Physically based modeling of water infiltration with soil particle phase

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One of the most important problems of hydrological forecasting in agriculture is to obtain a reliable estimate of effective irrigation. Infiltration is one of the variables which greatly influences the partitioning of irrigation water (especially rainfall) into surface runoff and subsurface flow and continues to occupy the attention of soil physicists and agricultural engineers. It is regarded as a very complex process with several contributing factors. Most infiltration models have been developed for clear water and have limited application to poorly structured soils due to slaking of aggregates and dispersion of clay. In order to address this limitation, an infiltration model was developed to describe the process of infiltration of muddy water into a uniform soil profile based on the Green-Ampt (G-A) flow theory. In addition, it incorporated the concept of surface sealing and allowed for a developing seal with time. The cumulative infiltration amount was found to be related to a constant which is a function of the particle diameter of the sediment, and describes the saturated hydraulic conductivity of seal. Thus, a modified G-A method for infiltration with transient seal formation is proposed that provides improved infiltration, surface sealing, and time-to-incipient ponding predictions during infiltration of muddy water. A laboratory column experiment was conducted to measure the cumulative infiltration amount of clear water (0 g) and soil suspensions made from 10, 20, 30 and 40 g fine sand (= 0.05 mm), silt (= 0.002 mm), and clay (= 0.001 mm) after 60 minutes. The set of the laboratory-measured data were used to evaluate the performance of the infiltration model in simulating cumulative infiltration amount. The model provided good overall agreement with the laboratory-measured data. The result of the study showed that the cumulative infiltration amounts predicted by the infiltration model were very close to the laboratory measurements for the different fluids and their various concentrations as evidenced by average values of the slope of the regression line between the measured and predicted data, coefficient of determination and root mean square error. The coefficient of determination R2 ranged from 0.9986 to 0.9998 with RMSE ranging from 0.00814 to 0.0793. The accuracy of the model’s prediction capability was in the order of 0 g > 10 g > 20 g > 30 g > 40 g for sand suspension, and 0 g > 10 g > 20 g > 40 g > 30 g for silt and clay suspensions. Overall, the predictability of the model was ranked using the R2 values in the order: Clear water > Sand suspension > Silt suspension > Clay suspension. The method also provides a good approximation of surface seal thickness, since direct determination of the seal thickness is difficult. Thus, a simple linear equation is proposed for the estimation of seal thickness based on the concentration and settling velocity of the sediment particle. Sand sediments produced thicker seals, even at relatively lower concentrations. Silt and clay sediments produced very thin seals, and their thicknesses showed no significant variations (P < 0.05) statistically at equal concentrations and time intervals. Additionally, the major factors responsible for the formation of the surface seals were identified as the size, concentration and settling velocity of the sediment, and the flow velocity of the fluid. The results of the study on saturated hydraulic conductivity were used to predict the relative time-to-incipient ponding of the various sediment surface seals. It was deduced that clear water would take the longest time and clay suspension the shortest time to cause surface ponding. The study showed that the Modified Green and Ampt Surface Sealing (MGASS) infiltration model can be effectively used to predict infiltration under variable moisture conditions and quantitatively analyse the effect of soil texture on surface sealing and the seal properties, as well as assessing the effect of sediment concentration on infiltration and surface sealing phenomena.
A dissertation submitted to the School of Graduate Studies,Kwame Nkrumah University of Science and Technology in partial fulfilment of the requirements for the Degree of Doctor of Philosophy in Soil Science.