Written by Dr. Niko Wanders
We often hear about droughts around the world including those recently in the U.S. and Brazil, which has threatened the water safety for this year’s Olympic Games. Despite their natural occurrence, there is still a lot that we do not understand fully about the processes that cause them and about how they impact our society and natural ecosystems. These topics are of great interest to scientists and engineers, and of great importance to policy makers and stakeholders.
The elusive definition of a drought
A drought can be broadly defined as a decrease in water availability below levels that are considered normal within a region. This means that droughts do not only occur in warm, sunny, dry countries but can take place essentially anywhere. What makes it hard to come up with a single, precise definition of a drought is that this below-normal water availability can be found at the different stages of the water cycle: precipitation, soil moisture (i.e. how much water there is in the soil), snow accumulation, groundwater, reservoirs and streamflow. Therefore, more useful definitions of drought conditions have to be tailored for specific sectors (e.g. agriculture or power generation) by focusing on the stage of the water cycle that is relevant for them (e.g. soil moisture for farmers, and streamflow for controllers of hydroelectric and thermoelectric plants).
Droughts can cover areas that range from a few thousand squared miles to large portions of a continent and can last anywhere from weeks to multiple years. Normally they start after a prolonged period of below-normal precipitation, sometimes in combination with increased evaporation due to high temperatures. This then causes a reduction in water availability in the soil, which can lead to lower groundwater and river levels as a result of decreased water recharge from groundwater aquifers into rivers. Snowfall is another important factor because it adds a steady release of water resources into streams throughout the Spring. When most of the precipitation comes as rain, it will wash out fast, leaving the Spring with dry conditions once again. The evolution of a drought through the water cycle is called drought propagation and normally takes multiple weeks to several months to take place.
So far this season, El Niño has been bringing some relief to the California drought. The current snow accumulation is above normal which is good news for this drought stricken region. The forecasts for the upcoming months look hopeful and it is likely that California will see some relief of the drought in the coming months. Nevertheless, it will take multiple years before groundwater and reservoir levels are back to their normal conditions, so the drought and its impacts will still remain for at least the coming years.
Droughts’ impacts on society
Extensive and long-lasting droughts can accumulate huge costs for the regions affected over time. For example, the ongoing California drought caused $2.2 billion in damage for the year 2014 alone. This is only an estimate of the damage to society in monetary terms, while the severe impacts on the region’s ecosystems are difficult to measure and quantify. As a result of the drought conditions, reservoir storages in most of California are at record low levels and strict water conservation policies have been implemented.
The severity of a drought’s impacts, however, depends greatly on the wealth, vulnerability, and resiliency of the region affected, including the degree to which the local economy and services rely on water. Despite the huge costs of the California drought, the U.S. is more capable of mitigating its effects and eventually recovering from it given the country’s general financial strength compared to many developing nations. According to reports by the United Nations and the Inter-Agency Standing Committee, an estimated 50,000 to 260,000 people lost their lives in the severe 2011 drought in the Horn of Africa, due to the fact that the financial means to provide food aid were not present and outside help started too late.
To have better tools to deal with these extreme events, several government agencies and institutes around the world have created drought monitors to track current drought conditions and to forecast their evolution. Examples are the Princeton Flood and Drought Monitors for Latin America and Africa, the U.S. Drought Monitor and the European Drought Observatory. These websites provide information on current drought conditions, which can be used to take preventive measures by governments and other stakeholders. Additionally, they can be used to inform the general public on current conditions and the need for preventive measures, such as conservation.
The power to affect a drought
Traditionally, droughts have only been thought of as a natural phenomena that we have to endure from time to time. However, a recent commentary in Nature Geoscience that included two Princeton contributors argued that we can no longer ignore how humans affect drought occurrences. For example, when conditions get drier from lack of rainfall, people are more likely to use water from the ground, rivers and channels for irrigation. These actions can impact the water cycle over large areas, affecting the water resources of communities downstream and of the local communities in the near future. In the case of California, the severe drop in groundwater levels has escalated in the last three years due to a combination of the extreme drought conditions and the resulting heavy pumping for irrigating crops. The extra water that becomes available from pumping of groundwater is only a temporary and unsustainable solution that will alleviate the drought conditions in the soil locally for a short period of time. Most of the irrigated water will evaporate and only a small portion will return into the groundwater. In the long run, these depleted groundwater resources need to be replenished to recharge rivers and reservoirs – a process that can take multiple years to decades. Furthermore, extracting groundwater in large amounts can lead to subsidence – a lowering of the ground levels – that can sometimes be irreversible and have permanent effects on future water availability in the region. Thus, through our actions we have the power to affect how a drought develops, making it necessary to rethink the concept of a drought to include our role in enhancing and mitigating it.
But it’s not all bad news. Last year I carried out a study with my collaborator, Dr. Yoshihide Wada, that found that sometimes human interventions can have a positive effect on the impact of natural drought conditions. This is most clear when we look at reservoirs that are built in many river systems around the world. It is shown that by building these structures the river discharge is more equally spread throughout the year. High flows or floods can be dampened by storing some of the water in the reservoirs, while this water can be used in the dry season or during a drought event to reduce the impact of low flows. This in itself opens up opportunities for regional water management that can help reduce the region’s vulnerability to droughts. Three limitations of the reservoirs are that they increase the amount of evaporation by having large surface areas, their benefits are limited in prolonged drought conditions simply because their storage is not infinite, and finally, they have a large impact on plants and animals in the downstream ecosystems (e.g. migrating fish species that need to swim upstream).
Drought in the future
Scientist have carried out many studies to explore what will happen to the characteristics and impacts of droughts in the future. Multiple research publications show that droughts will most likely increase in severity compared to the current conditions in many of the world’s regions with projected increases in human water demand, painting a stressful future. This then requires an adjustment in the way we deal with drought conditions, how we monitor and forecast these extremes, and how we consume water in general.
A short-term solution is trying to improve our monitoring and forecasting of these events so that we are better prepared. For example, additional improvements in meteorological and hydrological forecasts for conditions 3-6 months in advance would help operators manage their reservoirs in a way that would reduce the impact of upcoming drought events. These improvements require scientists to become more aware of the impact that humans have on the water cycle, which is a growing area of interest in recent years, but is definitely not standard practice.
Apart from increasing our possibilities to forecast upcoming drought events, we could also change our response to ongoing drought conditions by trying to be more efficient with the remaining available water. This could be achieved by using more efficient irrigation systems, building separate sewage systems for rainwater (that could be used for drinking water) and domestic and industrial wastewater (that is only reusable after severe treatment), and not cultivating crops that have a high water demand in areas with a natural low water availability. All these measures require long-term planning and willing government agencies and societies that would like to push and achieve these goals. Often a severe event (with significant damage) is needed to create the necessary awareness to realize that these measures are a necessity, such as the case in California that has resulted in new water laws and in Australia a few years ago.
Humans and the natural water system are strongly intertwined, especially in hydrological extreme conditions. Our impact on the water cycle is significant and cannot be neglected, both in normal conditions and under extreme hydrological ones. It will be important in the coming decades for us to learn how to responsibly manage our valuable water resources within a changing environment.
Dr. Niko Wanders is a Postdoctoral Research Fellow in the Civil and Environmental Engineering Department at Princeton working together with Prof. Eric Wood. His research interests include the study of the physical processes behind droughts, as well as the factors that influence their magnitude and impact on society. Niko received a NWO-Rubicon Fellowship to work on the development of a global sub-seasonal drought forecasting system. The aim of the project is to develop a system that cannot only forecast upcoming drought events, but also make reliable forecast on the drought impact on agricultural production, water demand and water availability for human activities.