A model for predicting water table fluctuations in layered soils

Graduation date: 1978Water tables in many soils of the Willamette Valley of Western\ud Oregon are often near the surface. This is because they are underlain\ud by a relatively impermeable layer at a shallow depth, and the rainfall\ud during the winter months greatly exceeds the evapotranspiration. The\ud high water tables combined with fine-textured soils cause the specific\ud yield of the soils to be extremely sensitive to the exact position of\ud the water table at any particular time.\ud Methods commonly used for predicting water-table fluctuations in\ud response to rainfall assume that the specific yield is a constant for\ud a particular soil. This assumption is not valid for many soils of the\ud Willamette Valley. However, an estimation of water-table fluctuations\ud is needed for the identification of drainage problems and for the\ud design of drains. To obtain a more satisfactory method of predicting\ud water-table fluctuations for such soils; models were constructed to\ud extrapolate data from observation of water-table fluctuations for short\ud periods of time.\ud Models were constructed for both naturally and artificially drained\ud soils. When the models were constructed using data for an appropriate period, i.e., when the hydraulic conditions of the soil profile were\ud similar to those for which predictions were made, the models gave results\ud in close agreement with observed water-table fluctuations.\ud The inputs required for the model are rainfall amounts as a function\ud of time and the initial water-table position. The output is the watertable\ud elevation as a function of time. The model also calculates an\ud index, evaluating the quality of drainage in the field, by taking into\ud account the elevation of the water table and the length of time that a\ud water table remains closer than 30 centimeters to the soil surface