Sally D. Logsdon, NSTL, 2150 Pammel Dr., Ames, IA 50011
Plant available water is often calculated as the difference between the lower limit (usually assumed to the water content at 1.5 MPa), and upper limit (often defined as field capacity, or the water left after drainage ceases). Field capacity often does not relate to water available to plants. Drainage continues in fine-textured soils for extended times, and plants continue to take up while during drainage. Also water moves laterally to lower positions in the landscape, which could increase the plant available water unless the water table is so shallow that rooting is limited. The objective of this study is to incorporate these landscape factors into a concept of plant available water. Soil water content and water table depths were measured at fourteen landscape positions over four years. Mean water content was determined to the water table depth. The lower limit, determined from soil texture, was subtracted to get available water. The profile available water was determined by rooting depth, which was set equal to 1.3 m (based on observation) or the mean water table depth, whichever was more shallow. These parameters were extended to yield transect positions based on correlations with elevation, slope, and depressional depth at the fourteen sites. In wet years soybean and corn yields were negatively correlated with available water but positively correlated with profile available water. In drier years the yields were positively correlated with both available water and profile available water. Further study is needed to relate lateral water accumulations with landscape features.