Saturday, 15 July 2006

Effects of Soil Slaking and Sealing on Infiltration – Experiments and Model Approach.

Juergen Schmidt, Technical Univ Freiberg, Agricolastr. 22, Freiberg, D-09599, Germany

Soil slaking and sealing are frequent features of many cultivated soils. The terms slaking and sealing refer to the breakdown of soil aggregates and the formation of a sealing skin which makes the soil surface less permeable. The physical processes of soil slaking and sealing are the result of the kinetic impact of raindrops on the soil surface and the translocation of soil particles by flowing water. When the drop impact forces exceed the internal cohesion of the impacted soil aggregates they break down into primary mineral particles. These particles are transported by surface runoff or washed into the soil surface layer. When deposited, the translocated particles could clog soil pores and form superficial layers characterized by higher bulk density and lower saturated hydraulic conductivity than the soil beneath. Due to the loss of soil water storage and infiltration capacities soil erosion and the risk of flooding are substantially increased. This paper refers to the EROSION 2D/3D computer model which simulates soil erosion by water on single slopes and small catchments. The runoff subroutine of EROSION 2D/3D uses a modified Green & Ampt infiltration equation in order to calculate rainfall excess. Because the Green & Ampt equation presupposes a rigid soil matrix the temporal variability of soil structure due to tillage, slaking and sealing, shrinking and swelling, biological activities etc. have to be considered by an additional empirical parameter which allows to calibrate the saturated hydraulic conductivity ks on the basis of measured data. n the EROSION 2D/3D model, this parameter is called a skinfactor, “Skf.” Values of Skf<1 reduce the infiltration rate, to take into account the effects of soil slaking and sealing as well as anthropogenic compaction. Values of Skf>1 causes a positive correction of infiltration rate, e.g. for the consideration of an increased infiltration into macropores due to soil shrinking, biological activity or tillage impact. If Skf=1 infiltration rate is obviously not affected by either slaking and sealing or macropores. Using experimental data from various soil erosion plots the temporal and spatial variability of the skin factor was estimated for different soils and tillage practices. Generally the results show that just after plowing, the hydraulic conductivity of the top soil is artificially increased and skinfactors are characterized by values >1. However, because of the weak stability of the loosened top soils they drop back to their original bulk density after a certain period of time. Generally linked to this process, is the formation of a compacted and less permeable skin at the soil surface. Accordingly, skinfactors decrease to values <1. So far skinfactors of 20 soils in the region of southeast Germany with various textures, land uses, tillage managements, and crops were collected over several years using a field-scale rainfall simulator. Taking these data into account, an appropriate estimation of the hydrologic effects of soil slaking and sealing can be achieved.

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