EFFECT OF RAINDROP IMPACT AND ITS RELATIONSHIP WITH AGGREGATE STABILITY TO DIFFERENT DISAGGREGATION FORCES

Abstract

A soil surface exposed to rainfall is subjected to processes of wetting and drop impact which can lead to the formation of a seal during the rainfall, reducing infiltration and increasing erosion by increasing runoff. The objective of this research was to evaluate the relationship between the effect caused by the drop impact and the aggregate stability of the soils when they are subjected to different disaggregation forces. The aggregates were subjected to cracking (by slow wetting), slaking (by fast wetting) and mechanical breakdown (by mechanical stirring after pre-wetting in ethanol). The effect of each process was evaluated by measuring the mean weight diameter (MWDsl, MWDf and MWDst, respectively) calculated as the sum of the mass fraction of soil left in the sieve after fractionation into four size classes, ranging from <0.25 to 2 mm, multiplied by the mean aperture of the sieve meshes and divided by the initial soil weight. The effect of water impact plus wetting was quantified by the saturated hydraulic conductivity of the seal (Ks) and the time necessary to reach this value. A relative sealing index (RSI) that measured the reduction of water intake caused by sealing was defined as the relationship between the minimum value of saturated hydraulic conductivity of the seal and that reached when the drop impact was avoided. The air-dry material rupture was evaluated with a penetrometer. The main soil characteristics that determine all these processes for the study soils were analysed. Most of the studied soils were very sensitive to slaking and mechanical breakdown, while they were stable when they were subjected to slow wetting. A significant relationship was found between the minimum saturated hydraulic conductivity (Ks) and the MWDst (R2=0.40, p<0.005), and between Ks and the MWDf (R2=0.69, p<0.05). In both treatments, slaking and mechanical stirring, the percentage of aggregates retained in the larger sieve mesh was also significantly correlated with Ks. This result could indicate that both processes are implicated in the disaggregation produced by drop impact, which contribute to seal formation process. The less stable soils had the lowest Ks value (<1 mm h-1), which was reached in a short period of time (<10 min). The high silt content and the low organic matter control the loss of aggregation by mechanical breakdown and the formation of the seal. The RSI values indicated a 200-fold reduction in water infiltration for some soils, caused by the formation of a seal.