Infiltration regulation and stability analysis of soil slope under sustained and small intensity rainfall

Rainfall infiltration depth and mode can severely influence slope stability.With the sustained rainfall,the influenced region of slope gradually expands.By using the Green-Ampt model to the soil slope,infiltration regulation was discussed under sustained and small intensity rainfall.And the infiltration rate of unsaturated soil was proposed according to the saturated infiltration theory.Because of the changing of initial moisture content in depth of slope,the saturated or unsaturated infiltration rate and depth could also be changeable with the sustained rainfall infiltration.Based on the principle of strength reduction,the calculation model of slope safety factor was established under different initial moisture contents and infiltration modes.Then,the slope stability was quantitatively analyzed through software FLAC3D.The calculation results of soil slope engineering show that there is a shorter period for slope stability under different initial moisture contents and unsaturated infiltration ways at the slope wetting front.The stability period of slope is 33.3%according to different initial moisture contents of wetting front less than that of the same initial moisture content of wetting front.And the slope is easier to fail under the unsaturated infiltration.The results agree well with the actual situation under sustained and small intensity rainfall.

Responses of soil moisture to vegetation restoration type and slope length on the loess hillslope

Soil moisture, a critical variable in the hydrologic cycle, is highly influenced by vegetation restoration type. However, the relationship between spatial variation of soil moisture, vegetation restoration type and slope length is controversial. Therefore, soil moisture across soil layers（0-400 cm depth） was measured before and after the rainy season in severe drought（2015） and normal hydrological year（2016） in three vegetation restoration areas（artificial forestland, natural forestland and grassland）, on the hillslopes of the Caijiachuan Catchment in the Loess area, China. The results showed that artificial forestland had the lowest soil moisture and most severe water deficit in 100-200 cm soil layers. Water depletion was higher in artificial and natural forestlands than in natural grassland. Moreover, soil moisture in the shallow soil layers（0-100 cm） under the three vegetation restoration types did not significantly vary with slope length, but a significant increase with slope length was observed in deep soil layers（below 100 cm）. In2015, a severe drought hydrological year, higher water depletion was observed at lower slope positions under three vegetation restoration types due to higher transpiration and evapotranspiration and unlikely recharge from upslope runoff. However, in 2016, a normal hydrological year, there was lower water depletion, even infiltration recharge at lower slope positions, indicating receiving a large amount of water from upslope. Vegetation restoration type, precipitation, slope length and soil depth during a rainy season, in descending order of influence, had significant effects on soil moisture. Generally, natural grassland is more beneficial for vegetation restoration than natural and artificial forestlands, and the results can provide useful information for understanding hydrological processes and improving vegetation restoration practices on the Loess Plateau

Characterising Vertical Redistribution on Irrigated Furrows in the Tukulu Soil

Subsurface soil water redistribution on the South African Tukulu, also referred as the Cutanic Luvisols in other countries, was evaluated following single run irrigation （20, 40, 80 and 160 L/min inflow rates） in 90 m furrows. Changes in soil water content （SWC） at three horizons were monitored using neutron water meter. Measurements were made every 10 m starting 5 m from the furrow inlet for 455 h. HYDRUS-2D software was used to estimate soil hydraulic parameters through inverse optimization algorithms for redistribution at the inlet, midpoint and furrow end. Optimized model parameters compared with initial estimates recorded satisfactory agreement between measured and predicted soil water content, despite spatial variability. Effective hydraulic conductivity （Keff） for 0-600 mm and 0-850 mm profile flow domains demonstrated linearity with SWC although inconsistencies under field conditions were inevitable. The underlying layer restricted gravity and augmented redistribution with Keff assuming a steeper gradient than normal. Conversion of KCff and soil water content into a ratio assisted in quantifying rate of redistribution at 0-600 mm and 0-850 mm profile depth. Vertical redistribution was found to be limited within the upper 600 mm depth thus providing the opportunity to develop furrow irrigation with confidence that water productivity is optimized.

Determining the Optimum Inflow Rates for Micro-flood Irrigation on the Tukulu Soil

The performance of micro flood irrigation （MFI） under different inflow rates was evaluated on 90 m closed ended furrows in the South African Tukulu soil. A single irrigation was used to characterise the surface and subsurface soil water distribution from the 20, 40, 80 and 160 L/min inflow rates treatments. Neutron access tubes were installed to a depth of 1 m at every 10 m distance interval starting at 5 m from the furrow inlet. Soil water content measurements were taken using the WaterMan neutron water meter. The HYDRUS-2D software was also used to provide insight on irrigated furrows soil water content and subsurface water distribution. The 20 L/min produced a stream flow that could advance up to the 60 m furrow distance. The stream flow from the rest of the inflow rates were able to reach the furrow end with the 180 L/min recording the fastest advance time of 23 min. The 20 L/min and 40 L/min had recession period of less than 7 min while the 80 L/rain and 160 L/min lasted more than an hour. Distribution uniformity （DU） at longer furrow distances was the highest from the 80 L/min and 160 L/min with the 20 L/min and 40 L/min recorded similar performances at shorter distances. The 40 L/rain was one of the smaller inflow rates that recorded the highest DU of 0.96 for the generated average infiltrated depth of the 30 m long furrow and therefore should be adopted for furrow distances of less than 60 m on the Tukulu soil.