Human land use, soil management and landscape hydrology – the real drivers of floods, droughts and heat waves?
There is no doubt that measures to combat climate change by reducing CO2 emissions are essential and have become a global political goal. However, focusing exclusively on this goal neglects other important mitigation measures that urgently need to be implemented. A team of authors from the Technical University of Munich and the Universities of Lancaster and Augsburg argues in an opinion paper that the restoration of hydrologically functional landscapes and soils should be considered equally important in mitigating climate change, particularly in relation to extreme events such as floods, droughts and heat waves, and in maintaining the basis for food and life.
Recognise land use-related climate change and take action
Floods, droughts and heat waves are increasing worldwide. This is generally attributed to climate change caused by greenhouse gases. However, at the global level, climate change does not lead to a reduction in precipitation nor does it sufficiently explain droughts, despite a moderate increase in evapotranspiration due to the rise in temperature as a result of global warming. Previous land use changes, in particular soil sealing, soil compaction and landscape drainage measures, are likely to play a greater role in water losses through runoff and thus in flooding and water scarcity. According to the authors, there is a need for greater awareness of the consequences of land use-driven climate change (figure). Measures to reduce soil sealing and compaction and to increase water storage in structurally rich landscapes (sponge landscapes and sponge cities) can all have a significant impact on the climate. They also include quick and easy measures that bring about a measurable cooling of the landscape (e.g. straw mulching after harvest (see below), i.e. no incorporation of straw after grain harvest).
Modelling predictions to derive recommendations
The greatest challenge here is likely to be in hydrology, where interactions between the atmosphere, soil surfaces and the soil itself are still largely neglected. As a rule, phenomena associated with land use that have a direct impact on water flows in the landscape (surface runoff and interflow, local evaporation, etc.) are insufficiently recorded. The size of a study area and its surroundings, for example, play hardly any role in many model calculations, such as those for evaporation. Land use is usually recorded in broad categories such as forest, grassland, arable land and settlement area – with parameter values that were partly derived decades ago and hardly reflect the unprecedented changes that have taken place in individual land use categories in recent decades.
Even the marked increase in rainfall erosivity (see "Worth Knowing"), currently the strongest CO2-induced climate change effect in rural areas alongside rising temperatures, is hardly captured by hydrological models. According to the authors, all conclusions on the effects of land use based on modelling must therefore be viewed with caution, regardless of the apparent certainty of the modelling results. Conversely, considering meteorological changes solely from the perspective of greenhouse gases is also distorted.
Conclusion
However, even if models fail and landscape experiments are not always possible, there is sufficient evidence that land use is a key driver of the problem and the solution to mitigating floods, droughts and heat waves. It is therefore essential to address changes in land use and management, as these will persist even with net-zero CO2 emissions and will make the world more vulnerable. However, given the strong influence of soil and land use on water and energy balances, there is potential to offset some of the negative effects of CO2 increases on terrestrial ecosystems. This option should be used more often to mitigate another important factor contributing to extreme events.
Worth knowing
Evapotranspiration
Evapotranspiration is the total loss of water from an area to the atmosphere through evaporation from, e.g. soil and water surfaces, and through transpiration from plants and animals.
Rainfall erosivity
Rainfall erosivity describes the ability of precipitation to loosen and transport soil particles through its impact energy. It depends on the intensity and duration of the rain and is an important factor in soil erosion. Heavy rainfall events with high kinetic energy can cause significant soil loss. Rainfall erosivity has approximately doubled in Germany over the last 60 years, increasing the risk of soil loss through erosion. Measures such as adapted soil cultivation and erosion control can help to reduce the negative effects.
The potential of straw cover
Straw covers would enable every farmer to maintain soil moisture for crops, as less energy from short-wave radiation would be available for evaporation. A study for France estimated that during the 100-year heatwave in Europe in August 2003, the change in albedo (i.e. the measure of a surface's reflectivity) caused by straw mulching of fields would have reduced the temperature nationwide by an average of 2 °C ( Davin et al., 2014). To achieve this, farmers would have had to leave the straw on the ground after the grain harvest instead of ploughing it under. The 2003 heatwave was the deadliest natural disaster in Europe in recent centuries, with more than 70,000 fatalities in Europe and around 20,000 in France alone ( Robine et al., 2008). Furthermore, straw cover would have contributed to a shorter, less intense drought.
This is because straw cover has a number of positive side effects: it reduces the loss of soil moisture. The capillary effect on the evaporation surface is reduced due to the physical barrier provided by the straw cover. In addition, a straw cover improves infiltration during heavy rainfall and thus reduces erosion. Due to the better thermal insulation, which reduces heat loss from the soil during the night, dew formation also increases.