Artificial Rainfall Events Research Articles

Performance of an unsaturated expansive soil slope subjected to artificial rainfall infiltration

A major infrastructure project, the South-to-North Water Transfer Project, has been proposed to carry potable water from the Yangtze River region in the south to many arid and semi-arid areas in the northern regions of China, including Beijing. The proposed 1200 km ‘middle route’ of the project is likely to be an open channel with a trapezoidal cross-section. At least 180 km of the proposed excavated canal will pass through areas of unsaturated expansive soils. An 11 m high cut slope in a typical expansive clay with medium plasticity in Zaoyang, close to the ‘middle route’ in Hubei, was selected for a comprehensive and well-instrumented field study of the effects of rainfall infiltration. Two artificial rainfall events were created during a month of field investigation and monitoring. Prior to the rainfall events, high soil suction was measured within the top 1 m of soil. This high initial soil suction created an upward flux of water and moisture. The abundant cracks and fissures near the ground surface played a key role in the soil suction–water interaction. After the rainfall, there was a delayed response of about 2 days before any significant changes of pore water pressure, earth pressure and ground movement were measured. A perched water table was deduced at a depth of about 1·5 m below the slope surface. Two distinct modes of down-slope deformations were observed: a cantilever deformation within the top 1·5 m at the midslope, and a deep-seated down-slope displacement at the toe of the slope. The observed continual soil swelling after the start of the rainfall appears to be governed by the low coefficient of permeability to water and the expansive nature of the soil. After heavy and prolonged rainfalls, earth pressure measurements suggest that passive failures may take place within the softened clay at shallow depths.

Performance of an unsaturated expansive soil slope subjected to artificial rainfall infiltration

A major infrastructure project, the South-to-North Water Transfer Project, has been proposed to carry potable water from the Yangtze River region in the south to many arid and semi-arid areas in the northern regions of China, including Beijing. The proposed 1200 km ‘middle route’ of the project is likely to be an open channel with a trapezoidal cross-section. At least 180 km of the proposed excavated canal will pass through areas of unsaturated expansive soils. An 11 m high cut slope in a typical expansive clay with medium plasticity in Zaoyang, close to the ‘middle route’ in Hubei, was selected for a comprehensive and well-instrumented field study of the effects of rainfall infiltration. Two artificial rainfall events were created during a month of field investigation and monitoring. Prior to the rainfall events, high soil suction was measured within the top 1 m of soil. This high initial soil suction created an upward flux of water and moisture. The abundant cracks and fissures near the ground surface played a key role in the soil suction–water interaction. After the rainfall, there was a delayed response of about 2 days before any significant changes of pore water pressure, earth pressure and ground movement were measured. A perched water table was deduced at a depth of about 1·5 m below the slope surface. Two distinct modes of down-slope deformations were observed: a cantilever deformation within the top 1·5 m at the midslope, and a deep-seated down-slope displacement at the toe of the slope. The observed continual soil swelling after the start of the rainfall appears to be governed by the low coefficient of permeability to water and the expansive nature of the soil. After heavy and prolonged rainfalls, earth pressure measurements suggest that passive failures may take place within the softened clay at shallow depths.

SIZE OF POINTERS FOR RESIDUE COVER MEASUREMENT

Comprehensive models for agrichemical transport necessarily include runoff predictions to partition rainfallbetween infiltration and runoff, as this ability is fundamental to predictions of chemical runoff and leaching. We compared GLEAMS, Opus, and PRZM-2 model runoff predictions with runoff measured in a precisely controlled field site used for chemical runoff studies. In 1992 and 1993, two 14.5 m 42.9 m corn (Zea mays, L.) field plots with 3% slope on Tifton loamy sand (fine-loamy, siliceous, thermic Plinthic Kandiudult) received six severe, artificial rainfall events over the growing season with each event consisting of a 25 mm hC1 rainfall for 2 h. Runoff was monitored continuously using a collector and flume. Model performance criteria included sensitivity analysis, graphical comparison and statistical analysis including mean, ratio of means, root mean square error (RMSE), and a paired difference t-test. Observed runoff averaged 20% of added rainfall. Lowest values occurred with freshly plowed soil or full canopy cover, while 24 to 34% runoff occurred when nearly bare soils had crusted over. Using an initial moisture condition-II curve number (CN) of 85, GLEAMS and Opus predicted runoff within 10%, overall, and produced a pattern of high and low runoff that closely followed observed. PRZM-2 overpredicted runoff by 90%, overall, and predicted its highest runoff when observed runoffwas lowest. Paired difference t-tests indicated a significant difference between measured and predicted runoff for PRZM-2 (p<0.001 at = 0.05), but none for GLEAMS (p = 0.761) or Opus (p = 0.194). Mean, ratio of means, and RMSE showed that GLEAMS and Opus performed better than PRZM-2. All three models were very sensitive to CN values which were empirical and subjective, but less sensitive to measurable soil physical properties. With careful parameterization, GLEAMS and Opus could be used to simulate runoff from similar row-crop and soil conditions.

GLEAMS*, OPUS, AND PRZM-2 MODEL PREDICTED VERSUS MEASURED RUNOFF FROM A COASTAL PLAIN LOAMY SAND

Comprehensive models for agrichemical transport necessarily include runoff predictions to partition rainfallbetween infiltration and runoff, as this ability is fundamental to predictions of chemical runoff and leaching. We compared GLEAMS, Opus, and PRZM-2 model runoff predictions with runoff measured in a precisely controlled field site used for chemical runoff studies. In 1992 and 1993, two 14.5 m 42.9 m corn (Zea mays, L.) field plots with 3% slope on Tifton loamy sand (fine-loamy, siliceous, thermic Plinthic Kandiudult) received six severe, artificial rainfall events over the growing season with each event consisting of a 25 mm hC1 rainfall for 2 h. Runoff was monitored continuously using a collector and flume. Model performance criteria included sensitivity analysis, graphical comparison and statistical analysis including mean, ratio of means, root mean square error (RMSE), and a paired difference t-test. Observed runoff averaged 20% of added rainfall. Lowest values occurred with freshly plowed soil or full canopy cover, while 24 to 34% runoff occurred when nearly bare soils had crusted over. Using an initial moisture condition-II curve number (CN) of 85, GLEAMS and Opus predicted runoff within 10%, overall, and produced a pattern of high and low runoff that closely followed observed. PRZM-2 overpredicted runoff by 90%, overall, and predicted its highest runoff when observed runoffwas lowest. Paired difference t-tests indicated a significant difference between measured and predicted runoff for PRZM-2 (p<0.001 at = 0.05), but none for GLEAMS (p = 0.761) or Opus (p = 0.194). Mean, ratio of means, and RMSE showed that GLEAMS and Opus performed better than PRZM-2. All three models were very sensitive to CN values which were empirical and subjective, but less sensitive to measurable soil physical properties. With careful parameterization, GLEAMS and Opus could be used to simulate runoff from similar row-crop and soil conditions.

Plugging Effects From Livestock Waste Application on Infiltration and Runoff

A rainfall simulator was used on repacked Waukegan silt loam and Hubbard loamy sand soil columns to determine the combined effect of rainfall and livestock waste application on infiltration and runoff. Dairy and swine waste slurries were either surface-applied or incorporated. Livestock waste application noticeably reduced the amount of runoff during a series of artificial rainfall events for all cases with the exception of swine waste incorporated into the silt loam soil. Loamy sand exhibited short-term plugging when both wastes were surface-applied with no incorporation. Surface-application of dairy waste on the silt loam soil apparently prevented formation of a surface seal and improved the infiltration capacity of the soil. Less surface-sealing in waste-applied columns may be attributed to increased organic matter on the surface of the soil that aided aggregate stability. Also, the waste particles protected the surface from the energy of the impacting raindrops.