Research question 5: What characterizes a robust drought risk system?

a higher manageability than systems with equal protection levels;

 Systems with ‘unbreachable embankments’ and a large, valley-shaped flood- prone area have a higher proportionality and a higher manageability than systems with traditional embankments that may suddenly breach;

 Measures aimed at impact reduction increase robustness when they reduce those impacts below the recovery threshold.

7.2.5 Research question 5: What characterizes a robust drought risk

system?

The drought cases (Chapter 5 and 6) have shown how different system configurations score on the robustness criteria. This gave insight into what enhances a drought risk system’s robustness. The insights are case-specific and therefore discussed per case. In the streamflow drought case, water is provided to municipal water users through a man-made reservoir. The amount of water supply could be increased by enlarging the reservoir’s storage capacity, for example by adapting its operating rules (raising water levels). Another option to increase supply is by accessing alternative water resources (e.g., groundwater wells, desalination plants or a pipeline to another reservoir). This would also increase the supply reliability and therefore the resistance threshold, because if one source is depleted, other sources are still available.

Demand reduction, for example by motivating industries and individuals to conserve water on a structural basis (thus not only during the drought), increases the resistance threshold as well as the manageability, because larger droughts are needed to cause water shortage, and a smaller demand (the amount people are used to) costs less to replace when supply is lacking.

Drought contingency plans may reduce the drought impact during a drought. It was demonstrated that hedging (temporary water conservation when reservoir is not empty yet), reduces the total impact of large drought events, but increases the impact of smaller drought events. The effect on robustness depends on how the criteria are weighed, because the resistance threshold is smaller than without hedging, but the manageability scores higher. Prioritization among different, competing water users

may also reduce the drought impact, but can only be applied in combination with hedging. It is recommended that future research explores the effect of prioritization. Drought insurance is an example of a measure that increases manageability by increasing the recovery threshold.

In the agricultural drought case, supply water originated from both rainfall and external river water, and was used for cultivated crops. Because rainfall varies in time, storage capacity is important. In this case, excess rainfall is stored in the soil, canals and water basins, where it is available for crops. A limited amount of external river water can also be stored in the canals by water level management. In this case study, the limiting factor of external water supply was the water quality (not the water quantity), and therefore (in-)dependence on this water resource was proposed as additional robustness criterion.

The resistance threshold can be increased by increasing the internal storage capacity6, but only when this water is available for all farmers. Local measures implemented by an individual farmer, for example soil management or installing water basins, only have a local effect on the storage capacity. They will therefore reduce the total impact of the drought, but they do not increase the resistance threshold of the region, since other farmers will still experience water shortage with the same drought magnitude.

Manageability is high when drought impacts are low over the full range of drought magnitudes. Drought impact is determined by the water demand and the economic function related to this demand. Water demand for agriculture is determined by the crop type. If the cultivated crops are diverse, thus sensitive to water shortage in different parts of the growing season, it is less likely that the entire crop yield of the region is lost in a single drought event. Crop diversity thus limits the impact and thereby enhances the manageability.

6

Note that increase of internal storage capacity qualifies as demand reduction in the Netherlands, because water supply is used to denote external surface water, for example from a river or lake outside the region considered. Measures that increase the storage capacity within the region will reduce the demand for this external water source.

Installing sprinklers will reduce the impact from precipitation deficit, but will increase the demand for external water and therefore make the system more dependent on the external water source, which is not a robust solution when this source is not reliable. The reliability of the external supply can be increased by enlarging the capacity of the contingency supply, which is in fact a different inlet point for the same river water that is less likely to be closed for quality reasons.

Comparing the results of the two drought cases shows that demand reduction has a positive effect on robustness, but not always for the same reason. In the agricultural drought case, reducing demand for external water (by increasing local storage capacity) is considered a robust solution, because it reduces the total impact, but in fact the total agricultural demand for water in general (including rainfall) is not reduced. Therefore it did not increase the resistance threshold. In the streamflow drought case, demand reduction (by encouraging water conservation) is considered a robust solution because it reduces the total drought impact and it also increases the resistance threshold. How certain types of measures affect different robustness criteria is thus case-specific. Summarizing, the following can be concluded:

 Systems with large storage capacity in comparison to the demand have a high resistance threshold;

 The resistance threshold can be increased by enlarging the storage capacity, if this water is available for all water users;

 Measures aimed at demand reduction reduce the impact and therefore increase the manageability;

 Drought contingency plans, including short-term measures such as rationing and prioritization, increase manageability, but they also reduce the resistance threshold.

 Diversity in drought sensitivity among water users (e.g., crop diversity) limits the impact and thereby enhances the manageability.