible scenarios construction should consider the main controlling factors of the hazard severity (e.g. river discharge, maximum water level, and sea level rise). Monitoring information is crucial in supporting risk management. 465 Timely information will allow the updating of consequence and risk criteria, and hazard scenarios, and will support mitigation and adaptation strategies definition. https://doi.org/10.5194/nhess-2020-422 Preprint. Discussion started: 5 February 2021 c Author(s) 2021. CC BY 4.0 License.
As main conclusions, this study presents a risk assessment approach that can be replicated in other agricultural estuarine areas. The approach incorporates scientific-based knowledge of the hazard processes and is suitable to support decisionmaking at a local level. The consequence descriptors considered can be adapted according to local specificities and updated 470 in time to reflect the evolution of hazard, exposure and vulnerability conditions. At first sight, the extent of the information required to the approach application can be pointed out as a limiting factor. However, the complexity level in both consequence evaluation and criteria definition can be adapted to the available information and tools. Complex numerical models can be used, as in the application to Lezíria Grande presented herein, giving greater scientific robustness to the results. In the absence of this possibility, consequence evaluation and criteria definition can rely on expert judgment 475 supported by past events information. Finally, the risk assessment approach showed to be appropriate to support the discussion of potential mitigation and adaptation measures for risk level reduction, mainly when the possible impact of climate change in risk levels is considered. As future work, the approach is foreseen to be applied to other estuarine agricultural areas and the possible incorporation of further discussion from stakeholders.
Code and data availability. The model SCHISM is publicly available at https://github.com/schism-dev/schism.git. The 480 model input files and data are not provided due to the confidentiality of the data.
Author contribution. Conceptualization of the risk assessment approach: PF, MR and ABF, with contributions from AF.
AF obtained the local data and other information from the risk owner. Application to study area: MR implemented the numerical model, performed the simulations and treated the results for water salinity; ABF implemented the numerical model and performed the simulations for water levels, and PF treated the results for risk assessment. Discussion: all authors 485 contributed. Manuscript preparation: PF prepared the manuscript with contributions from all authors.
Competing interests. The authors declare that they have no conflict of interest.