This study was undertaken as an attempt to develop a simplified approach to a complex problem- the prediction of soil moisture under corn which, at times, may have a limited moisture supply. Soil moisture under corn was estimated for June, early August and early November by using April, June and August soil-moisture survey data as the starting points. All computations were made on the basis of the amount of plant-available water present. To obtain this value, both the field capacity and the wilting point of a soil need to be known. Precipitation amounts were added to the data for soil moisture supply after deducting a value for runoff. An antecedent precipitation index, which 'Varied with the season, was used to compute runoff.
For the April-June period, evaporation was estimated as 0.1 inch per day for as long as any available water was present in the top 6 inches. After early June, evapotranspiration was estimated by using open-pan evaporation data as the measure of the potential for evaporation. This was multiplied by a factor to account for crop development and, when necessary, by a factor that considered any moisture stress present. The product of these values gave the actual evapotranspiration for each day. The water used in evapotranspiration was removed from the shallow depths early in the season, with a gradual increase in the depth of extraction as the season progressed. By Aug. 1, water use was assumed to take place to a depth of 5-feet. Water use was apportioned among the different depths active in water absorption, with the largest percentage coming from the shallow depths.
Shaw, R. H.
"Estimation of soil moisture under corn,"
Research Bulletin (Iowa Agriculture and Home Economics Experiment Station): Vol. 34
, Article 1.
Available at: https://lib.dr.iastate.edu/researchbulletin/vol34/iss520/1