Kinetic energy of rainfall as driving force of soil detachment and transport

Soil erosion processes are highly dependent on the rainfall energy which is a function of the size and falling velocity of raindrops. Interactions between raindrop size, velocity and shape, wind speed and the intensity of the thunderstorm control the power of the erosive storm. As raindrops increase in size, their terminal velocity and momentum increase. Increasing raindrop size increases their kinetic energy which affects soil detachment and transport processes. Limited data on raindrop characteristics are available; most studies were performed in a laboratory or field setting covering only a few storms. Objectives of this study are to 1) quantify and compare drop size distributions of rainfall events at comparable locations in Austria, Czech Republic, and New Zealand, 2) derive rainfall kinetic energy (KE) - intensity (I) relationships for rainfall erosivity estimation, 3) evaluate temporal/seasonal distribution of the KE-I relationship, and 4) investigate the relationship between rainfall erosivity and soil erosion. At several sites in Austria, Czech Republic and New Zealand disdrometers will be installed to determine the erosive force of rainstorms. Relationships between rainfall kinetic energy and intensity will be determined with respect to seasonal/monthly variability. For those sites with measured wind data, the impact of wind speed and direction will be evaluated. Next to the disdrometers small erosion plots will be installed to register the erosive force of the rainfall events. Based on the obtained results we will check and possibly modify and adapt the existing correlations between rainfall energy and soil detachment and transport.

Soil erosion processes are highly dependent on the rainfall kinetic energy, which is a function of the size and fall velocity of raindrops. Interactions between raindrop size, velocity and shape, wind speed and the intensity of the rainfall control the power of the erosive event. Objectives of this study are to 1) quantify and compare drop size distributions of rainfall events at comparable locations in Austria, Czech Republic, and New Zealand, 2) derive rainfall kinetic energy (KE) - intensity (I) relationships for rainfall erosivity estimation, 3) evaluate temporal/seasonal distribution of the KE-I relationship, and 4) investigate the relationship between rainfall erosivity and soil erosion. Field measurements were carried out at four sites in Austria (Mistelbach and Petzenkirchen), Czech Republic (Prague) and New Zealand (Christchurch). The device-specific measurements and the discrepancy in rainfall kinetic energy estimation from multiple disdrometers clearly demonstrated the effects of using different disdrometer types on the estimation of rainfall erosivity. This had commonly not been taken into account in previous studies using disdrometer data for rainfall erosivity estimation. A correction factor thereby proved useful to ensure improved comparability of kinetic energy between multiple disdrometers. Measurements under natural rainfalls at the three sites showed that splash erosion can be described as a linear function of the rainfall intensity. Temporal variation in rainfall characteristics are typical of natural storms. This explains why cumulative rainfall kinetic energy underestimated splash erosion. However, by dividing with the rainfall duration, better results were obtained. Spatial rainfall characteristics were shown to have an effect on the splash erosion between sites, although a full investigation into this aspect was hindered by the use of different disdrometers at the sites. The effect of initial soil surface conditions and the impact of kinetic energy on splash erosion was investigated also under simulated rainfall. The largest decrease in splash erosion rates was seen for samples with high initial water content and a partly developed surface seal. Surface seal formation was measured through a decrease in soil saturated hydraulic conductivity described by a power function of increasing cumulative rainfall kinetic energy. These samples showed also surface ponding which protected the surface from further raindrop impact. Soil samples with dry-crusted surfaces and fully developed surface seals had lower erodibility and infiltration capacity. However, samples without surface seals, which were exposed to lower intensity rainfall, presented higher soil detachment capacity due to lower cohesion of soil particles. The results showed that splash erosion is highly affected by the initial soil conditions. Continuous measurement of rainfall parameters at high temporal resolution is essential in estimating soil erosion and in describing the processes behind it. This should be taken into consideration not only in erosion experiments, but also in soil erosion prediction models.