Morphometric methods for simulation of water flow

Water flow in structured soils is strongly governed by the occurence of macropores. In this study emphasis was given to combined research of morphology of water- conducting macropores and soil physical measurements on bypass flow. Main research objectives were to: (i) develop and improve soil physical methods in such a way that hydraulic effects of macropores in soils are accounted for, and (ii) quantify soil structure in such a way that bypass flow phenomena can be realistically represented in simulation models.In chapter 2 a modified crust test is presented, to be used for measuring the unsaturated hydraulic conductivity near saturation. The test will be referred to as the suction crust infiltrometer. Since contact between soil and crust is perfect the suction crust infiltrometer is particulary suitable to express the effects of macropores on conductivity. Only one crust is used as fluxes through the crust occur at different heads, which makes the method less laborious and more convenient to use than the former crust test.Chapter 3 combines physical measurements of soil water potentials (using small transducer equipped automated tensiometers) and bypass flow with morphological data obtained from staining patterns. Tensiometers in close contact with water-conducting macropores reacted very quickly during the start and at the end of a bypass flow event. Other tensiometers showed slow reactions indicating a location deep inside structure elements. The various tensiometer reactions were used as a point count to identify occurence of soil physical processes such as internal catchment and bypass flow. Mass balances showed that 52 percent of the applied water left the soil cores through bypass flow while an estimated of 33 percent contributed to internal catchment.In chapter 4 a sensitivity analysis on processes affecting bypass flow was carried out. A simulation model was used to determine the relative impact of different soil physical properties and boundary conditions on different soil types. For the light textured soils surface infiltration was the most important term in the water balance. For heavy textured soils drainage through macropores was the most important mass balance term. Lateral absorption was only a minor fraction in the total mass balances. Surface infiltration is a crucial parameter in bypass flow and is mainly dependent on rain intensity, initial pressure head and the conductivity of the soil matrix.In chapter 5 an empirical equation was developed which relates total amount of measured outflow, derived from 5 bypass experiments, to fractal dimensions and volume fractions of methylene-blue stained macropores. The total amount of bypass flow was successfully regressed on measured time lag for initial breakthrough at the bottom of the soil column. This study indicates that morphology of flow paths is an important factor in the process of bypass flow. A physico-morphological approach for measuring and simulating bypass flow on 15 undisturbed soil columns was described in chapter 6. The impact of various physical boundary conditions on bypass flow was measured with a computer-controlled measuring device. Macropore geometry was characterized using fractal dimensions of methylene-blue stained macropores. Initial breakthrough at the bottom of the soil cores was predicted with a pedotransfer function based on fractal dimensions and methylene blue stained volumes. Measurements showed that geometry of water conducting macropores prevailed over soil physical properties with respect to water absorption along macropores walls. Soil physical and morphological data were used in a simulation model which predicted drainage at the bottom of the cores with significant precision. In chapter 7 bypass flow and nitrate leaching at the Kandelaar experimental farm was measured in drain outflow over a period of 4 years, using an automated monitoring set up. Bypass flow towards ground water exceeded sometimes 80 percent of precipitation. Simultaneously measured nitrate concentrations in drainage water increased during bypass flow, indicating that bypass flow is associated with an increased nitrate load to ground water. A catch crop grown directly after slurry application could reduce nitrate concentration in drainage water up to 75 percent. This study also showed the inability of suction cups in structured clay soils to provide representative samples for soil solutes.Chapter 8 describes the field scale modelling of bypass flow on the Kandelaar experimental farm. The LEACHW model was extended with a module that described bypass flow and tortuous water transport in macropores. This model was used in a Monte-Carlo analysis to calibrate the model parameters and to investigate the sensitivity of the various model parameters. Explained variances for the various terms in the water balance were always significant. Simulation of measured ground water levels and drain discharge from an independent year using the calibrated parameter set appeared successful