In a first of its kind effort, researchers from all over the world have quantified soil erosion due to rainfall by collecting data from 63 countries to prepare a 'rainfall erosivity' map of the world. The study, published in Scientific Reports, is a joint effort of 31 scientists from over 20 countries all over the world.
Erosivity is the tendency of rainfall to cause soil erosion. Displacement of soil from its original place is called erosion. Rainfall, rapidly flowing water like streams and rivers, wind, or even mass movement of land like landslides or avalanches can cause erosion. Rainfall alone causes more than 50% of soil erosion in the world. Erosion takes away the nutrient-rich topsoil from the erosion site, and in the long run, leads to desertification. The other side of erosion is eutrophication - excessive richness of nutrients - caused by the eroded soil being deposited as sediments in water bodies like lakes and rivers, choking them in the process. This excess of nutrition in the water causes the unabated growth of vegetation and algae, resulting in reduced dissolved oxygen in the waterbody.
The erosivity factor
Erosion causes a lot of ecological damage too and impacts economies dependent on local resources. "When the water of the flooded Brahmaputra flows downstream in Bengal, it disrupts the fishing reservoirs of the region. The local fishermen, thus, lose their means of livelihood," says Dr Nabansu Chattopadhyay of the Indian Meteorological Department, Pune, and a co-author of the study.
Erosivity of soil is the combined effect of the duration of rainfall, its magnitude and intensity. "For example, if a place gets 100 cm rainfall in 24 hours, but it is quite uniformly divided, such rain may not cause soil erosion. However, suppose there is 10 cm rainfall in a mere one hour, then soil may get washed away," points out Nabansu. To determine erosivity, it is also essential to take into account the previous erosivity events that have occurred over a long period of time. Erosion is more likely to happen when the soil is already saturated with water and a heavy downpour occurs. Water can no more be absorbed into the soil, and soil then flows downstream with water.
Erosivity factor, evaluated based on the duration of rainfall, its magnitude and its intensity, is essential for assessing the extent of soil erosion by water. This also helps in calculating the risk of floods and preventing natural disasters. Suitable mitigation measures can then be taken to prevent losses to lives and property. The researchers of this study collected data from 3,625 precipitation stations all over the world and created a Global Rainfall Erosivity Database (GloREDa). The data came from rain gauges spread over different continents, in different climatic conditions, during different time periods and of different accuracies. Rainfall measurements at short duration, once every 30 minutes, was taken for evaluation.
The number of years for which data was available also had a large variation; some gauges had only 10 years of data, a duration much shorter than the 22 years recommended for evaluation of erosivity. The researchers had to interpolate this data suitably in the framework used for this assessment. Uncertainty is also introduced in erosivity values due to the variability of rainfall, its duration, its magnitude and its intensity in areas lying between different climatic zones.
The final map that the researchers came up with indicates that the highest erosivity values are located in Southeast Asia, Central Africa, South America, Central America and the Caribbean islands. The lowest erosivity was found in Siberia, West Asia, Northern Africa, Canada and Northern Europe. The researchers observed that spatial pattern of erosivity values corresponds to the extreme rainfall events observed in those areas. An analysis by the climate zone indicated that the tropical climate group that included tropical rainforest and monsoon climatic types showed highest erosivity. The cold climate group, that includes the subarctic region had lowest erosivity values.
Mitigation measures
Another major achievement of this study is the setting up of a global erosivity dataset with a spatial resolution of about one km that is now available. The researchers credit this to the availability of rainfall measurement at short duration, the growing computing power and the development of sophisticated geostatic models. "We acknowledge that this achievement was only feasible through the scientific cooperation between scholars from all over the globe," say the authors. The global erosivity map is now publicly available, and researchers can use it to model soil erosion at a national, continental or global level. This can certainly help plan erosion mitigation measures.
One way to conserve what is left of the soil is to increase the time of concentration of the runoff or reduce the velocity of the runoff. This allows more water to sink into the soil, and prevents it from carrying the soil along. Building terraces and bunds are one way to prevent the runoff at a smaller scale. On a larger scale, dams and retention reservoirs could also be planned, opine the researchers. Biological measures include planting vegetative strips, protective bushlands and forests in regions prone to increased soil erosion. Certain agricultural practices like contour ploughing, where the land is ploughed across a slope following its elevation contour lines, and mixed cropping, where two or more of plants are grown simultaneously in the same field, could also help.
For a country like India that is largely dependent on agriculture, soil plays a very important role in driving the economy. So what are implications do the findings of this study have? The researchers believe that soil conservation efforts in India can hugely benefit from the erosivity map. "The Ministry of Agriculture in India undertakes multiple soil and water conservation projects. With a view to better monitor rainfall, installation of automatic rainfall measurement gauges is planned," shares Nabansu. With more such systematic measures, perhaps soil erosion could be a thing of past.
(The author is with Gubbi Labs, a
Bengaluru-based research collective)
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