How to adapt to a drier future: A review of adaptational strategies to decrease drought vulnerability in the Netherlands

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How to adapt to a drier future: A review of adaptational

strategies to decrease drought vulnerability in the Netherlands

Writer: Lea Maas- 10729372 (UvA) Supervisor: N. Wanders (UU)

Datum: 28-11-2020 Examiner: F. de Vries (UvA)

Assessor: C. Hoorn (UvA)

Abstract

Severe, long droughts like the one that hit the Netherlands in 2018 will become more likely due to climate change. What can the Netherlands do to increase their resistance to future droughts? In this review I give an overview of the adaptational strategies that already have been implemented or are suggested to reduce drought vulnerability in the Netherlands. The appropriate strategy will depend on the needs of the specific region and therefore, I made a distinction between coastal zones, low inlands and the high sandy soils to simplify assigning suitable methods. The methods discussed in this review focus on increasing storage in the soil to create a larger buffer for the summer, increasing freshwater availability, optimizing water-use efficiency and using regional waste- or brackish water as alternative source. Currently, several parties are investigating the appropriate solutions for their respective regions, and the adaptation process is in full swing. This review could aid in understanding the phenomenon of drought, how climate change increases drought risk and what adaptive measures could offer solutions for the different regions. Though many more factors need to be taken into account when developing your own adaptational strategy, this review provides a starting point.

Introduction

In 2018, the world faced the fourth warmest year since 1880 and during the summer, Europe was affected by a severe and long drought. The warm temperatures and the lack of precipitation during the summer caused the soil moisture levels to drop drastically in the Netherlands (Buitink et al., 2020; Haan, 2020; Peters et al., 2020). The summer of the following year, broke the heat records for the second time in a row and the ground water levels were barely able to recover (Haan, 2020). The drought had a large impact on the agricultural sector; the damages by drought and low groundwater levels were estimated between 0,9 – 1,65 billion euros (Beleidstafel Droogte, 2019). Freshwater shortages were present in all agricultural areas and some areas in the West faced salinization. The prolonged heat caused the groundwater levels to drop to such a level that it threatened the freshwater intake and even caused lake IJselmeer to show increased salinity levels (Haan, 2020). The lack of soil moisture was also a problem for nature areas and it aggravated the naturally occurring soil subsidence in the West (lowering of the ground due to the shrinking of dry peat and clay (Haan, 2020; van de Ven et al., 2011)). While the Netherlands are historically well equipped in dealing with floods and water surpluses, the drought of 2018 and its economic impact were a stark reminder of the vulnerability of the Netherlands to droughts.

Future climate projections show that certain areas in Western Europe (including the Netherlands), will likely face more extreme temperatures and experience stronger and more frequent droughts in the future (Spinoni et al., 2018; Teuling et al., 2013). The full extent of the impact of future climate change is not completely known, but research has shown that in the most likely scenarios, the Netherlands will face wetter winters, warmer and drier summers with an increased risk of both floods and extreme droughts. Droughts like the one in 2018 are expected to have a return time of 10 years instead of 30 years and the Netherlands might need to prepare for this future scenario (Beleidstafel Droogte, 2019).

In order to decrease their drought vulnerability, the Netherlands need to structurally change the way they handle water. As a delta area, the main aim of Dutch water governance has historically been towards managing floods and keeping “dry feet”(Özerol et al., 2016). To adapt to the increased risk of droughts, new water governance might need to integrate flood and drought governance and balance water surpluses in winter with storing enough water for the dry summers. Many adaptational methods to decrease drought vulnerability have been suggested and are currently being tried out; from storing winter precipitation to be used in summer, using drip-irrigation in the South-West of the Netherlands to more unorthodox plans like

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building a lake on the border between the Netherlands and Germany (Markus, 2020; van Duinen et al., 2015). Which brings us to central question of this review:

Which adaptational strategies could be used in the Netherlands to decrease drought vulnerability?

The goal of this review is to give an overview of the potential adaptational methods, while taking into account the regional differences within the country. Neither the impact of climate change nor droughts are evenly distributed over the country (Philip et al., 2020; Wong et al., 2013). And as a result of these regional differences, the potential adaptive measures need to be tailored to the specific regions. The elevated sandy grounds in the east have a shortage of freshwater in the summer; they are dependent for freshwater on rainfall and have a high risk of agricultural droughts in the summer(Haan, 2020; Weijers, 2020). In other areas with a lower altitude, salinization and further soil subsidence due to low groundwater levels are the more immediate problem(Haan, 2020; van de Ven et al., 2011; van Duinen et al., 2015). In order to answer the main research question, the following sub-questions will need to be addressed first:

a) How will climate change impact the risk of droughts in the Netherlands?

b) What are the regional differences in drought risk within the country that should be included in drought management?

In order to answer these questions, the review will start with a theoretical background on the phenomenon of drought: the definitions, causes, consequences and linkages between environmental factors and the water balance. Next, I will give an overview of how climate change will likely impact the water-systems in the Netherlands and how this increases the risk of droughts, followed by an overview of the key factors described in literature, that explain the differences in regional impact. Lastly, I will provide an overview of the different possible adaptational methods that could decrease drought vulnerability.

Materials & Methods

A literature search was conducted on Web of Science, Scopius, UvA library, Google Scholar and the database from University of Wageningen using the keywords below and various search methods: Best Match, Snowball and citation search methods. Data from the IPCC was used as a basis for projected climate change. To analyze and summarize the adaptational drought strategies, I analyzed literature, governmental reports and reports from research institutes (Deltares, STOWA, ZLTO, KvK, TWO, Spaarwater, Acacia, DAW).

Keywords: hydrological drought, adaptation, precipitation, climate change, freshwater, salinization,

drainage, evotranspiration, groundwater, irrigation, soil moisture, groundwater, drought risk, vulnerability, The Netherlands

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1. Theoretical Framework on Droughts

Drought can loosely be defined as a below normal water availability for a certain amount of time for a specific region(Naumann et al., 2018; Van Loon & Laaha, 2015). It is a complex natural hazard that is becoming an increasing global problem. Though a precise definition of a drought is contested, it generally indicates a shortage of water at a certain point within the hydrological cycle. As the name suggests, the water constantly moves (cycles) through the system; it can temporally be stored in lakes, groundwater and soil moisture, and the amount of available water is influenced by fluxes (Van Loon, 2015). Inputs can be river charge or precipitation while water can also leave the system through evotranspiration or run-off (Teuling et al., 2013).

1.1 Drought Types

Depending on the impact of the water shortage on the water-cycle, society and nature, they often make a distinction between certain drought types:

o Meteorological drought is defined as a prolonged lack of precipitation. When the prolonged rainfall deficit (meteorological drought) persists, it can develop into other types of drought.

o Hydrological drought: We speak of hydrological droughts when temperature or rainfall deficiencies have such an impact on the hydrological cycle that the balance is negatively impacted (Van Loon, 2015). The lack of inflow into the hydrological system results in lower water levels elsewhere in the water cycle. Hydrological droughts occur later than meteorological and soil moisture droughts but can be linked to a longer lasting impact on the water-system and society (Van Loon, 2015).

Examples of hydrological drought are below level groundwater or declining lake levels. (Van Loon & Laaha, 2015; Van Loon, 2015; Weijers, 2020).

The latter two drought types are defined by their impact on vegetation and society:

o Soil moisture drought or agricultural drought occurs when there is not sufficient soil moisture available for the vegetation which often results in crop failure and economic damage (Sepulcre-Canto et al., 2012; Van Loon & Laaha, 2015).

o Socio-economic drought is more related to the impact of persistent drought on society; it occurs when there is insufficient available fresh water to meet the demands of this sector and it can lead to economic damage (Belal et al., 2014; Sepulcre-Canto et al., 2012).

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1.2 Drought propagation

A precipitation shortage can progress through the water-system and develop into a hydrological drought; a

phenomenon called drought propagation. The development of a hydrological drought does not necessarily occur after one year of intense rainfall deficit, but it could also occur after several years of slightly lower rainfall (Hellwig et al., 2020; Weijers, 2020).

The drought propagation steps can be seen in figure 1. It often starts with a precipitation deficit; the effects of this shortage reduce the run-off from the system which then leads to soil moisture deficiencies. These low soil moisture levels then lead to reduced streamflow and even low

groundwater levels. At this point the meteorological drought has propagated into a hydrological drought. During this last phase of low ground water levels, saltwater can intrude the groundwater and cause salinization. With every step of the drought propagation processes, the response is delayed and the probability of occurrence decreases (Hellwig et al., 2020; van Engelenburg et al., 2018; Van Loon & Laaha, 2015; Van Loon, 2015; Weijers, 2020).

1.3 Indicators and Risk assessment

Droughts can have large impacts on nature, economy, agriculture and societies as a whole. To assess the drought probability, severity and impact, certain descriptive parameters (drought indicators) can be used. Examples of drought indicators are meteorological data (precipitation, temperature, wind), hydrological parameters (groundwater or soil moisture levels, river fluxes, snow catchments) or vegetation data. There is a wide range of drought indicators and indices (combination of indicators) available to asses drought risk and severity. Belal et al., (2014) give an overview of several of the most common indices that can be used to assess drought risk through Remote Sensing. These indices use various data sources like precipitation (PDSI & SPI), vegetation (CMI, NDVI) or snow-packs and waterflow data (SWSO & RDF) to assess the state of the water balance (Belal et al., 2014; Kumar et al., 2016). In stable climates like the Netherlands,

precipitation deficiencies are most often the cause of droughts and as a result, often used as indicator (Van Loon, 2015).

The risk of drought depends on the intensity of the hazard, the exposure (the extent of the phenomenon) and the vulnerability of the exposed area. The vulnerability will likely depend on other factors like soil type, vegetation, available water sources, geo-hydrological processes and land-use of the exposed area. The next two chapters will give a quick overview of how climate change influences the risk of drought in the Netherlands and the main regional differences in vulnerability.

Figure 1 Visual representation of drought propagation. Meteorological drought (precipitation deficit) progresses through the terrestrial water cycle. Figure represents how the response of the steps of the drought propagation is delayed. (source: Van Loon, 2015)

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2. Climate Change impact

In the last century, we have already seen changes in temperature and precipitation patterns (Klijn et al., 2012). Climate change will likely have an impact on the hydrological cycle and the likelihood of droughts in the Netherlands. In this section I will summarize how climate change is projected to impact several

components of the hydrological cycle in the Netherlands. This will largely be based on the KNMI’14 reports and on the Intergovernmental Panel on Climate Change (IPCC) results of 2013.

2.1 Precipitation

The IPCC presents four scenarios for future climate change. According to these scenarios, overall future precipitation will likely increase in the future which will partly compensate increased evaporation due to rising temperatures(Jacobs et al., 2010; KNMI, 2014). The bulk of this precipitation will fall in the winter with a higher chance of extreme weather outpours. The summers will likely be drier with longer periods of no rain which will result in more precipitation shortages. The extent of these shortages will largely depend on to what extent temperatures will rise in the future. In one scenario (G) temperatures rise will be moderate (+2 °C) while high (+ 4°C) in the other scenario (W). In scenario G the summer precipitation actually increases with 3% while it could decrease with 20% in the worst case scenario W+ (Klijn et al., 2012). In figure 2, the average precipitation shortage of a 35-year period in these two scenarios is compared with the current situation.

2.1.1 Sea-Surface Temperatures

Changes in precipitation patterns due to climate change will likely not be evenly distributed. Philip et al. (2020) have shown that there are differences in precipitation patterns between coastal regions and more inlands. Along the coast, no decreased summer precipitation trend could be found which could be due to the effect of sea-surface temperatures (SST) on the coast. Lenderink et al. (2008) have shown that due to increased SST, precipitation could increase in the coastal areas. More inlands, summer precipitation will likely decrease and a trend in increased agricultural droughts can already be observed according to Phillip et al. (2020).

Figure 2 Comparison of average meteorological precipitation shortages over a period of 35 year in the summer months in mm. Current situation (left) is compared to scenario G (moderate temperature increase) in the middle and on the right scenario (W). Source: Klijn et al. 2012

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2.2 Temperatures

Global temperatures have already risen by 1,5-2 degrees compared to the 20th_{century. A further increase}

of 1,5-4,5 degrees is expected between 1990-2100 depending on further anthropogenic emissions (KNMI, 2014). Higher temperatures and solar radiation will likely result in more evotranspiration (transpiration and evaporation). While there is not much research on the role of evotranspiration in drought development (mainly due to a lack of long-term quality evotranspiration data), it is certain that higher evotranspiration rates can cause strong water losses (Teuling et al., 2013; Weijers, 2020). Teuling et al. (2013) investigated the role of evotranspiration in drought development in four different catchments within Europe and found a positive relationship between evotranspiration and below average water storage during summer

droughts. This trend can be explained by what is described as the “drought paradox”; less precipitation corresponds with higher evotranspiration rates. The increased evotranspiration could be explained by the high temperatures, high solar radiation and lack of cloud cover during periods of no rainfall. This amplifying effect of evotranspiration on drought impact increases with naturally wetter climates and could play a role in drought development in the Netherlands (Teuling et al., 2013)

2.2.1 Stomatal CO2_response

Worthwhile mentioning is the potential feedback-loop of stomatal closure due to increased CO2_levels.

Increased atmospheric CO2_{levels can reduce the stomatal opening in plants and reduce transpiration rates,}

which will lower overall evotranspiration (Kruijt et al., 2008). What the extent of the effect of stomatal closure will be is still uncertain, but research by Kruijt et al., (2008) has shown that in rough, taller vegetation, it can reduce evotranspiration up to 15%. Overall, reduced evotranspiration rates due to stomatal closure could impact the soil moisture losses.

2.3 Changes in the Atmosphere

Several authors have investigated the role of circulation patterns like the North-Atlantic Oscillation (NAO), Eastern Atlantic/Western Russian (EAWR) and Scandinavian (SCAN) patterns on drought development (Hannaford et al., 2011; Kingston et al., 2015; López-Moreno & Vicente-Serrano, 2008). Especially the NAO might play a key role in drought development in the northern half of Europe(Kingston et al., 2015). The circulation patterns- drought relationship in Europe is a complex one, but according to Kingston et al., (2015), the weakening of western circulation can be linked to drought onset which can be caused by a combination of different circulation patterns.

The impact of climate change on the European wind regime is not entirely certain. In the Netherlands, climate change will likely not have a large impact on the wind regime according to the KNMI’14 reports. In the two scenarios where climate change will change circulation patterns, the Netherlands will likely experience more Western winds during winter and more Eastern winds in the summer which could cause warmer and drier summers (Kingston et al., 2015).

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2.4 River Charge

Temperature and precipitation changes have an impact on the amount of water that flows out of the catchments. For example, warmer temperatures in the Alps will cause more molten snow to be

transported by the Rhine while less snowfall in winter will likely decrease river charge. Future river discharge projections differ depending on climate scenario and calculation methods (Jeuken, Hoogvliet, et al., 2012; Klijn et al., 2012).

According to KNMI’14 scenarios, the river discharge will increase in the winter: 15-30% increase from the Rhine and 10-25% from the Meuse. River charge will likely decrease in the summer months; up to 15% for the Rhine and up to 30% for the Meuse in the most extreme scenario (Rijkswaterstaat, 2019). The river system (see figure 3) is an important source of freshwater, especially for inland areas. Freshwater shortages in the summer are often compensated with water from the river system. The reduced river charge in the summer might contribute to an increased drought risk.

2.5 Run-off rates

Run-off rates can play a relevant role in the development of droughts. After a rainfall shortage, the first response is generally a reduced run-off rate (see figure 1). Teuling et al., (2013) found that storage levels are an indicator for run-off levels. So, even though persistent reduced run-off rates can lead to hydrological droughts, initially it can actually mitigate the impact of precipitation deficits.

2.6 Human Land-use

Another factor that can greatly impact the hydrological cycle and drought development is human land-use. Humans extract great quantities of water for irrigation, industrial or household purposes. In the past 50 years, human water consumption has increased global drought intensity and frequency (by 20 in Europe) according to Wada et al., (2013). The impact of extraction for human consumption differs per region but has been shown to significantly decrease streamflow and intensify hydrological droughts. Considering human water consumption is only expected to increase in the future (largely due to the increased water demand for irrigation), its influence on drought intensity and development should not be left out (Alcamo et al., 2007; Wada et al., 2014; Wada et al., 2013; Wanders & Wada, 2015).

Figure 3 Visualization of main waterways in the Netherlands. Source: Klijn et al., (2012)

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3. Regional Differences

Future drought management will need to incorporate the many regional differences that exist within the Netherlands regarding drought impact and drought vulnerability. First of all, the impact of climate change will likely not impact the Netherlands equally (see chapter 2). Secondly, drought vulnerability will be influenced by several environmental factors (soil type, vegetation and altitude), human influences and other geo-hydrological processes.

Many reports and papers that discuss adaptive measures, focus specifically on one region, waterboard or catchment and suggest tailored measures. To simplify assigning suitable methods, it was decided to divide the Netherlands into three regions that roughly face comparable drought issues: the high sandy grounds, the low- inland and coastal areas. The

characteristics of these regions and their dominant processes and problems will be explained briefly in this chapter.

3.1 High Sandy Soils

The high sandy soils are mostly located in the East and South of the country (figure 4). This area is, in contrast with the rest of the country, characterized by its higher altitude, deeper groundwater levels and mainly coarser sandy soils (Geertsema et al., 2011). What differentiates its hydrology from the rest of the country is that the high sandy soils have no direct access to the rivers. As a result, they are dependent on precipitation for their freshwater supply which makes this region especially

vulnerable for summer droughts and precipitation deficits (Bressers et al., 2016).

While the system is currently very dry, the

sandy soils actually have a large water-holding capacity and the area used to be much wetter. Large-scale human reclamation activities between 1850-1980, changed the area drastically. Deep

drainage systems were constructed to dewater the area and the groundwater levels were purposefully lowered. Additionally, the hillslopes increase run-off and during intense rainfall (more likely due to climate change), precipitation intensity will increase and water will have less time to infiltrate the soils (Bressers et al., 2016, pp. 181–201; Geertsema et al., 2011).

Considering the drainage construction and limited freshwater supply, many of the adaptational methods suggested in this area focus on retaining more water in the system and using water more efficiently.

Figure 4: Representation of the location of the high sandy soils in the Netherlands (yellow) and low-Netherlands (while). Map shows location of main river system and main locations of saline sea-water seepage and brackish waters. Adapted from(RIVM, z.d.)

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3.2 Coastal Area

The coastal areas are largely located in the West and North of the country and include the Wadden-Islands. The main differentiating characteristic of this area is the process of “saline seepage” (seawater infiltrating the groundwater). As large parts of the Netherlands are located below sea-level, the upwards groundwater flow can infiltrate groundwater, lakes and ditches and turn them brackish. This saline seepage is

exacerbated due to climate change, sea-level rise and a reduction of freshwater recharge in the winter (de Louw et al., 2010; Jeuken, van Beek, et al., 2012; van Duinen et al., 2015; Velstra et al., 2011). In order to establish an efficient water management to combat drought and freshwater shortage, the threat of the salinization of freshwater sources needs to be taken into account.

3. 2.1 Saline seepage

The depth of the saline interface can be seen in figure 5. De Louw et al., (2010) identifies 2 main seepage systems: 1) sub-recent transition areas like in the North and South-West deltaic area and 2) seepage in deep polders more inland. What is typical for system 1) is the shallowness of very saline water: 50-100% at 0-5meter depth. According to de Louw et al., (2010), this distribution is due to sea-water inundation in the Holocene and human land-reclamation. Deep polders on the other hand are often more inland and face upward saline seepage from the groundwater due to their low altitude. They are often quite below sea-level, and the upward flow of salt groundwater causes an upward saline seepage (see figure 6). Especially in areas with

soil-subsidence, upwards saline seepage might become an increasing problem (Jeuken et al., 2013)

3.2.2 Freshwater lenses

In these areas with saline saltwater, agriculture is largely possible due to the existence of freshwater lenses that are located above the salt groundwater. They are formed by precipitation and for some agricultural areas without access to the rivers, these freshwater lenses are the main freshwater source. They can mainly be found in very low areas like deep polders and in the South-West deltaic area. De Louw at al. (2010) even described the province of Zeeland as a salt sea with several floating freshwater islands. This fresh layer is often the only barrier between the saline groundwater and their often not-salt resistant crops. Without these freshwater lenses, saltwater can infiltrate the root area

due to capillary rise and damage the crops (Tolk, 2012). The thickness of these freshwater lenses is influenced by evotranspiration, precipitation and is very vulnerable to drought. Several adaptational methods that will be discussed in chapter 4, focus on protecting or enlarging these freshwater lenses, simultaneously increasing drought resistance and protecting against saline seepage.

Figure 6 Schematic visualization of upwards saline seepage threatening the freshwater lens. Source: de Louw 2010 Figure 5 Map showing the depth of saline groundwater

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3.3 Low inlands

What will be classified as the low inlands of the Netherlands can be differentiated from the other two regions mainly because they do not have issues with freshwater supply or salinization. It includes Flevoland, Drenthe and parts of Groningen and Friesland. As the name suggests, it is located mostly below sea-level and geologically different from the high sandy soils. Unlike the high sandy soils, the low inlands have access to freshwater from either the main river system or IJselmeer and have many smaller streams that function as storage (Wuijts et al., 2013). At first glance more similar to the coastal regions, the low inlands have no issue with saline seepage but some areas struggle with subsidence (Jeuken, Hoogvliet, et al., 2012). Adaptational methods suitable for this region will be discussed in the next chapter.

4. Adaptational Strategies

Here the adaptational strategies are presented, categorized according to the overall goal: 1) Increase storage in the soil, 2) increase availability 3) agricultural adaptation 4) use of alternative sources.

4.1 Increase Storage soil

4.1.1 Drainage

Many sub-surface drainage structures were built in the Netherlands to lower water-levels and discharge rainwater (see chapter 3.1). Several methods focus on adapting the existing drainage systems to increase the amount of water stored within the system. By increasing water storage during the winter, more water becomes available during the summer. This method has two other potential benefits; it can help combat salinization and slow subsidence in peat areas (Jeuken, Hoogvliet, et al., 2012; Ritzema, 2015; van Bakel et al., 2007; Velstra et al., 2011).

Drainage can be used to increase the freshwater lenses that protect against the saline groundwater. Acacia (a Dutch consultancy company) has completed several pilots where this technique was successfully implemented (Acacia Water, 2019; Tolk & Velstra, 2016). On the other hand, adjusting drainage can slow down subsidence because higher groundwater levels reduce the oxidation processes responsible for a significant part of the subsidence(Jeuken, Hoogvliet, et al., 2012).

Considering the wide range of potential applications for drainage, this method has potential for all three regions. Many small-scale pilots have tested several drainage applications. I will give a short overview of the most discussed drainage types:

• Remove Conventional drainage: Conventional drainage (CD) focusses on removing water and lowering the groundwater table. Especially in the East, it has been suggested that simply removing the existing drainage system can decrease its drought vulnerability (Bressers et al., 2016).

• Drainage New-Style: Drain systems can be used to increase water storage through a method called Drainage New Style (DNS). With DNS, the drainage pipes are placed at a shallower depth and closer to each other to raise groundwater levels (see figure 7) and ensure only peak discharges will be discharged (Bressers et al., 2016; Jeuken, Hoogvliet, et al., 2012; van der Zee et al., 2017).

Figure 7 Schematic representation of how different drainage systems impact groundwater levels. Adapted from van Bakel et al., 2007

Ditches function as drainage

Deep sub-surface drains

Shallow subsurface drains

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• Adaptable drainage: Drainage systems can often not be altered when placed and water will automatically be discharged even in dry periods. With adaptable drainage, the groundwater levels can be changed by adjustable pipes or by using dams to set a certain water-level that will be maintained throughout the area. Water will only be drained if the water-level is above this set thresh-hold. This thresh-hold can be raised to store more water in the sub-surface to prepare for dry periods(Jeuken, Hoogvliet, et al., 2012).

• Climate adaptive Drainage: New types of controlled drainage have made it possible to retain more water in the soil and control water discharge. One of these methods is Climate Adaptive Drainage (CAD). As can be seen in figure 8, the CAD system consists of connected sub-surface drainage pipes that collect the water in a reservoir where the water-level can be controlled. This system ensures peak charges can be reduced and rainwater can be stored for drier periods. Adjusting the reservoir levels can be done manually or can be coupled to weather-monitoring systems that can anticipate weather changes. CAD-systems are more and more replacing conventional drainage system and several pilots (drains2buffers, Spaarwater, DROP) have been completed where reservoir water was re-used for irrigation or used to strengthen freshwater lenses (Bressers et al., 2016; Jeuken, Hoogvliet, et al., 2012; van der Zee et al., 2017).

4.1.2 Landscape rearrangements

In order to retain more precipitation, some simple landscape rearrangements have been suggested: Removing ditches, raising water levels within rivers and making streams broader and shallower. This could increase water retention within those areas dependent on rainwater (Blom et al., 2008; Bressers et al., 2016; Deurloo, 2017; Geertsema et al., 2011). Building retention areas in parcels or flood planes could also help store rain- and water surplus (Oude Essink et al.,2010). Another option could be to build physical constructs to prevent discharge and raise river levels.

To give river-water more time to infiltrate the soils and replenish the groundwater levels, several authors suggest letting the rivers re-meander and overflow in the winter (Blom et al., 2008; Geertsema et al., 2011; Roth & Warner, 2007).

4.1.3 Increase water-holding capacity soil

Another way to ensure more water can be retained within an area is by improving the water-holding capacity of the soil. Several sources (Beleidstafel Droogte, 2019; Geertsema et al., 2011; Tolk, 2012) suggest improving the soil structure to increase water infiltration and reduce run-off. The water infiltrating capacity of the soil can also be improved by reducing the working of the soil and by increasing the organic matter content. ZLTO, STOWA and several other parties have researched several methods to improve the infiltrating capacity of the soil and several successful pilots have been completed (e.g. Wel goed water Blijven geven by ZLTO).

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4.2 Increase Freshwater Availability

Drought vulnerability can be reduced by ensuring there is fresh water is available to compensate for shortages during dry periods. As a result, many parties focus on increasing or securing new freshwater resources. Several government reports discuss how to optimally connect river branches and better

distribute river-water to areas in need (Haan, 2020; Rijkswaterstaat, 2019; Tolk, 2012). For areas where this is not an option, mainly the high sandy soils and several coastal areas, the following adaptational methods were found:

4.2.1 Rainwater basins

Rainwater storages are efficient ways to store winter precipitation. This can be done in above ground basins and lakes or by raising the water-levels in rivers and other surface-waters.

Precipitation can also be stored below ground, a method used by greenhouse agriculture for quite some time already(Allied Waters, z.d.; Jeuken, van Beek, et al., 2012). Through drainage-systems,

precipitation is actively infiltrated into the soil, where the water is stored in the space between the grains. During dry periods, this supply can be used for irrigation. Even though this is not a new technique, several pilots are investigating the suitability for their project region. In Texel they managed to create rain-water storages and in Noord-Brabant they successfully stored roof-water below grounds(Acacia Water, 2019). 4.2.2 Alternative Below-ground Storages

Freshwater can be stored below-grounds with several methods: through Creekridge infiltration, Aquifer Storage and Recovery (ASR), the Fresh-Maker and coastal supplements. These methods are especially suited for the high sandy soils and coastal areas because water can be stored even in areas with saline groundwater (Tolk, 2012)

• Creekridge Infiltration

Within raised sand-ridges, a freshwater bubble can be found, naturally formed by precipitation. This bubble is actually a thick freshwater lens and due to several techniques, the size of this lens can be increased. Several adjustable drainage methods can be used to recharge the lens with freshwater from other areas. If sufficient winter precipitation is collected within the ridge, water can be extracted during dry periods(Tolk, 2012). Within the Go-Fresh project, several successful creek-ridge storage pilots were completed in the province Zeeland (south-West deltaic area).

Figure 9 Creekridge (Kreekrug) Infiltration system during summer. Fresh water was stored in the creekridge during winter and extracted for irrigation purposes in current figure. Adapted from https://www.zwdelta.nl/

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• Aquifer Storage and Recovery

With the Aquifer Storage and Recovery (ASR) method, fresh water is injected deep within an aquifer. Through this injection, a freshwater bubble can be formed in an otherwise saline groundwater environment. Freshwater from various sources can be injected in this deep aquifer through vertical drainpipes or wells during periods of surplus. As the name of the methods suggest, freshwater can then be recovered from this aquifer during dry periods(Tolk, 2012; Zuurbier et al., 2016)

• Freshmaker:

The Freshmaker technique reduces the upwards flow of salt groundwater to increase the size of the

freshwater lens (see figure 10). This method has been tried by Go-Fresh in coastal areas and could be useful in areas with salinization problems. The upwards flow of salt groundwater is reduced by placing drainage deep within the soil (10-20 m) so only salt ground water is drained away. This lowers the level of salt groundwater and a larger freshwater lens can form. Freshwater can also be injected into the top layer to increase the freshwater lens (Tolk, 2012; Zuurbier et al., 2016).

• Coastal supplements

An option mentioned by Oude Essink (2010) and implemented by Coastar from Allied Waters, is to increase the dune size or place sand deposits along the coast to increase or create freshwater lenses. The

freshwater lens in the sand deposits would increase in size due to precipitation and could help reduce coastal saline seepage and provide larger freshwater supplies.

Figure 10 Fresh-Maker strategy. Freshwater is injected in top layer and saltwater extracted during winter to increase freshwater lens. During summer freshwater can be used for consumption while saltwater extraction prevents saline seepage. Source: Zuurbier et al., 2016

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4.3 Agricultural Adaptation

The agricultural sector was hit particularly hard by the 2018 drought and remains quite vulnerable to climate change induced droughts (Peters et al., 2020; Tolk & Velstra, 2016). To adapt to these drier conditions, farmers might need to change their usual methods. I will discuss two strategies that could help the agricultural sector reduce their vulnerability: 1. More efficient water-use. 2. Crop choice.

4.3.1 Water-Use efficiency

Several authors suggest reducing sprinkler irrigation (as much is lost through evaporation) and promote more efficient drip-irrigation (Geertsema et al., 2011; Tolk & Velstra, 2016). Drip-irrigation is often used in countries with dry climates. Water is administered very specifically to the rootzone of the crop, either below- or above grounds, to reduce water loss through evaporation or run-off (see figure 11). The moisture levels surrounding the roots can be kept optimal for crop growth with very little water. By using weather forecasts and moisture sensors, they can very precisely determine how much water to distribute depending on crop type and soil characteristics (Jeuken, van Beek, et al., 2012; van Duinen et al., 2015).

Research from Acacia (Spaarwater project) found that in dry years, drip-irrigation can increase the yield by 22% (Hulshof et al., 2019). Drip-irrigation has already been successfully implemented in several areas within the Netherlands. Notable mentions are projects Spaarwater in the Wadden, Deltadrip in the South-West Deltaic and the DROP 2016 and 2018 projects from DAW and LTO in Groningen, Drenthe and Friesland (Özerol et al., 2016; Tolk & Velstra, 2016).

The sub-irrigation method is often coupled to the CAD-system discussed in section 4.1. With CAD,

drainage water is collected in a reservoir which can then be re-distributed to the rootzone of crops during the summer using the same drain systems. Sub-irrigation is used to increase the groundwater levels so moisture levels in the rootzone remain high (figure 11). Water loss through evaporation is low considering water is distributed below grounds. The Spaarwater research project (Hulshof et al., 2019) estimates that water-use efficiency can be increased to 85% depending on the water holding capacity of the soil. Sub-irrigation has been tried out in several successful pilots (e.g. Wel goed water BLIJVEN geven) in Limburg and in Noord-Brabant.

4.3.2 Crop types

To adapt to a drier climate, several authors have suggesting switching to more heat and salt resistant crops(Jeuken, Hoogvliet, et al., 2012; Tolk & Velstra, 2016; van Duinen et al., 2015). The first option would be to switch to species that need less water or have a slight salt tolerance (quinoa, samphire, aquaculture). Jeuken, Hoogvliet et al., (2012) mention the option of increasing crops’ salt tolerance so they can be irrigated with slightly brackish water as farmers sometimes already accidently use brackish water for irrigation (Tolk, 2012; Tolk & Velstra, 2016). In order to increase the salt and heat resistance of crops, it might also be possible to breed and/or genetically adapt species to be more drought resistant (Jeuken, Hoogvliet et al., 2012). To reduce loss through evaporation, several authors suggests using less evaporating vegetation. Replacing coniferous species with less evaporating deciduous species could be an option for the natural environment (Geertsema et al., 2011).

Figure 11 Visualization of drip-irrigation, below-ground (left), above ground (middle) and sub-irrigation (right). Source Hulshof et al., (2019)

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4.4 Use of alternative sources

In areas with insufficient freshwater resources, other unsuitable resources could provide an alternative. The first option could be to desalinate brackish or saltwater for irrigation purposes. This option could especially be useful for the coastal areas as they already deal with salinization problems. Secondly, sewage water or factory affluent could be re-used after treatment. This second option could be especially

interesting for areas with no direct freshwater resources like the high sandy soils in the East and South but with an extensive industrial sector(Allied Waters, z.d.; Jeuken, Hoogvliet, et al., 2012; van Duinen et al., 2015; Wuijts et al., 2013).

4.4.1 Desalinization

Several techniques exist to desalinate salt/brackish water. The advantage of desalinization is that regional brackish water can be turned into a new freshwater source. The most used desalination method is reverse osmosis (RO)(Jeuken, van Beek, et al., 2012; Wuijts et al., 2013). Other methods are Reversed Electrodialysis (RED) and Ion Exchange Resin (IER) softener. RED is based on the transport of ions through membranes under an electric field (Banasiak et al., 2007). The IER technique is based on transporting brackish water along membranes where the hard ions get exchanged for softer salt molecules, softening the saltwater (Stuyfzand & Raat, 2010). A new desalination method was devised by Vitea in 2019: membrane-capacitive deionization electro-desalination (CapDI) (Jeuken, van Beek, et al., 2012). Many of these new techniques could have potential and several pilots and smaller plants are implementing and testing these techniques(Jeuken, Hoogvliet, et al., 2012). Considering the high quality demands for drinking water, desalinized brackish water is most often used for irrigation and industrial purposes (Stuyfzand & Raat, 2010).

4.4.2 Re-use affluent

Normally, industrial wastewater is discharged into the surface water after the most abundant chemicals and dangerous micro-organisms are removed. It has been suggested that with extra treatment steps and further removal of chemicals and nutrients, wastewater can be re-used for agriculture or industrial purposes. Rietveld et al., (2011) gave an overview of the possibilities for using wastewater for either drinking-water or industrial and agricultural purposes. Wastewater can be cleaned more effectively through several multi-barrier treatment steps (RO, ultra-filters, sand filters). While it is possible to create ultra-pure water, demands for drinking water are very high and wastewater has a high risk of bacterial contamination. Rietveld et al. (2010) and Jeuken, Hoogvliet et al. (2012) argue that treated affluent would be most suitable for the industry or agriculture.

Several trials where wastewater gets treated and re-used have been implemented in the Netherlands. In the South of the Netherlands the F2Agri and the “boer-bier-water” projects used affluent from Bavaria for local irrigation with success(Jeuken, Hoogvliet, et al., 2012).

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Discussion

In this review, I have explained the concept of drought and summarized the impact of climate change on drought risk in the Netherlands. Considering the regional differences within the country, three different regions were defined: coastal regions (salinization issues), the high sandy soils (main issue is dependency on precipitation) and the lower inlands; no salinization but subsidence in some regions. It is expected that the risk of droughts will increase in the coming decades and therefore, the Netherlands will need to have strategies in place, tailored for every region. I gave an overview of different methods that have been suggested in literature, tested or already implemented. A

summary of these strategies, their main benefits and the regions for which they qualify most can be seen in table 1.1

1_{The numbers in superscript correspond with the following authors; 1. (Geertsema et al., 2011); 2. (Blom et al., 2008); 3. (Rietveld et al.,}

2011); 4. (Bressers et al., 2016); 5.(Jeuken, Hoogvliet, et al., 2012); 6. (van Duinen et al., 2015); 7. (Tolk, 2012); 8. (Velstra et al., 2011); 9. (Oude Essink et al., 2010); 10. (Allied Waters, z.d.); 11. (Wuijts et al., 2013); 12. (van der Zee et al., 2017); 13. (Deurloo, 2017); 14. (Roth & Warner, 2007); 15. (Beleidstafel Droogte, 2019); 16. (Ritzema, 2015); 17. (van Bakel et al., 2007); 18. (Acacia Water, 2019); 19. (Tolk & Velstra, 2016); 20. (Jeuken, van Beek, et al., 2012); 21. (Banasiak et al., 2007); 22. (Stuyfzand & Raat, 2010); 23. (Özerol et al., 2016); 24. (Hulshof et al., 2019); 25. (van den Boomen & Kampf, 2013); 26. www.boerbierwater

Strategies Where Effects?

Increase Storage in Soil

Integrate Flood/drought management (flood

plains11_{, re-meander rivers}1, 2, 11, 14 Low inlands & _{High sandy soils}1 Increase infiltration river-water 1, 2, 14

Less flood risk11, 14

Adjusting Drainage system6, 7

Remove drainage4

Drainage New style (DNS)1,4, 5,6, 7, 12

Climate adaptive drainage (CAD)4, 5, 7, 12, 19

Adaptive drainage 5

All three Freshwater availability

High Sandy soils1, 4_{Increase groundwater level}17, 18, 19

Low inlands Stop subsidence5, 16

Coastal areas Counter salinization7, 8, 16, 18

Physical constructs (dams, retention areas)1, 2, 4, 9,

11, 13 All three Fresh water availability 11

Coastal areas Stop saltwater infiltration9

High Sandy Soil1

Low inlands Retain rain & river-water

1, 2, 4, 9, 13

Increase Storage capacity soil1, 2, 7, 10, 15 _{All three} _{Increase water storage soil}1, 2, 7, 15

Increase Freshwater availability

Rainwater basin7, 11, 28, 19, 20

Aquifer Storage & Recovery (ASR)6, 7, 9

Creekridge Infiltration6, 7

All Three11 _{Freshwater resource}

The Freshmaker6, 7, 9, 10 _{Coastal areas} _{Strengthen fresh water lenses}6, 7, 9, 10

Coastal supplements 9, 10 _{Coastal areas} _{Fresh water resource}9, 10

Slow salinization10

Precision Agriculture

Sub-irrigation9, 19, 23, 24_{& drip-irrigation}1, 6, 7, 19, 20 _{Agricultural areas} _{Increase water-use efficiency}

Heat resistant crops1, 5, 6, 7, 9 _{Agricultural areas} _{Higher drought resistance sector}

Salt resistant crops5, 6, 7, 9 _{Coastal areas} _{Irrigation with brackish water}7, 9_,

higher salt/drought resistance sector

Alternative Source

Desalinization11, 20, 21, 22 _{Coastal areas} _{Freshwater resource}

Affluent Treatment3, 5, 25, 26 _{Industrial regions} _{Freshwater resource}

Table 1 Overview of the adaptive measures described in chapter 4. The left column shows the type of adaptive measure (number in subscript indicates reference (see Note1_{)). The middle column shows for what region the measure could have most potential based on the division made in chapter 3.}

Coastal areas: Areas with salinization issues; High sandy soils: geological distinctive due to their sandy soils and higher altitude; Low inlands: areas at lower altitude with no direct salinization issues; All three: the method has potential for all three regions. Agricultural areas; measure has potential for agricultural sector overall. The column on the right indicates the benefits of the adaptive measure mentioned by the author.

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Uncertainties towards Climate Change

One of the complexities in developing drought strategy, is the lack of certainty on how climate change will develop (Jeuken, van Beek, et al., 2012). But it is expected that climate change will have an impact on future weather patterns and river charges. We can expect more extreme floods and intense rainfall in the winter with high run-off rates(Beleidstafel Droogte, 2019). During summer, both evotranspiration and precipitation will likely increase(Klijn et al., 2012). This will not immediately lead to precipitation shortages, but more precipitation will fall along the coast,

increasing the drought vulnerability in the sandy high grounds in the East, that are dependent on this rainfall for freshwater(Geertsema et al., 2011; Lenderink et al., 2008). This is confirmed by Phillip et al. (2020) who has already observed an increased trend in agricultural droughts in the East. River charge will also decrease in the summer which could become problematic for the areas that use river water to compensate for shortages in the summer(Klijn et al., 2012; Rijkswaterstaat, 2019). When droughts occur, they can be further intensified by increased water consumption and due to the amplifying effect of higher evotranspiration(Teuling et al., 2013). Overall, the risk of drought seems to be increasing and the Netherlands needs to undertake action in order to minimize its drought vulnerability.

Describing the regional differences

No strategy would suit the whole country and in this review, I divided the country in three regions: The coastal zone includes areas at lower altitude that face salinization issues, the low inlands are defined as the lower regions with no salinization issues, while the high sandy soils can be

differentiated due to their sandy soils and higher altitude. This division is too simplistic to fully describe all the regional differences in regard to water management but could be useful to describe the drought issues and corresponding measures.

First of all, this division was chosen due to the results of the literature analysis. Most literature and adaptive measures focused on either coastal areas(Allied Waters, z.d.; Oude Essink et al., 2010; Tolk, 2012; van der Zee et al., 2017; Velstra et al., 2011; Jeuken, van Beek, et al., 2012), the high sandy soils (Blom et al., 2008; Geertsema et al., 2011; Hendriks et al., 2014) or on another specific area (Bressers et al., 2016; Weijers, 2020) but this division represents the focus in much of literature that was found.

Secondly, this division simplifies assigning suitable strategies; the drought issues in the specific regions are often coupled to un-going geo-hydrological processes captured in this division;

salinization and subsidence issues can both be linked to a lack of freshwater in the system(de Louw et al., 2010; Haan, 2020). As a result, drought strategies for coastal regions often focus on

strengthening the freshwater lenses and combating saline seepage (see table 1). The drainage system in the high sandy soils is so effective that too much water is drained away(Bressers et al., 2016). It will come as no surprise that many adaptational methods there focus on reversing the effective drainage and retaining precipitation (see table 1). While not completely accurate, this distinction was useful for describing the dominant drought issues and corresponding adaptational strategies. Every water-board, farmer, stakeholder would need to look objectively at their own situation to find its optimal adaptational strategy.

Implementation process

Many adaptive initiatives are taking place at different scales. While river water distribution is often arranged at regional scale (Rijkswaterstaat, 2019), much of the adaptive strategies were implemented at a smaller scale. There is much cooperation between research institutes, water-boards, companies and farmers to find the appropriate solutions for their regions and interests (Özerol et al., 2016).

But, it seems like a great deal of independence is expected from small stakeholders to implement adaptive measures themselves (Bergsma et al., 2012; Gupta et al., 2016). There is a great body of literature available investigating how individuals respond to the increased drought risk and what influences their adaptive response (van Buuren et al., 2015). Several articles focus on

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cost-benefits analyses (Acacia Water, 2019; Schaap et al., 2013), threshold analyses (van Buuren et al., 2015) or use stakeholder processes and uncertainty levels to investigate farmers’ adaptive responses (Schaap et al., 2013; Thissen et al., 2017). This emphasis on individual responsibility could slow-down the implementation process as van Duinen et al. 2015 found that money is often a limiting factor for farmers. Both Bergsma et al. (2012) and Gupta et al. (2016) argue that a too decentralized approach could create challenges for the adaptive response; conflicting interests may occur, and it could become unclear who has the responsibility to implement due to the many stakeholders at different scales.

Collaboration will be very important to structurally adapt to climate change and implement adaptive measures. Van Duinen et al. (2015) found that joint implementation of measures is cheaper and more effective because it considers the hydrology of the entire region. To support the

implementation process, the government should supply more financing, support joint-implementation initiatives and provide guidelines regarding the larger hydrological context.

Lastly, only a few reports (Blom et al., 2008) considered the impact of these adaptational methods on nature and ecosystems. The methods discussed in this review could seriously impact nature and ecosystems; adjusting groundwater levels could influence the entire hydrology and ecosystem of a region. Before large scale implementation, the potential impact of these methods on nature should be investigated and considered first. This could be more easily achieved in a more centralized drought management.

Conclusion

The risk of drought is steadily increasing, and the Netherlands needs to adapt. In this review I gave an overview of the potential adaptational strategies for reducing drought vulnerability in the

Netherlands. Finding the appropriate regional solutions is complicated but I strived to provide the tools to properly understand the drought problems and regional differences in impact.

This review showed that the Netherlands will likely see wetter winters, warmer and drier summers with a higher change of floods and droughts. As a country, we will likely have higher temperatures and more precipitation, though focused on the coast and in extreme rainfall events. In the summer, both river-charge and inland precipitation are expected to decrease threatening the freshwater supply of the regions dependent on it.

Actual drought impact will then depend on several factors like freshwater accessibility, environmental factors and geo-hydrological processes like salinization and subsidence. To take these regional differences into account, I made a distinction between coastal areas that face salinization, the high sandy soils in the East that are dependent on rainwater and the lower inlands with access to the river system but subsidence in some regions.

To effectively adapt, future drought management needs to be included in overall water management. Several methods (landscape re-arrangement, adjusting drainage, sub-surface storages) can ensure more rain- and river water is stored in the soil as a buffer for the summer. I discussed several options to increase drought resistance in the agricultural and industrial sector: increasing water-use efficiency (precision agriculture), adapting crop choice and methods for treating affluent and brackish water to make them suitable for consumption.

Finding the most suitable strategy to adapt is a complicated and expensive affair. Despite this, the risk of droughts is increasing and the urgency to adapt has been felt by many. Many research projects and pilots have been undertaken and though challenging, I found that the process of reducing drought vulnerability is well underway. This review could be helpful in understanding the increased drought risk in the Netherlands and the potential adaptive measures to increase drought resistance. With continued collaboration between different levels and stakeholders and if we keep sharing knowledge and continue to implement pilots, it is a challenge we can face.

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