A Simplified Water Accounting Procedure to Assess Climate Change Impact on Water Resources for Agriculture across Different European River Basins

Abstract

European agriculture and water policies require accurate information on climate change impacts on available water resources. Water accounting, that is a standardized documentation of data on water resources, is a useful tool to provide this information. Pan-European data on climate impacts do not recognize local anthropogenic interventions in the water cycle. Most European river basins have a specific toolset that is understood and used by local experts and stakeholders. However, these local tools are not versatile. Thus, there is a need for a common approach that can be understood by multi-fold users to quantify impact indicators based on local data and that can be used to synthesize information at the European level. Then, policies can be designed with the confidence that underlying data are backed-up by local context and expert knowledge. This work presents a simplified water accounting framework that allows for a standardized examination of climate impacts on water resource availability and use across multiple basins. The framework is applied to five different river basins across Europe. Several indicators are extracted that explicitly describe green water fluxes versus blue water fluxes and impacts on agriculture. The examples show that a simplified water accounting framework can be used to synthesize basin-level information on climate change impacts which can support policymaking on climate adaptation, water resources and agriculture.

1. Introduction

In many regions in Europe, climate change impacts on water resources are threatening the sustainability of the agricultural systems [1,2]. The increased occurrence of drought and water scarcity is predicted in many regions throughout Europe [3,4,5], and recent events over the last ten years have demonstrated that drought episodes typically for Southern European countries are expanding to Eastern and Western Europe [6,7]. Thus, policies and legislation are needed to mitigate the related risks and to adapt to climate change impacts [8,9]. For this, European-level decision makers need information on how climate change impacts affect water resources for all sectors, particularly agriculture, especially in the most drought-prone and/or water scarce regions in Europe.

The European Union (EU), in coordination with its Member States, is committed to support the implementation of the United Nation’s (UN) 2030 Agenda for Sustainable Development, and it strives towards improved water efficiencies and climate adaptation, among other targets, as are stated in the Sustainable Development Goals (SDGs). The European Water Framework Directive also requires solutions for water scarce conditions. One of the major goals of EU policy is to make agricultural water management across Europe more efficient [10,11], although it is not always clear how efficient water use shall be achieved. While some stakeholder groups think of solutions in terms of irrigation efficiencies, e.g., [12], others rather think of solutions in terms of crop water productivity, e.g., [13], reuse of drainage water, e.g., [14] or irrigation with wastewater, e.g., [15,16] for which legislation is currently being drafted by EU member states. Whatever the strategy is, there is an urgent need for information at the European level to feed into reports and monitoring efforts on past performance. Even more importantly, however, policy-relevant indicators on water availability, use, and water stress are needed for the future, under consideration of climate change [17,18].

To assess climate change impacts on water resources across Europe, several assessments have thus far been done using large-scale hydrological models [19,20,21,22,23]. These global models have the advantage that information is generated based on one single methodological approach, which makes it relatively easy to compare between different locations, e.g., [24]. On the other hand, they typically require lots of computational resources, have rather larger grid sizes (typically 10–50 km) and do not always converge, e.g., [25,26]. A more important limitation is that global approaches do not include local details in the water resources system [18] that can sometimes be crucial, including dams, diversions, groundwater abstractions, water harvesting schemes, spring water use, sub-surface drainage systems, and specific vulnerable water users with high priorities (wetlands, etc.), which often challenge drought monitoring and management [27,28]. On-farm irrigation and drainage systems are usually strongly simplified in these models or even entirely absent with no distinction between micro and flood irrigation systems. Additionally, these modelling systems, due to their global nature, do not consider local knowledge (from local experts and stakeholders) on the already occurring impacts or on the adoption and impact of national or EU regulations, such as the EU Water Framework Directive.

River basin-level climate change impact assessments are nowadays available for many regions in Europe [29,30,31,32]. Often, these studies are done or commissioned by the local water authorities to support the river basin management plan [33,34]. However, these studies are hardly useful for European-level decision making, simply because the outcomes of these studies are not reported to Brussels. Additionally, the chosen indicators are oriented towards the local decision-makers.

Accounting frameworks allow for a wide variety of data to be synthesized so that regular information and indicators are produced and can feed into decision-making processes. The underlying principle of these approaches recognizes that while there may be discrepancies between different data sources as well as data gaps, decisions need to be made based on the best-available standardized information. Eurostat has adopted the international standard for environmental accounting, which is the System of Environmental-Economic Accounting (SEEA). It was produced and released under the auspices of the United Nations Statistics Division and also includes accounts for water [35]. Several of the accounts in this framework (CO₂ emissions, energy flows, etc.) are already obligatory for EU countries. Eurostat has plans to develop water accounts, and some experiments are being done on the national level, e.g., [36]. On the other hand, the European Environmental Agency has also piloted several studies at the basin-level, which yielded accounts for several basins in Spain [37,38,39,40,41] and Italy [42]. Outside Europe, Australia has already successfully implemented a water accounting framework and uses it actively to support decision-making [43,44].

However, these efforts have thus far focused on reporting past water resources, yielding very comprehensive, detailed, and location-specific data-intensive studies. Only very few basin-level studies exist in which water accounting methodologies were applied for future conditions [45,46]. For describing future conditions and assessing local options to make water resources management more efficient and sustainable, water accounting has hardly been used. Given the uncertainties in future predictions, it is recommendable to use a simplified water accounting framework instead of an original one that relies on measurements and includes high levels of detail which are redundant in future studies. Additionally, a simplified framework will make the tool more accessible and interpretable by a large group of users.

The objective of this work is to demonstrate the application of a single water accounting framework to synthesize outcomes of climate change impact studies for five different European basins. The focus lies on future water resources for agriculture (green and blue) and the presentation of outcomes by means of a candidate set of impact indicators.

2. Materials and Methods

2.1. The Water Accounting Framework

A water accounting framework which is increasingly used and supported by the United Nations Food and Agriculture Organization and the Asian Development Bank is called Water Accounting Plus (WA+) [47,48,49]. This framework was largely inspired by the pioneering work of water accounting conducted by Molden [50], which used a water balance approach to classify inflows and outflows into various categories to provide information on the quantity of water depleted by various uses and the amount available for further use. The WA+ framework was designed to be mainly fed with the remote sensing of local water management and farming practices to assess water availability and water use by land use types. The inclusion of remote sensing data lowers the requirements on ground-based data and is a good example of standardized data collection and sharing. The active program of the European Space Agency (ESA) on agricultural water management is an essential asset (Goetz et al., 2017).

The simplified framework proposed here was based on the comprehensive WA+ framework, because of (1) the land-use and agriculture focus and (2) its emphasis on the distinction between green and blue water. The resource base sheet from Molden (1997) and WA+ was re-designed, simplified and finalized during a workshop in which all experts from included river basins participated to make sure that the framework was versatile and straightforward enough to be applied to the various case study basins. The modifications principally consisted of the aggregation of certain flow and stock components in order to limit the redundancies, based on the principal of parsimony.

The proposed water accounting framework makes uses of the increasingly used terminology green water and blue water, originally coined by [51] to distinguish, respectively, (1) soil water that is available for evapotranspiration and comes from direct rainfall and (2) water that is available for evapotranspiration and other consumptive uses by withdrawals from surface or groundwater sources. The main components of the resource base sheet are shown in

Table 1. The main components of the resource base sheet.

Inflows	Outflow	Others
Precipitation Interbasin transfer Upstream tributaries Lateral groundwater inflow Desalination	Green water evapotranspiration Blue water evapotranspiration Utilizable outflow Committed outflow Environmental outflow Non-utilizable outflow Interbasin transfer Lateral groundwater outflow	Total water withdrawal Agricultural water withdrawal Surface storage change Subsurface storage change

. Appendix A provides a description of the components.

As in the WA+ framework, the evapotranspiration (ET) flows of the resource base sheet make distinctions between key land-uses types:

• Protected natural area (PNA)
• Non-protected natural area (NNA)
• Rainfed cropland (RC)
• Irrigated cropland (IC)
• Other managed water use (industry, services, households) (OMW)

More details on the water accounting framework can be found in Appendix A.

2.2. Case Study Approaches for Five River Basins

This work includes five river basin case studies in Europe (see Figure 1). A summary of the basins is given in

Table 2. Summary table of the case study basins.

Abbreviation	Case Study Basin	Country	Climate (Köppen Classification)	Total Area (km2)	Agricultural Area (%)
						SEG	Segura	ES	Semi-arid (BSk)/Mediterranean (Csa)	18,870	45
JUC	Jucar	ES	Semi-arid (BSk)/Mediterranean (Csa)	22,187	35
LCO	Lake Como	IT	Humid continental (Dfb)	4500	31
MES	Messara	GR	Mediterranean (Csa)	400	63
RHI	Delta Rhine (only NL)	NL	Temperate oceanic (Cfb)	25,347	55

. Four of the basins are in Southern Europe. Three of the basins have a Mediterranean climate and are water scarce. All case study basins have a relatively high share of land used for agriculture. In the Mediterranean basins, agriculture consumes most of the available water (green and blue). In the Lake Como basin, these values are relatively low, as the study area includes the wide Alpine catchment of the lake (about 4500 km²) with the irrigation district located in the downstream part of the system and covering an area of about 700 km². In the Delta of the Rhine basin, most water is used to maintain water levels and flush the system, mainly in order to prevent peat dike breaches to reduce salinity intrusion. Green water is mostly used by rainfed cropland, as the largest part of the agricultural area is rainfed.

Each study basin has its own hydrological model and data sources. The simulations, analysis and synthesis into the water accounting framework for each study basin were performed by the local experts that managed the local toolset. For the historic period, the hydrological models were calibrated with 20–30-year meteorological data (rainfall and temperature) and corresponding streamflow observations. In the study basins where water resources are highly regulated, simulations from the hydrological model of river flow were then used as input into a water resources system model. For the climate change scenarios, the models were forced with future climate projections from climate models (details in next section).

Details on the modelling and data sources per basin can be found in Appendix B. Here, only a summary is given for each river basin. For the Segura river basin, the Spain02 meteorological dataset was used to simulate the historic period using the hydrological model SPHY (Spatial Processes in Hydrology). The simulated stream flow from SPHY was used in the water resources system model WEAP (Water Resources and Evaluation Planning) to simulate water demands, supplies, storage, and releases from the reservoirs. More details can be found in Appendix B.1.

For the Jucar basin, data from the rainfall-runoff model PATRICAL [52] for the baseline period 1980–2012 were used: The same data that were used as for the Jucar River Basin Management Plan. The SIMGES module (Andreu et al., 2007) from the AQUATOOL Decision Support System Shell (DSSS) (Andreu et al., 1996; Andreu and Ferrer-Polo, 2009) was used to simulate supplies, demands, water allocation, and reservoir operations. More details can be found in Appendix B.2.

For the Messara basin, rainfall data from fourteen weather stations were used. The Sacramento hydrological model (SAC-SMA) was used to simulate the hydrological flows after Koutroulis et al. (2013) and Tsanis and Apostolaki, 2009. The water balance of the reference period was validated with observations provided by the Directorate of Water of the Decentralized Administration of Crete. Future projections affecting the water balance were based on the work of Koutroulis et al. (2016). More details can be found in Appendix B.3.

For the Lake Como basin, the water accounts were developed by using an integrated model (for details, see Giuliani et al., 2016) that included three main components: (1) The hydrological model Topkapi-ETH, a spatially distributed and physically based model for the watershed of Lake Como; (2) the Lake Como operational model describing the lake dynamics by a mass-balance equation; and (3) an agricultural district model which simulates the dynamic processes of the Muzza irrigation district including the water balance, crop growth and yield. More details can be found in Appendix B.4.

For the Delta of the Rhine river basin, the water accounts were developed using the Dutch National Hydrological Model (NHM), which contains four coupled models to assess changes in ground and surface water flows (for more information, see Lange et al., 2014). The results were validated with water accounting results of the Central Bureau of Statistics of The Netherlands (Graveland et al., 2017). More details can be found in Appendix B.5.

2.3. Future Horizons and Projections

The water accounts were made for an historic period and two future periods. The exact time-window for the three periods depended slightly on the case study but were within the following time windows:

• Historic baseline (BL): 1980–2015
• Near-future (NF): 2030–2060
• Far future (FF): 2080–2100

All case studies assessed the two “Representative Concentration Pathways” (RCPs) that are commonly used for climate impact assessments: (1) RCP4.5: Emissions peak around mid-century being approximately 50% higher than 2000 levels, then declining rapidly over 30 years, and then stabilizing at half of 2000 levels, associated with moderate population and economic growth; and (2) RCP8.5: Most pessimistic scenario in which emissions continue to increase rapidly through the early and mid-parts of the century. Population growth is high, reaching 12 billion by century’s end. The scenarios assessed for all case study basins are summarized in

Table 3. Summary table of the scenarios studied for the case study basins.

ID	Horizon	Emission Scenario 1	Name 1
1	Baseline		BL
2	Near Future	RCP4.5	NF45
3	Near Future	RCP8.5	NF85
4	Far Future	RCP4.5	FF45
5	Far Future	RCP8.5	FF85

¹ For Delta Rhine, only the Dutch Delta Scenario WARM2050 was available, which corresponds to near future RCP8.5.

.

Socio-economic development was considered by including the future demands and population growth, as indicated by the SSP2 (Shared Socioeconomic Pathway 2), also sometimes called the “middle of the road” scenario. Additionally, infrastructural developments that are currently considered in the respective river basin management plans were included in the simulation, as far as information was available.

2.4. Indicators

Based on the water account sheets, a wide range of indicators could be assessed. For this work, a selection was made focusing on indicators that characterize four key aspects of river basin water resources and agriculture:

• Green water availability and blue water availability: Relative change compared to the baseline period.
• Green water use for agriculture and blue water use for agriculture: Relative change compared to the baseline period.
• Blue water dependency-indicators, expressed as blue water availability and use compared to total availability and use
• Water stress-indicators expressed as green and blue water use compared to water resource availability.

Table 4. Definition of the indicators used.

Type	Name	Derived from
Water availability	Total Water Availability	Precipitation + Interbasin transfer + Upstream tributaries + Lateral groundwater inflow + Desalination
	Blue Water Availability	Total water availability − Green water evapotranspiration
Water use	Green Water Use Agriculture	Evapotranspiration from Rainfed Agriculture + Evapotranspiration (only green) from Irrigated Agriculture
	Blue Water Use Agriculture	Evapotranspiration (only blue, originating from field external supplies) for Irrigated Agriculture
Blue water dependency	Blue Water Available versus Total Water Available	Blue Water Available/Total Water Available
	Blue Water Use versus Total Water Use	Blue Water Use (all sectors)/Total Water Use (green and blue)
Water Stress	Green Water Use versus Total Water Available	Green Water Use (all land-uses)/Total Water Available
	Blue Water Use versus Blue Water Available	Blue Water Use (all sectors)/Blue Water Available

shows how these indicators were calculated based on the components of the water account resource base sheet (see Appendix A for a definition of the components).

3. Results

The water accounts (resource base sheet) were developed for each river basin based on the data and toolset used in each basin. The resulting sheets can be found in Appendix C. Based on these accounts, the indicators listed in the previous section were assessed. These indicators are presented and discussed here to illustrate the potential of the water accounting framework to extract policy-relevant indicators from these multiple accounts. The results presented here focus on the relative change compared to the baseline period; for the absolute numbers per river basin, please refer to Appendix C.

3.1. Water Availability

Figure 2 shows how water availability is affected by climate change in the five river basin areas. The figure shows the percentage change of the indicator values for the future periods compared to the baseline (historic) period. The four climate change scenarios (two horizons, two RCPs) are shown.

As can be seen in Figure 2, green water availability will decrease for the three Mediterranean basins (Segura, SEG; Jucar, JUC; and Messara, MES). For the near future, decreases of a few percent are expected, although for the Messara basin, this trend is not clear. For the far future, expected decreases are substantial: Around −20% for the most pessimistic emission scenario RCP8.5. For Lake Como (LCO), no significant trend could be observed, and both positive as well as negative changes are predicted, depending on the scenario. These results mostly depend on the predicted dynamics of glaciers and snowpack, which are expected to melt in the near future, thus increasing water availability, and then disappear in the far future for the RCP8.5. For the Delta Rhine basin area (please note only NF85 was available for this case study), a slightly positive trend was observed related to increased annual rainfall.

Blue water availability is even more affected by climate change in the Mediterranean basins. For the near future, reductions between 10% and 20% are predicted. For the far future, decreases are predicted around −30%. Again, for the Lake Como basin, no negative or positive trend could be observed in this indicator. For the Delta Rhine basin, blue water availability is expected to increase slightly.

3.2. Water Use for Agriculture

Changes in water availability lead to changes in consumptive water use (evapotranspiration) and thus likely to changes in productivity. Hydrological simulations were done for all basins to assess water use across the main landcover classes, assuming no climate adaptation. Climate change impacts on water use in agriculture due to changes in water availability are shown Figure 3. The water accounting framework aggregated flows per main landcover class and distinguished between rainfed and irrigated agriculture. Green water is relevant for rainfed and irrigated agriculture, while blue water applies only to irrigated agriculture. Figure 3 shows the relative change compared to the baseline scenario.

For the near future, changes in green water use are in the order of a few percent, except for the Lake Como basin, where higher decreases are projected. For the far future, the RCP4.5 scenario shows similar decreases as in the near future, but the RCP8.5 scenario shows substantial decreases in water use, ranging from −10% to −20% (for the Delta Rhine basin, a far future scenario was not available).

Blue water use in (irrigated) agriculture decreases slightly more than green water Use. The trends across the river basins are similar to green water use. The reductions in consumptive water use shown here are caused by a reduction in the amount of water that is available to crops, considering the crop growth season and crop water demands. These reductions, if not counteracted with adaptation measures, are typically related to a reduction in productivity and thus imply an economic impact.

3.3. Blue Water Dependency

Irrigated agriculture relies on blue water availability, which in turn relies on total water availability in the basin and upstream green water use by rainfed agriculture and natural land-uses. The fraction of blue water in the total water resources availability is informative for the vulnerability of irrigated agriculture to consequences of climate change. Figure 4 shows how blue water availability and blue water use compare to total water availability and water use for the baseline (historic) scenario and the four future climate scenarios. As an example, a value of 0.2 for the availability indicator would mean that 20% of the renewable water resources availability becomes available as blue water and could thus potentially be used for irrigation or other blue water uses (domestic, industrial, etc.). A value of 20% of the use indicator would mean that 80% of the water could be consumptively used as green water, and 20% could be used as blue water.

Figure 4 shows that for most future scenarios and basins, the values of both indicators decrease slightly relative to the baseline. The availability indicator is only shown for the three semi-arid Mediterranean basins (SEG, JUC and MES): For the other two basin areas, blue water availability is very high (above 0.8), and, thus, changes in this indicator are less likely to affect the agricultural sector. For all three semi-arid basins, a smaller fraction of the total water available is expected to become available as blue water under the future scenarios.

Due to reduced availability, the fraction of total water use that is used as blue water also slightly reduces. For the LCO basin, green water use upstream is expected to increase slightly, causing blue water/total water use to decrease. Again, impacts are likely to be small because water is not a scarce resource. For the Delta Rhine basin area, blue water use is expected to increase because of additional withdrawals from surface and groundwater bodies to meet higher water demands, as well as an increase of blue water evapotranspiration in wetlands.

3.4. Water Stress

Figure 5 shows two indicators that relate water use with water availability. Use-to-availability indicators can be informative for the stress level of a basin and are often used in water resources planning as well as climate impact assessments, mostly for blue water (second indicator in the figure). The proposed framework also allows for the assessment of a similar indicator for green water (first indicator). For both indicators, higher values are indicative for a higher level of water stress in the basin.

Figure 5 shows only the values for the semi-arid Mediterranean basins. For the other basins, the water stress indicator values are relatively low and below the threshold (0.2) at which a basin is typically considered to be in a stress condition [53]. Thus, for the Como and Delta Rhine basin areas, this indicator is not informative on possible climate change impacts on water resources, and other stress indices that consider, e.g., seasonal variability, should be considered.

For the green water indicator, values for the three basins are around 80%, confirming the relatively high share of water consumed by rainfed agricultural systems and natural land-uses in basins in this climate zone. No clear trend could be observed in this indicator, some scenarios show a slight increase, and others show a slight decrease, possibly due to competing trends in evaporation and precipitation under future climate conditions. For the blue water indicator, there are large differences among the three case study basins. For the Segura river, this indicator shows a decrease in the near future, which results from the assumption that non-renewable groundwater abstraction will be eliminated, as required by the European Water Framework Directive. However, for far future scenarios, the stress indicator increases again. For the Jucar basin, a small downward trend could also be observed because of the same reason. For the Messara basin, this value monotonically increases in the future to values higher than 1, indicating water use rates exceeding renewable water availability and, thus, the use of non-renewable water sources.

4. Discussion

A key question for designing policies related to climate change impacts on water resources is how the increased water deficits will partition between green water and blue water. This is important as it can determine, together with information on vulnerabilities, whether policies should focus more on natural areas and rainfed agriculture or on irrigated agriculture and other economic water-reliant economic sectors. The results presented here for the five river basins show that climate change impacts will decrease both green water as well as blue water availability and use for the Mediterranean basins. Decreases in blue water availability and use seem to be higher and more consistent. For the two non-Mediterranean basins, there are no clear trends. For Europe, Orth and Destouni [54] showed that droughts are likely to reduce blue water stronger and faster than they reduce green water; however, they found that this depends strongly on the climate zone, and that in drier climates, this effect is less clear. Other researchers have found similar outcomes [55,56,57,58], although it has appeared to be challenging to untangle the interdependent link with land-use change [59,60]. Water accounts by land use class solves this challenge.

The results for the Mediterranean river basins also show that blue water dependence will increase under climate change. Huang et al. [61] predicted that this will typically happen in semi-arid regions. At the same time, water stress may also increase in particular basins. Indicators extracted from accounts, such as the ones presented here, can be used to inform on targeting measures that are needed to increase the water productivity (WP) of agricultural systems and become more efficient while at the same time not compromising water used for environmental purposes [9,31,62,63]. Spain has dedicated substantial efforts to increase WP in basins like Jucar and Segura, although in some cases, these investments have ultimately led to an overall increase in water consumption [64]. Currently, Dutch aid policy is targeting a 25% increase in WP over the next decade, promoting adaptation measures that increase WP in various countries outside of the EU. A similar policy may be necessary at the EU level in order to mitigate the consequences of climate change on water resources available for agricultural production.

The water stress indicators extracted from the water accounts and presented here give an incomplete characterization of water stress, as water stress should typically be described by a variety of indicators that describe intensity, duration, vulnerability and other features. However, the blue water indicator presented here is often used (in a similar form) as a first approximation of water stress in basins and is one of the key SDG indicators (SDG Indicator 6.4.2). It is also used by the EU for reporting on water stress, referred to as the water exploitation index [65,66]. Vanham et al. [67] gave various recommendations for improving the monitoring of this type of indicators, which included a better recognition of the connection green–blue water and a clear distinction between net and gross abstractions. For future assessments of these indicators, the proposed framework allows for indicators that integrate these improvements.

The main purpose of the results presented here is exemplifying the use of a simplified water accounting framework to synthesize multiple individual climate change impact studies on water resources. Presenting indicators that aggregate several basins in a single figure or table allows for the comparison of trends and the detection of whether trends are consistent among different studies or similar basins. These indicators can also be used to identify hotspot regions that require specific attention. Such hotspot regions can be examined with the same framework, as water accounting is independent of scale. Future water accounts allow for a wide variety of data to be synthesized so that regular information and indicators can be produced and can feed into decision-making processes. At the same time, by definition, accounting recognizes that there may be discrepancies between different data sources as well as data gaps; however, experts that know the subject of study (i.e., river basin) can deal with that and extract the relevant features for decision-making.

Obviously, the indicators here are only a very small subset of what could potentially be presented and be useful for decision-making at the EU level. Even from the current information collected for this work, more indicators could be extracted, e.g., indicators that compare water uses among sectors and land-uses, indicators that compare gross versus net abstractions, and indicators on alternative water sources (basin transfer, desalination, etc.). Additionally, the indicators presented here were based on long-term averages, while climate change impacts on water resources are often felt most during dry spells: These can become more intense or longer, so they can be important to consider when looking at water scarcity [68,69]. Especially in (sub-)humid climates, annual trends can mask seasonal effects that can be very relevant. For example, in the Dutch part of the Rhine basin, annual precipitation is expected to increase in the future scenario, while summer precipitation is likely to decrease. These nuances have yet to be taken into account in the water accounting approach. However, the water accounting framework is flexible enough to select the time periods for aggregation in such a way that it accurately reflects the basin characteristics. It allows for the creation of extracting indicators based on inter-annual and intra-annual information, if relevant. Such indicators can be particularly relevant for determining maximum amounts for allocation to irrigation, not compromising other users and the environment.

Another venue for further extension and improvement is the use of a simplified water accounting framework, as presented here, for assessing the effectiveness of climate adaptation options across multiple basins. A harmonized set of indicators based on standardized water accounts needs to be agreed upon. Additionally, more information on productivity, biomass, and yield can be added to allow for the estimation of future water productivity and assess related interventions. Finally, it is recommended to compare the bottom-up approach presented here based on local tools, knowledge, and phenomena with the typical top-down approach for pan-European assessments using global hydrological and water resources models.

5. Conclusions

This work presents the application of a single simplified water accounting framework to assess the climate impacts on water resources availability and use across several European basins in a standardized way. Such a framework allows for the comparison of various independent and area-specific studies at the river basin scale and studies that integrate key local features and local knowledge that can be essential when assessing and synthesizing climate change impacts across various areas.

Water accounting methodologies are typically applied for reporting on statistical data about the availability of water resources and water use by sectors. This paper demonstrated a simplified framework harmonized across several river basins which used a bottom-up approach where local knowledge, modelling approaches, and datasets were incorporated with a focus on climate change impacts. Top-down approaches using global hydrological models can be ambiguous in decision making because they do not represent fully local key water-related and socio-economic issues and priorities. The framework proposed here can be used to assess future conditions and possible adaptation strategies across several basins, enabling the integration of local criteria and processes (biophysical, socio-economic, legal) that can be critical.

Using basin-specific information from various sources and for different areas also means that it becomes important to be transparent about the methods, datasets and models used. In order to improve on the interpretability and coherence among case studies, a large effort at the EU level is needed to distribute earth observation and climate model datasets. The current Copernicus program can support this and can improve the homogeneity among the individual basin-studies, as researchers and practitioners will increasingly use these sources to inform their analyses and modelling. This will lead to more standardized input data on land use, crop types, evapotranspiration and soil moisture, among others. The use of these EU-level or global datasets for regional and basin-level studies can benefit local decision-making, but we argue that there is a two-sided benefit, as it can also allow for a better integration of local knowledge on local phenomena in EU-level decision making.

Furthermore, the case studies presented in this paper show the importance of embedding green water use and availability in water accounts to fully represent the needs and future prospects of agricultural water use, water productivity and water allocation.

Understanding climate change impacts across blue and green-water fluxes throughout regions and sectors in Europe is essential for ensuring food and water security and for developing early-warning and adaptation systems in support of society and ecosystems. The proposed methodological framework can be useful to support European policies and decisions in this matter.