Water restrictions under climate change : a Rhone-1 Mediterranean perspective combining ‘ bottom up ’ and ‘ top-2 down ’ approaches 3

Drought management plans (DMPs) require an overview of future climate conditions for ensuring long 13 term relevance of existing decision-making processes. To that end, impact studies are expected to best reproduce 14 decision-making needs linked with catchment intrinsic sensitivity to climate change. The objective of this study 15 is to apply a risk-based approach through sensitivity, exposure and performance assessments to identify where 16 and when, due to climate change, access to surface water constrained by legally-binding water restrictions may 17 question agricultural activities. After inspection of legally-binding water restrictions (WR) from the DMPs in the 18 Rhône-Méditerranée (RM) district, a framework to derive WR durations was developed based on harmonized 19 low-flow indicators. Whilst the framework could not perfectly reproduce all WR ordered by state services, as 20 deviations from socio-political factors could not be included, it enabled to identify most WRs under current 21 baseline, and to quantify the sensitivity of WR duration to a wide range of perturbed climates for 106 22 catchments. Four classes of responses were found across the RM district. The information provided by the 23 national system of compensation to farmers during the 2011 drought was used to define a critical threshold of 24 acceptable WR, related to the current activities over the RM district. The study finally concluded that catchments 25 in mountainous areas, highly sensitive to temperature changes, are also the most predisposed to future 26 restrictions under projected climate changes considering current DMPs, whilst catchments around the 27 Mediterranean Sea, were found mainly sensitive to precipitation changes and irrigation use was less vulnerable 28


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Baseline climate data were obtained from the French near-surface Safran meteorological reanalysis (Quintanachanges and on critical thresholds above which the system starts to fail to identify possible maladaptation strategies

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The risk-based framework adopted contains three independent components (

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up framework is to define performance metrics and associated critical thresholds relevant for the system 229 of interest. In the case of our study, these thresholds will make it possible to distinguish duration of 230 water restrictions, which are unacceptable for users,

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(iii) Exposure, as defined by state-of-the-art regional climate trajectories superimposed to the climate 232 response surface. The exposure measures the probability of changes occurring for different lead times 233 based on available regional projections.

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All the components of the framework together contribute to the vulnerability of the system (including its 235 management) to systematic climatic deviations.

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The sensitivity analysis was conducted applying a water restriction modelling framework. Climate conditions 237 were generated applying incremental changes to historical data (precipitation and temperature) and introduced as  complexity, only the irrigation water use will been examined here, since it is the sector which consumes most 260 water at the regional scale, with a critical threshold defined for this single water use.

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Exposure to changes here is measured using regional projections, visualized graphically by positioning the 262 regional projections in the coordinate system of the climate response surfaces and identifying the associated likelihood of failure relative to Tc. Note that, to update the risk assessment, only the exposure component has to 264 be examined (including the latest climate projections available onto the response surfaces).

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The conceptual lumped rainfall-runoff model GR6J was adopted for simulating daily discharge at 106 selected

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finally introduced in the GR6J model an exponential store in parallel to the existing store of the GR5J model.

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Considering additional routing stores is consistent regarding the natural complexity of hydrological processes, and

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The GR6J model has six parameters to be fitted (Fig. 5

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A WRL(d) time series is created as the median of wrl(t) for each 10-day period;

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The WRL(d) value is set to zero if preceding 10-day precipitation total exceeds 70% of inter-annual 339 precipitation average( precipitation correction).

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The WRL modelling framework was verified in the 15 evaluation catchments (Table 1)  14 suggesting that more than 85% of the observed non-events were correctly simulated by the WRL modelling 365 framework. The median of WRL Sensitivity score with HYDRO is around 45%, indicating that for half the 366 catchments, less than 45% of observed events are detected based on HYDRO discharges, but this raises to 68% of 367 events detected when WRLs are simulated based on GR6J discharge. Using GR6J is more effective for detecting 368 legally-binding restriction than using observed discharges while it is less efficient for predicting periods without 369 restriction for most of the catchments. There is a compensatory effect, which is not easy to detect graphically since 370 Sensitivity scores are more sensitive than Specificity scores due to the reduced number of observed days with 371 adopted restrictions. No evidence of systematic bias associated with catchment location or river flow regime was 372 found: northern (blue) and southern (red) catchments are uniformly distributed in the Sensitivity/Specificity space.

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Sensitivity and Specificity scores using HYDRO as benchmark in the contingency

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Simulations were made on near pristine catchments and thus water uses are unlikely to be the main reason. Other 390 causes of higher Sensitivity scores obtained when simulated discharges are used as input have been investigated in 15 score using HYDRO, demonstrating that the choice of the monitoring indicator and regulatory thresholds is 394 probably not involved. A "smoothing" introduced by the hydrological modelling was also suspected but 395 autocorrelation in observed and GR6J simulated VC3 time series was found very similar. Future works may re-396 investigate these aspects. They will need to explore new ones (e.g., the way WRL is derived from the daily values 397 wrl for each 10-day period) using a longer verification period with not necessary uniform but fixed regulatory    ].
(2) 438 with P0 and T0 + AT mean annual changes in precipitation (1) and temperature (2)  and to extend the prospective scenarios over the RM district. Thus, the water restriction modelling framework 477 considers, in this application, the "Business-as-usual" scenario, which assumes that only minor change in water catchments. Despite unrealistic, maintaining the current conditions allows assessing the impact of climate change 480 regardless of any other human-induced changes. The advantage is that results are easier to understand and to 481 embrace by stakeholders than those obtained with complex multi-sectorial scenarios they may not identify with.

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The response surfaces are exemplified on three of the 15 evaluation catchments (Table 1, Fig. 9):

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The Argens River basin, along the Mediterranean coast, severe low-flows occur in summer and actual 500 evapotranspiration is limited by water availability in the soil,

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The Ouche River basin, in the northern part of the RM district, has a typical pluvial river flow regime under to summer climate conditions dominating changes in low-flows.

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The visual inspection of response surfaces shows that:

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To further the regional analysis and help sensitivity assessment at un-modelled catchments, basin descriptors

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The risk-based framework has been applied to the irrigation water use since annual net total water withdrawal 588 for agriculture purposes is ranked first at the regional scale. Note that in the Rhône-Méditerranée district around 589 90% and 10% of water used for irrigation originate from surface water and groundwater, respectively. To 590 complement water needs irrigators may also have access to small reservoirs (storage capacity usually less than 1 591 Mm 3 ). Most of the reservoirs are filled by surface water in winter and release water later in the following summer.

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Water restrictions are not imposed to these reservoirs but it is assumed here that during severe drought events the 593 majority of them are empty and thus the existence of potential sources auxiliary to surface water on the conclusions 594 has limited influence on the conclusions.

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We assumed here that irrigated farming is globally under failure if the duration with limited or suspended 596 abstraction is above a critical threshold Tc that causes insufficient water for crops. The catchment or area i will be 597 considered more vulnerable than the catchment or area j if the likelihood of failure (i.e., exceeding Tc) for 598 catchment or area i is more than the likelihood of failure for catchment or area j. The critical threshold Tc is a value 599 of total number of days with legally-binding water restrictions that needs to be fixed. To move closer to reality and   and 2071-2100 were superimposed to the representative response surfaces to assess the risk of failure (Fig. 4).

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Finally the vulnerability resulting from the combination of the three components sensitivity, performance and

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The map displaying the class membership could be a convenient tool for local authorities to discuss the spatial (see the southernmost department in Fig. 10). Therefore, any investigation on DMPs at the department level 702 disregarding this heterogeneity will be biased. The sensitivity analysis provides information for local authorities 703 to better understand the differences in catchment responses to observed droughts in areas, which fall within their 704 responsibility. For instance, water management in basins of Class 4 could be more problematic during a year with a severe heat wave while it could be more problematic for a year with a pronounced precipitation deficit for 706 catchments of Class 1. It is likely that the differences in the impact of droughts on WR will persist if stakeholders 707 do not question the assumption of a uniform definition for the hydrological indicators within the department.

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DMPs have been recognized in the PNACC as relevant water management tools and our findings have also 709 implications for adaptation strategies. We have shown that the climate change effects could be felt more acutely adaptation strategies on additional water storage may lead to maladaptation since natural inflows will probably 721 decrease, and delay the mutation of agricultural practices and conservation measures. In addition, there is actually 722 no guarantee that these reservoirs will be filled and that their storage capacity will be enough to cope with severe 723 droughts.

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The RM Water Agency has taken other the objectives of PNACC at the regional scale and has initiated an

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The analysis of the past and current DMPs in the RM district shows a decision-making processes highly 739 heterogeneous in terms of both low-flow monitoring variable and regulatory thresholds. In reality, the WR 740 statements follow a set of rules defined in the DMPs (which can be simulated and reproduced automatically) but 741 also expert judgment or lobbying from key stakeholders -which are not accounted for in the WRL modelling 742 framework put in place here. However, the post-processing of GR6J outputs allows detecting more than 68% of 743 severe alerts (more severe than level 1), making the developed framework a useful tool. Our study is a first step 744 towards a comprehensive accounting of physical processes, but does not capture socio-economic factors, also 745 critically important and reaches out to interdisciplinary for completing the modelling framework designed here.

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The study at the regional scale illustrates an expected difficulty to simulate accurately a regulatory framework.

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Further improvement is not expected in enhancing hydrological models but in reproducing decision-making 748 processes. The overall performance could be improved by scrutinizing the minutes of the drought committees to 749 better understand the weight of the stakeholders in the final statement.

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The sensitivity analysis and the related response surfaces suggest that basins located in the Southern Alps are 751 the most responsive basins to climate change and that those experiencing a high ratio P/PET are found the less 752 responsive. The classification method CART has been applied to 106 responses surfaces associated with 106 753 gauged basins and leads to four classes with different sensitivity. The key-variables known at un-modelled but 754 gauged catchments can be introduced in the decision-tree to finally predict the assignment as a first guess to one 755 of the four classes. Water managers are thus encouraged to monitor in priority and more accurately temperature and/or precipitation when and where the sensitivity of their catchments is found the highest. This may mean efforts 757 to reinforce field instrumentation within these key catchments.