A Soil Moisture Budget Model Accounting for Shallow Water Table Influences

Soil moisture balance programs developed on well-drained soils were found to be unsatisfactory for a soil underlain by shallow water tables, a condition typical of about 9 million acres of cropland in Indiana. Capillary rise past a 105-cm root zone boundary was estimated as the difference between estimated evapotranspiration (ET) and changes in soil moisture under corn (Zea mays L.) on a tile-drained Typic Argiaquoll at West Lafayette, Ind. during three growing seasons, 1971–1973. Capillary water was found to supply an average of 27% of the ET in periods with little or no precipitation. Computer model estimates showed capillary water to furnish about 17% of the total ET over a 100-day period from 49 days before silking to 50 days after. Evapotranspiration was based on measured pan evaporation adjusted with crop development and moisture stress factors from the literature. Soil moisture in the root zone was measured by neutron counting and expressed as deficits from a variable holding capacity which was allowed to change in time depending upon the depth of the shallow water table. Water table levels were measured in open wells, and water table changes were statistically related to the estimated amounts of capillary rise for use in the model. The factors used to estimate capillary rise were the soil moisture deficit in the root zone and depth of the water table. The derived relationships with those obtained from literature sources and assumptions regarding runoff and recharge were programmed in a computer model for simulating the daily moisture status and changes in the corn root zone. Model inputs were pan evaporation, precipitation, soil moisture characteristics, corn silking date, and initial soil moisture conditions. The model was found to closely track measurements of both soil moisture and water table depths in four independent seasons: early and late plantings in 1970 and 1974.