Cooperation under conflict: participatory hydrological modeling for science policy dialogues in the Aculeo Lake

: Hydrological modeling tools can support collaborative decision processes by visually displaying hydrological systems connections, uncertainties and gaps, as well as conflicting preferences over water management strategies. Nevertheless, many challenges remain in the real 10 application of these technical tools to successfully implement, capture, and communicate to non-experts the complexities of coupled human hydrological systems and effectively support science policy dialogues. The disappearance of a 12 km 2 lake in the Aculeo basin in Chile led to increasing socio environmental conflicts over the causes and effects of the water scarcity. A traditionally WEAP based hydrological model study aiming to explore the causes of the lake desiccation, was 15 transformed into a multiple question driven socio hydrological modeling process to help answer the diversity of questions instigating conflict. The surface aquifer hydrological model tested a subset of socially accepted management strategies under two climate change scenarios, showing that combining more low impact, but socially acceptable adaptation measures such as using the out of season irrigation surplus (March to May), improving irrigation efficiency for agriculture 20 industry and decreasing the grass surface in new urbanizations, would allow to recover up to half the Lake water volume even under a pessimistic climate change scenario. As presented in this article, flexible approaches and research agendas could better support the exploration of synergies towards collaboration and production of useful and socially acceptable hydrological models and water management strategies. 25


Introduction
Sound science is necessary to support decision making where population, economic and climate change have aggravated conflicts over water (Poff et al., 2016(Poff et al., , 2003. However, on top of the scientific uncertainties impacting water stationarity (Galloway, 2011;Kiparsky et al., 2012;Milly 30 et al., 2008), societal complexities make water related problems "wicked", given their competing and mutually exclusive deep human values and aspirations that are not resolved with technical and economical strategies (Nie, 2010). Addressing water problems from a technocratic and solely governance point of views, is to disregard the political and transformative power of water (Boelens et al., 2016;Melsen et al., 2018). In this challenge, the hydrological scientific community is 35 searching for approaches to better show the social ecological interconnections (Mauser et al., 2013;McMillan et al., 2016;Salter et al., 2010), and the different types of knowledges that can contribute to a science policy dialogue (Nardi et al., 2021).
However, science policy interactions and processes, are neither simple nor straightforward (Hegger et al., 2012;Scott et al., 2012). First, science society collaborations require non-scientists 40 with a deeper understanding of the complex interactions and behavior between the natural, economic and social processes (Pahl-Wostl, 2006), and a better understanding of the uncertainties and risks attached to the scientific process (Jasanoff, 2003;Röckmann et al., 2012), a requirement that has been found to be difficult to address by scientists (Kahan, 2010;Nisbet, 2009;Somerville et al., 2011), as well as a main barrier to collaborative efforts towards engaging diverse 45 stakeholders in decision making (Larson et al., 2009). Then, science society collaboration requires scientists that can not only adjust the scientific process and results to different timeframes, and different needs of science policy interactions (Rice et al., 2009), but also recognize the deeply political impact of communicating scientific results (Budds, 2008;Dabelko, 2005).
In this science policy challenge, there is an academic and practical need for a diversity of 50 photographs showing the before/after outcome, as well as of trucks distributing water, were used in diverse national and international media to discuss climate change and water governance in Chile 1 . To respond to the water scarcity, and specifically the Aculeo Lake desiccation causes and 80 possible solutions questions, it was necessary to develop a basic surface-aquifer hydrological model. The hydrological modeling study collided and strongly collaborated with a participatory process called the Voluntary Agreement for Watershed Management (AVGC in Spanish) -a dialogue process usually implemented by the Chilean Government in the context of Climate Change international agreements. Consequently, the traditional hydrological modeling process had 85 to be transformed into a collaborative modeling process in order to confront and include as most opposed views as possible.
In this article, the modeling of strategies and results are described and explored. Additionally, insights from behind the scenes during the Aculeo Lake modeling process are presented as a guideline for participatory modeling and transdisciplinary efforts in contexts of high conflict and 90 poor information on the hydrological system. This is important in Chile, as the combination of surface and underground hydrological modeling software described in this article is implemented in National watershed management to develop the first set of Strategic Planning at the Watershed Level throughout Chile. Therefore, the importance of this study to shows how to use this tool to work towards effective collaboration and mutual learning, while disseminate their usefulness for 95 transdisciplinarity in hydrological and water resources management.

The Aculeo water crisis
The Aculeo Lake basin is a 200 Km 2 sub basin of the Maipo River in the Metropolitan Region, Central Chile (Figure 1), a mainly agricultural zone 50 km south from Santiago, the capital of Chile, but also, was one of the most iconic touristic hotspots of the Metropolitan region of the 100 5 country. Runoff from several creeks from the upper basin (2000 m.a.s.l.) flow into the Aculeo Lake located in the middle of the valley. Around 526 mm of rainfall is the annual average (data from 1960-2016), 94% of which is received in autumn-winter (April to September) and 6% is spring-summer rain (October to March). This is a heavily intervened basin, as many agricultural basins in Central Chile (more on this below). The lake should naturally drain to the Aculeo creek 105 (also called Santa Marta or Santa María) towards the Huiticalan creek, but a small detour infrastructure keeps water from naturally flowing. Agriculture and livestock have been the main productive activities in the Aculeo valley, since 1660 when this was a private large state (hacienda). During this time, the Aculeo Lake basin also went through a physical transformation from a natural basin to an anthropogenic watershed with a series of channelization for different productive activities. The first Agrarian Reform in 1962 Law No. 15,020, redistributed land among peasants until the military coup of 1973 (Bellisario, 2007;115 Órdenes and Díaz-Diego, 2018). During the military government, the 1981 Water code transformed water in shares to be transacted in a free market (Bauer, 2004;Madaleno and Gurovich, 2007), given by the General Water Directorate (DGA) to anyone who asks as long as there is availability. The 1981 Water Code had slight adjustments in 2005 with the establishment of water ecological flow restriction for new water rights, a fee in the case of non-use of water rights 120 and the obligation to report transactions on water rights, but its essence is still primarily market based.
The Aculeo Lake drying process started in 2010, until it went completely dry in 2018. The megadrought (Garreaud et al., 2017(Garreaud et al., , 2019, has manifested as a sequence of years with 25% to 45% precipitations deficit affecting Central Chile since 2010 in terms of reduction in streamflows 125 and increasing evapotranspiration processes. Nevertheless, several uncertainties remained related with the human management factor in the water scarcity. In this regard a hydrological modeling study was commissioned to explore the potential causes of the lake desiccation (Barría et al., 2021).
The hydrological study used a surface (WEAP software) and groundwater (MODFLOW software) hydrological model to explore possible solutions to restore the Aculeo Lake or alleviate the water 130 scarcity.
As it should be expected, the lake desiccation exacerbated conflicts among users, including additional indirect conflicts, such as livestock wandering and later dying because of the lack of water, creating problems for ecosystems and later major sanitary issues. This ecological and social problem, led the DGA to announce in 2018 a set of measures to reverse the situation of the lake, 135 ensure human consumption for the population living nearby, and strengthen the monitoring and control of water extractions. Following, the Sustainability and Climate Change Agency (ASCC in https://doi.org/10.5194/hess-2021-643 Preprint. Discussion started: 17 January 2022 c Author(s) 2022. CC BY 4.0 License. Spanish) was mandated by the Environmental Ministry to work on an AVGC. The AVGC is a process in which different private and public organizations (including academia 2 ) voluntarily work to identify different actions and strategies in which to collaborate to address a basin challenge (e.g. 140 a private company may support a civil organization to find funds for water conservation). Water scarcity was one of those problems in discussion, which also further pushed interaction with the hydrological modeling study. Although these two processes were designed to be implemented in parallel, giving the level of conflict and large number of uncertainties, we saw an opportunity actively participate in the AVGC process and move towards a more transdisciplinary hydrological 145 modeling.

WEAP hydrological model
The surface-groundwater model used for the hydrological balance analysis was the Water Evaluation and Planning System (WEAP) (Yates et al., 2005a(Yates et al., , 2005b. WEAP is a semi distributed 150 hydrological modeling software tool that uses nodes and links in a graphical platform, allowing to incorporate inputs (e.g. climatic information, land use/land cover) and obtain outputs in a watershed (e.g. evaporation, infiltration, extractions), to estimate the water balance and simulate different possible scenarios. This system has been used in participatory processes to study climate change adaption options (Bhave et al., 2014), ecosystem services assessment (Yates et al., 2005a) 155 or the economic impact of water agricultural policies (Varela-Ortega et al., 2011). The aquifer and its connection to the lake and other catchments, is represented by a node that gathers its hydrogeological characteristics, analyzed as part of the hydrological study project (Barría et al., 2021).
The development of the WEAP model, and the water balance is explained in detail in two technical 160 reports (Barria et al. 2019, Bluedot, 2020, as well as in a peer-reviewed paper that explains the causes of the Aculeo Lake desiccation (Barría et al., 2021). To perform the water balance, the WEAP model uses as input data the historical configuration of the land use/land cover (LULC), and its changes, in addition to historical climatological information. Also, the hydrological modelers need to properly translate the conceptual understanding of the hydrology into the model. Hydrological modeling was very difficult given scarce information and inconsistencies between official and non-official information, therefore visits and interviews were used to collect, contrast 185 and/or incorporate on the ground information to complement official sources. One of the main challenges, among many, was the short fluviometric records in the basin (i.e. less than seven years for only one station), the non-existence of previous detailed groundwater studies. For more details regarding the WEAP model construction, calibration, validation, the water balance and the aquifer information, we would like to refer the readers to Barria et al. (2019), Bluedot, (2020), and Barria 190 et al., (2021).

Iterative approach to hydrological modeling of the Aculeo basin
Following the methods section is divided in two main moments that guided the modeling process: 1) discussions on the problem causes, and 2) discussion on the solutions. Although both questions are related, as the elimination of cause can be considered a solution, this article only expands in 195 the participatory evaluation and hydrological simulation of solution strategies related with water management. Results from the climatic 'causes' (e.g. precipitation decrease due to megadrought) modeling process along with climate scenarios sensitivity analysis, are better described in Barria et al., (2021).

Hydrological model construction 200
In the line of the participatory modeling literature and the categories of participation by (Basco-Carrera et al., 2017), the Aculeo Lake modeling followed a consultation during the modeling stage and a discussion during the scenario analysis, going back to a co-design to refine the model structure and input used in the modeling stage ( Figure 2). Besides the basic elements of the hydrological model required by the modeling environment (e.g. Digital Elevation Model, inflows 205 and outflows records, among others), the participation of this team as members in the discussions of the Voluntary Agreement group, resulted in a diversification of questions and possibilities for the parallel modeling process. On one hand, these instances were used by the modelers (this article authors) to identify additionally questions that were pressing and causing suspicion and conflict among participants, such as belief that water diversions upstream was not only illegal, but also 210 causing the lake desiccation. Second, these instances were also used by stakeholders to question and challenge the accuracy and validity of the hydrological model being constructed, which in turn forced the modeling team to challenge the official information and search for alternatives sources and approaches to find answers, such as the official versus the real number of water wells. In this process, contrasting opinions on both the causes of the water scarcity and the need to rehabilitate 215 the Lake hinted at the level of underlying tension in this community (Ocampo-Melgar et al., 2021).
Contrary to Bhave et al. (2014) where the objective was to evaluate alternatives, in this project the larger effort was to understand the hydrology to construct the model to evaluate causes of the water scarcity. Modeling water management alternatives was a necessary addition after constructing the model, given the level of conflict and scientific uncertainties that both teams (AVGC and 220 hydrological modeling) encountered when exploring collaboration towards facing water challenges. Voluntary project participation in those collaborative meetings were then, a mean to refine the model, but also to combine local with scientific knowledge to corroborate that the model was answering the right and required questions for the collaboration endeavor. The iterative interactions were essential to co-produce a hydrological model and list of potential water 225 management strategies inspired in the continuous stakeholders' questions and local knowledge, as well as critical questioning to the hydrological tool and modeling process as a political and decision making instrument.
Finally, to evaluate the sustainability of the strategies, a business as usual climate change scenario (Representative Concentration Pathway RCP 8.5, Van Vuuren et al., 2011) and a moderate 230 scenario (RCP 4.5) were also considered in the water balance simulations. To assess the effectiveness of these strategies, a reference scenario, without considering them (without management strategies) under climate change was also implemented.

Figure 2.
Main questions and iterative science society stages that fed the modeling process in 235 terms of design, but also in model resolution and critical inquiry of information gaps.

What management strategies could alleviate the crisis?
As the hydrological model structure was being built with input from the stakeholders, a participatory evaluation of the different management strategies that could reduce the water balance deficit was conducted. A list of eight water management strategies (e.g. water use by the 240 agricultural sector), as well as four institutional support ideas management strategies were collected from the AVGC debates and extreme positions presented by the different stakeholders.
Even though some of these "solutions" did not have a legal, nor social support or possibility of being implemented, it was decided to let the stakeholders consider their pertinence. This list was presented to 25 individuals from nine stakeholders groups participating in the AVGC process 245 (Table 1) to elicit their interest or concerns about the strategies (Table 2). During these interviews, stakeholders were requested to comment and suggest changes, as well as to give a 1-3 value to each idea, where 1 was a very bad idea, 2 an acceptable idea, and 3 an excellent idea. Variations and considerations on each ranking were also gathered to make sure the wording of each strategy was understood. 250 Table 1. Stakeholder groups interviewed in the Aculeo Lake basin. Tourism camp sites owners Private areas around the lake rented for camping and recreation. Water for human consumption, as for gardening irrigation and pools came from private wells, as they are not usually associated with the APRs.

Livestock association members
Arrieros or transhumance livestock producers (an old tradition in mountain ranges following the seasonal change of pasture and water).
In Aculeo, as in many parts in Chile, arrieros do not own water nor land, but pay an old "tribute" to landowner in the form of non-paid day of work per head of cattle passed through their land.
New building sector Organizations of either real state groups or new built areas ("condominium" of second homeowners that populated areas around the lake). These organizations have their private anonymous societies for water distribution that own underground water shares. However, the lake was important for recreational activities and affected their real state value.

What is the impact of those solutions in the Aculeo water scarcity?
Once an initial hydrological structure for the Aculeo basin was complete (inflows and outflows), a subset of the "Action/Strategies" presented in Table 2, were simulated in the WEAP Aculeo 255 Basin Model: • Action 5. Agricultural irrigation efficiency: The improvements in agricultural water efficiency is commonly used in Chile to increase irrigation area. The strategy here is to increase water efficiency, but without increasing agricultural area. The interviews allowed to corroborate that industrial agriculture already has high irrigation efficiency, but there 260 still are important amounts of annual crops with water efficiencies of 50%. Two scenarios were simulated in WEAP, Scenario 1 increased the water efficiency of annual crop to 70%, while Scenario 2 increases to 85%.
• Action 6. Rural house grass gardens reduction: An increase of second home owners during the last decade coincided with the water crisis, generating discomfort and suspicion 265 among traditional inhabitants about their preference for grass gardens, being the cause of the lake disappearance. Eliminating rural house gardens was perceived by the hydrological modeling group (the authors), as a good strategy before the interviews. However, people that used to work in other agricultural activities are currently working as gardeners, hence eliminating gardens might have a quite big impact. After interviews, simulation used 270 recommendations by Bown and Fuentes. (2019), which is 20% of the gardens with grass, 30% the shrubs of intermediate consumption and 50% of the surface with cacti species, stones, or xerox type garden, with no irrigation. This scenario incorporates both the need to reduce water consumption and the peoples need of not eliminating grass due to the impact it may have to their jobs. 275 • Action 10. Elimination of water diversions: This watershed has a long history of occupation, which has resulted in numerous manmade water structures, some are not even known by its owner due to their antiquity. Historical aerial photography and legal information were analyzed on a specific manmade stream diversion channel up in basin that according to a conservation with a social movement, through those diversions some 280 people had been stealing water from the watershed. Although from a water balance perspective the diversion (legal or not) is an extraction old enough to be considered, there were sensible tensions that needed an answer. The diversion channel was visited, followed by interviews with the Agriculture and Livestock Service and review of 1956 property documents borrowed from the Real State Conservative, confirming the legality of those 285 water shares. Even though diversion was confirmed to be legal, and eliminating it means the expropriation of Water Use Shares (WUS), an extreme measure in Chile given that water shares are real state asset, the strategy was simulated to respond to stakeholders' concerns. This potential strategy considers the diversion of water not being used in some months of the year by other nearby basins (which would make it feasible without actually 290 expropriating WUS) in two inflow scenarios, one with 700 l/s and the other with 1,000 l/s, from March to May. In fact, there is a project currently being under study called "Aguilino Chanal" that may have a similar goal.
Other strategies, although important to improve management, were not contributing in a significant 295 way into the overall water budget (actions 1, 2, 3, 4, 12 in Table 2), or were not possible to simulate given the level of information needed (action 8). In other cases, actions were not simulated due to not acceptance by stakeholders. This was the case of industrial farming prohibition, which the majority of the interviews showed to be a none acceptable strategy, due to its economic impact.

Participatory evaluation of water balance strategies
Individual interviews and group discussions with stakeholders showed that the most "extreme" solutions may not necessarily have the majority support, even without considering the possible legal and technical limitations. In Table 2, a summary of individual rates or evaluations of each strategy is presented. 305

2) Support for the Rural Drinking Water
Organizations that provide drinking water "It is necessary to avoid another water crisis."

100%
There are institutional and financial changes that have been assessed for the State agency to better support rural organizations

3) Implementation of underground Water Communities (Comunidad de Aguas Subterráneas CASUB in Spanish)
"It is necessary but does not solve the illegal water extractions problem."

89%
Is the formal organization to manage aquifer water resource. There are experiences in other basins. While this project was being conducted, the Water DGA was starting conversations on this line.

4) General Water Directorate (DGA) strengthening
"It is necessary, but also to define new and different roles than only inspection and fining."

89%
There are institutional and financial changes that have been assessed in different reports for the DGA to better conduct their supervision role in Chile.

5) Agricultural water use efficiency
"It is necessary, although it does not solve the problem of those small agricultures who quit, due to the drought." 100% It requires investment on infrastructure and human capabilities.

6) Urban water use efficiency (rural house grass gardens elimination) 3
"It is necessary but should not be mandatory nor extreme. Better to support voluntarily reductions as it will affect jobs for local people."

77%
It has been applied voluntarily by individuals, but never as a Municipal regulation.

7) Industrial farming prohibition
"This is a terrible idea that will affect agricultural jobs."

44%
No precedents for this type of measures. Except in cases where the market forces a crop change for economic reasons.

8) Proration of water shares between legal users
"It is fair and necessary. Although it does not consider illegal water extractions."

100%
It is a strategy being used in several basins in Chile to exchange water between users when one of them does not need it.

9) Intra basin transfer by connecting water diversions for human consumption in critical season
"It is not a solution for human consumption as it does not have enough quality. Could be a solution to have some water in the lake." 100% There is a project and different ideas, but none has been applied before.

10) Elimination of water diversions (buying water shares)
"It can be unconstitutional to expropriate water shares. Should only be bought and for human consumption only."

100%
This type of strategy has been carried out in severe cases of water scarcity and overexploitation of water bodies, where the State has had to buy water shares to meet conservation or drinking water needs, as in the case of the Ligua-Petorca basins.

11) Water transfer from other basins
"It is the only solution for the lake, but it can be very conflictive as there are no other basins nearby with extra water."

89%
There are some small projects in the same basin, but no transfers from other basins have been done before. It will require large agreements and if done it will have social and environmental impacts.

12) Water reuse
"It is a survival strategy that has been already implemented voluntarily; but does not solve the problem."

89%
It requires financial support for infrastructure and human training.
Additionally, to the percentage of low bad rating of some of the cases (e.g. water reuse), it is important to notice the representativeness of the stakeholder group that gave the low rating (e.g. 310 may be an important economic sector), as well as the justifications given for their evaluation, as this information points out to aspects that could facilitate their implementation and avoid future conflict. This was the case of strategies that are implemented at a household scale, and therefore could require government support to alleviate the economic burden mentioned by citizens. At the same time, results showed that sometimes the most mentioned or publicized strategy while in the 315 heat of conflict, may be recognize as a not very good option while discussed in private (e.g. industrial farming prohibition).

Simulation of water balance strategies
As described in section 3.1, a hydrological and a hydrogeological model were implemented to contribute with water balance information under different water management scenarios (Barria et 320 al., 2021). Although there are 12 strategies gathered and evaluated (Table 2) during the participatory activities, only three of them are directly related to the water balance of the basin and could be modelled by the WEAP tool. Therefore, according to section 3.2.3, the participatory modelling considers: 1) the diversion of water not being used for some months of the year by the agriculture users of the Aguilino Channel (Aguilino), 2) Agricultural irrigation efficiency, 325 increasing the efficiency of annual crops from 50% to 85%, and 3) Rural house grass gardens reduction, implemented as follow: 20% of the gardens with grass, 30% the shrubs of intermediate consumption and 50% of the surface with cacti species, stones, or xerox type garden, with no irrigation. Figure 3 shows the lake water volume simulated under the historical, and climate change scenario 330 as a result of the water balance of the basin. We used approximately 100 general climate models (GCMs) simulations from ~30 models, under the RCP 4.5 and RCP8.5 scenarios (Van Vuuren et al., 2011), which were bias corrected using the Quantile Delta Mapping method (QDM, Cannon et al., 2015).
According to the simulations, by implementing the three adaptation measures under the historical 335 climate ("Hist. adapt.", panels a, b, e and f of Figure 3), the volume of the lake would increase to around 34 Mm 3 (56% of the lake volume) during the Megadrought. Note that as presented by Barria et al. (2021), the Aculeo Lake is completely desiccated since year 2018. Comparatively, although the three strategies contribute to the water balance, as presented in panels c) and d) of Figure 3, the linkage to agricultural channel adaptation measure, has the largest contribution to the lake water volumes under the climatic scenario. Among the three strategies, the contribution of the Aguilino for the 2050-2100 period under the RCP8.5 scenario, represents about 83% of the increase in the water volume, which is around ten times more than the contribution of 355 the agricultural water use efficiency and the garden water use efficiency.
Although the two strategies oriented to improve the irrigation efficiency in the basin have a comparatively lower impact on the water balance at the basin scale, they can be part of a combination of strategies socially accepted by the stakeholders proved by the high ranking obtained, which could contribute to increase awareness of the complex water scarcity problem and 360 to develop preparedness under climate change and its impacts. allowed for the identification of a combination of strategies that are of moderate impact, but of higher local acceptability than the large structural options. On the contrary, a solely top-down hydrological modeling would not have considered less efficient solutions, due to their relative 375 more moderate impact in terms of the lake. From this experience, we have insights for science society initiatives involving hydrological modeling under limited information, and when underlying conflicts may demand a more collaborative process: a. Addressing the right questions: Co-producing knowledge can help scientists to understand how their science could be more useful to respond stakeholders' needs to 380 different decision scales (Corburn, 2003). The participatory modeling of the Aculeo basin showed that there is important information that can be extracted from local users to improve model accuracy and usefulness. For example, the Aculeo process showed that information (in this cases strategies preferences) obtained by individual interviews result very different than group conversations when discussion are used to show power 385 over other stakeholders, while in individual conversations, stakeholders may smooth their comments. However, there is also a large chance of finding very different and contradicting information that is impossible to contrast, such as when there is a fine legal interpretation in between, or where there are no possibilities to confront the reality due to lack of data or time and funds constraints. 390 b. Managing uncertainties: For some authors, the process of making the uncertainty transparent weakens the model usefulness and creates misunderstanding between scientists and non-scientists (Hisschemöller et al., 2001;Webler et al., 2011). In our experience, it is better to discuss uncertainties as early as possible, as assumptions and information gaps will show eventually. Including local decisionmakers in the process 395 of finding alternative solutions to those gaps may help empower them and make them participant of the modeling. As proposed by many authors before, complex problems where there is a high uncertainty in knowledge, distribution of power and ambivalent goals, are better approached through participatory and deliberative methods to discuss different possible narratives, than a single participatory modeling or top down policy 400 approaches (Pellizzoni, 2012;Stirling, 2006;Wise et al., 2014).
c. Approaching positions: The use of hydrological models for supporting decision making in the Aculeo participatory process, showed that model acceptance was not related with model complexity. The acceptance and legitimacy of model increased once stakeholders noticed their knowledge and opinions were incorporated, which may or 405 may not increase model complexity. When opinions are later confronted with the modeling results, an increased understanding may help bring closer originally opposite opinions, as it has been previously showed in deliberative processes that focus in understanding the problem and learning about different facts and values, while avoiding never ending discussion on the scientific uncertainty (Hermans et al., 2007). 410 However, as we found, in some cases, when conflicts have surpassed a dialogue limit, and when many economic and political interests are involved, not even the best available science will make their positions closer. while avoiding choosing a side. Nevertheless, as we found in this experience, maintaining neutrality is extremely difficult in situations of high conflict, especially in these times, as expressed by Pellizoni (2012), when science and technology are charged with growing social expectations, but are greeted with equally growing skepticism or hostility. Especially when the decision is over a politically contentious 420 topic with uncertain science, there is the high probability that lack of credibility ("we don't believe this"), legitimacy ("the process has been corrupted"), and/or salience ("science answered the wrong question") of the results complicate the use of science for policy making (Cash et al., 2006). As Vogel et al. (2007) stated, "When scientists neglect-even if unintentionally-the political and strategic nature of scientific 425 knowledge, and the political context in which it is produced, they can be faced with uncomfortable and challenging situations for whose navigation many are ill-equipped".
Recognizing the political role of the hydrological model is part of making the process and us, as researchers, better at identifying better ways to address science society dialogues. 430 Main lessons from this research are summarized in a five steps participatory modeling engagement guideline to use when conflict and uncertainties are high. This is only a guideline for hydrological modelers, but not for the whole participatory process that should have a proper facilitation and different instances for dialogue and deliberation. As suggested by the literature on participation and science production, this guideline aims at being as flexible, horizontal, 435 transparent and open dialogue as possible, to co-produce a hydrological model that is accepted by the community, and later used for water resources management (Fraser et al., 2006;Garcia et al., 2016;Pellizzoni, 2012;Scott et al., 2012;Voinov and Gaddis, 2008).
• Step 1. Collect questions: In an initial stage, it is important to collect all pressing questions 440 that stakeholders would like to have an answer and a discussion about the limitations and capabilities of the hydrological model. No questions should be eliminated due to an initial presumed lack of information. Only differentiate plausible from impossible questions to answer, to reduce the list to those that are related with a socio hydrological modeling, otherwise this type of participatory processes can become an endless process. This step is 445 important to set the basis and limitations of hydrological modeling, but also to understand the context.

•
Step 2. Divide questions on stages for the conceptual modeling development and for the simulation stage: remaining questions need to be grouped and organized in variables for the model, in terms of Causes, Links and Solutions. It is necessary to consider enough 450 flexibility to be able to modify the conceptual model later, as other inquiries or questions can arise in the process that may change our conceptual view of the basin. In this step, regardless of the pressure, it is important to be unbiased and avoid rising expectations on the identification of "culprits", as this does not contribute to the necessary dialogue, nor represents the complexity of socio hydrological systems. 455 • Step 3. Evaluate information availability and quality: Once questions have been reduced and sorted, it is important to evaluate if there will be enough official or scientific information for the model. It is important to include stakeholders in this step by letting them know the information gaps, as well as the alternative methods and assumptions that have to be adopted to be able to proceed. This will help the hydrological model acceptance 460 within the community, as well as helping in opening doors to alternative information that the community or other stakeholders may have that one did not originally knows, which helps both finding a technical solutions and the overall acceptance of the process.

•
Step 4. Share model design and assumptions: Once data has been evaluated, there are assumptions to be made to proceed with the simulation process. Communicating scientific 465 uncertainties and gaps can be used to test possibilities with them, for example, what scenarios should be tested and how. Claiming that there is not enough official information, will not be enough when conflict is high.
• Step 5. Communicating and discussing results: modeling results may not leave everybody happy, as there are conflicts underneath that an answer won't diffuse. As 470 Goleman (1989) has discovered once an idea is conceived, of how the world works, one tends to incorporate information that validates that idea and discard contradictory information, hence people whose original ideas does not match the results will tend to discard your study. In this case, understanding the context, conflicts, perceptions and values involved are important to design the model, but also to find the most sensitive way 475 to inform results (Abels, 2007). In many cases, modeling will be a bureaucratic step in a political decision defined earlier (as when modeling is used to finance a reservoir), then understanding the impact of our results is as important as being scientifically unbiased (Babidge, 2016;Budds, 2008).

480
The participatory modeling implemented in the Aculeo basin was key to navigate throughout the complex situation, understand and recognize local actors' opinions, and concerns in the model structure, and in the identification of strategies. Specifically, the participatory identification and evaluation of strategies allowed to better adjust the hydrological model to answer questions that were causing suspicions and further conflicts among stakeholders. The same process was also 485 important for the identification of a combination of strategies that were of moderate impact, but of higher local acceptability than the large structural options. The surface-groundwater model tested a subset of socially accepted management strategies under two climate change scenarios, showing that combining more low impact, but socially acceptable adaptation measures such as using the out of season irrigation surplus (March to May), improving irrigation efficiency for agriculture 490 industry and decreasing the grass surface in new urbanizations, would allow to recover up to half the Lake water volume even under a pessimistic climate change scenario. Possibly, a solely topdown hydrological modeling would not have considered less efficient solutions, due to their moderate impact in recovering the Lake.
However, as the experience was not completely successful in terms of engagement, this article 495 also shows that hydrological modeling requires now, more than ever, funding transdisciplinary approaches both in its construction, but also in its application as it is key to achieve boundary objects and credible, salient, and legitimate processes for decision making. As exposed in the Aculeo basin, especially when contentious water related conflicts are high and attribution of climate change impacts are uncertain, collaboration in the hydrological modeling process is key in 500 finding management options that could contribute to both answering the problem, as well as to understand the conflict. This, however, may not be necessarily enough to reduce conflicting positions, that are constantly stirred according to personal interests.