Experimental Study On The Effects Of Physical And Chemical Conditioning On Runoff And Erosion

Many soils in arid and semi-arid regions have unstable structure which makes them susceptible to seal formation. Seal formation at the soil surface has been found to have big effects on soil erosion and infiltration rate. Seal formation is the result of two complementary mechanisms: (â… ) Physical disintegration of surface soil aggregates which are affected by drops impact energy and by the wetting rate of the soil surface (both are controlled by rain intensity and/or by inflow rate of concentrated flow) and (â…¡) Physichemical dispersion of soil aggregates which is affected by soil solution. Based on these two mechanisms, many researchers on physical methods (surge flow) and chemical control (using soil amendments Gypsum, PAM) have been explored to keep aggregates stability and prevent dispersion.Surge irrigation has been found to be effective in reducing infiltration and erosion. Many studies have recognized that aggregates stability was attributed to effects of interrupted flow, but it is difficulty to predict. In most PAM applications to the soil surface, PAM dissolved in irrigation water was applied. This practice is not possible in rain-fed agriculture because water for dry PAM dissolution is not available and PAM is not readily soluble. Labor and water needed for PAM dissolution and spraying makes PAM application in dry land farming uneconomical.Six types of the soils from Israel were used to investigate the effects of surface application of dry granular PAM mixed with mine gypsum (MG) or phosphogypsum (PG), the soils with different ESP and the effects of prewetting rates and types of flow (continuous flow or interrupted flow) on intake and erosion through experiments of rain simulation and miniflumes in a lab. The following results were obtained: Surface application of dry granular PAM mixed with MG or PG achieved the results as good as spray of PAM solution at the surface, increasing final infiltration almost by 4 folds as much as that of control, reducing erosion by 70-85%. Increasing the ESP of the soils decreased IR and increased runoff and erosion in the control; the magnitude of these changes depended on soil type, being the greatest in the loamy sand and the least in the calcareous loam. Spreading PAM mixed with PG or just PG was effective in maintaining high final IR and low runoff and wash erosion levels compared with the control. Use of PAM mixed with PG resulted in higher final IR and lower runoff levels than just PG. Conversely, with respect to wash erosion, PAM mixed with PG and PG alone had comparable effects on soil loss in the loam and clay~HH. In the loamy sand and the clay~E, use of PAM mixed with PG resulted in lower erosion levels than spreading only PG.In the soils wetted by concentrated flow (I), prewetting rates did not have effects on cumulative intake of two soils (silt loam and sandy clay) in continuous flow. Prewetting rates had a significant effect on that of the sandy clay in interrupted flow, but a negligible effect on silt loam. In the soils wetted by spray (II), infiltration decreased with an increase in wetting rates in sandy clay. 120 mm h"1 is a critical value at below that cumulative infiltration decreased with an increase in wetting rates and above that no further decrease in cumulative infiltration took place when wetting rates increased. Wetting rates had no effect on cumulative infiltration either applying continuous or interrupted flow in sandy loam.In wetting (I), erosion was affected differently by prewetting rates for two soils. In the sandy clay erosion increased by 320% when wetting rates changed from low to high rates while increased only by 30% for the silt loam. In wetting (II), erosion increased rapidly when applying the rates higher than 120 mm h"1 and no change on erosion took place at the rates low than 50 mm h"1 in sandy clay. The wetting rates had a relative small effect on erosion in silt loam.Interrupted flow reduced intake and erosion to sandy clay compared with continuous flow in wetting (I) and the reduction decrease with an increase in wetting rates. Applying interrupted flow to silt loam significantly reduced erosion but had small effect on intake. The observations are similar to that found in the most studies before. However, in the wetting (II), controversial results were founded that interrupted flow increased intake to sandy clay and erosion to silt loam compared with continuous flow. The study revealed the mechanism responsible for the finding is aggregate stability and determined interaction between wetting rates and interrupted flow in quantity.The results obtained in the dissertation will provide guide to prevent runoff and erosion in rain-fed areas and to improve irrigation uniformity in irrigated land.